Full text of "The theory and practice of electrolytic methods of analysis"

REESE LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA
Class

vrv^fv-v^^yr^^

ELECTROLYTIC METHODS
OF ANALYSIS

ORGANIC CHEMICAL MANIPULATION. By J. T. HEWITT,

M.A., D.Sc., Ph.D., Fellow of the Chemical Societies of London and
Berlin, Professor of Chemistry in the East London Technical College.
With 63 Illustrations. Crown 8vo. 272 pp. 7s. Qd. net.

' The work will be of great service to many teachers of practical chemistry.'

ENGINEER.

STEEL WORKS ANALYSIS. By J. O. AENOLD, Professor of

Metallurgy, Sheffield Technical School. With 22 Illustrations sind
Diagrams. Crown 8vo. 105. Qd.

'Everything that a steel- works analyst may fairly be called upon to examine
lace in this volume.' NATUREJ

finds a p

EXPLOSIVES, The Manufacture Of. A Theoretical and

Practical Treatise on the History, the Physical and Chemical Properties,

and the Manufacture of Explosives. By OSCAR GUTTMANX, Assoc.

M.Inst.C.E., F.I.C. With 328 Illustrations. In 2 vols. Medium 8vo. 2. 2s.

' Mr. Guttmann's important book.' ENGINEER.

' In the work is given such an exposition of the manufacture as to enable
one to gain a fair grasp of the subject.' ENGINEERING.

THE ELECTRO -PLATERS' HANDBOOK. A Practical

Manual for Amateurs and Young Students in Electro-Metallurgy. By
G. E. BONNET. With Full Index and 61 Illustrations. Second Edition,
Revised and Enlarged, with an Appendix on Electro-typing. 3s.

'An amateur could not wish for a better exposition of the elements of the
subject.' ELECTRICAL REVIEW.

London: WHITTAKEE & CO.

THE THEOEY AND PRACTICE

ELECTEOLTTIC METHODS

ANALYSIS

DR BERNHARD NEUMANN
M

ASSISTANT LECTURER ON METALLURGY AT THE TECHNICAL
SCHOOL AT AACHEN

TRANSLATED BY

JOHN B. C. KEESHAW, F.I.C.

WHITTAKER & CO.

WHITE HART STREET, PATERNOSTER SQUARE, LONDON
AND 66 FIFTH AVENUE, NEW YORK
1898

I'lUNTED BY

SPOTTISWOODE AND CO., NEW-STREET SQUARE
LONDON

TRANSLATOR'S PREFACE

IN the preparation of the English translation of Dr.
Neumann's work on Electrolytic Methods of Analysis a
comparatively small number of alterations or corrections
have been found necessary ; and the Author's consent has
been in every case obtained for the few that have been
made.

The portion of the original work dealing with primary
arid secondary batteries, the dynamo, and the thermo-pile
has been omitted. The electro- chemical equivalents have
been recalculated upon the most reliable figures for the
atomic weights of the elements. The Translator has added
a few notes to the original text where such notes were
considered to be of utility. These notes are in every case
included in square brackets, in order to distinguish them
from the original text. The translation is provided with a
fairly complete subject index, and a name index, which it
is believed will increase the value of the book to those
engaged in original research in this branch of chemistry.

With regard to the question of current supply, it may
be pointed out that city and town laboratories can obtain
the current required from the supply mains of the electric-
lighting companies.

The installation of small transformers for reducing the
current voltage to five volts, and where necessary for
converting alternating currents into direct ones, is a com-
paratively simple matter. When continuous work is con-
templated, the addition of one or two storage batteries
would be requisite.

VI TRANSLATOR'S PEEFACE

Three warnings may be given to those who through the
instrumentality of this book are led to make practical use of
electrolytic methods in their laboratories for the first time :

1. Electrolytic methods should only be used when
decided advantages in time or accuracy will result. The
practical examples given in Part III., D, clearly indicate
the principles upon which electrolysis can be used with
advantage in analytical work. The Author's concluding
paragraph on page 245 wisely states that it will 'always be
found most convenient to combine the chemical and electro-
lytic methods of separation.' The attempt to carry out
complete analyses by means of electrolysis, or to use
electrolytic methods for the determination of metals more
conveniently estimated by gravimetric or volumetric
methods of analysis, will generally result in failure, and
may lead the chemist who has not had any experience of
the advantages to be gained from these new methods, when
rightly applied, to confine himself more rigidly than before
to the older methods of analytical work.

2. The greatest care and attention must be given to
the precautions, mentioned on page 85, relative to the elec-
trodes. Many failures of electrolytic methods in the hands
of students and novices could, no doubt, be traced to neglect
of these very elementary conditions of successful work.

3. The conditions as to current density, temperature,
and E.M.F. mentioned in the detailed descriptions of the
various methods in Part III. must be strictly observed.
Slight variations in these conditions will in many cases
suffice to entirely alter the nature or character of the deposit.

In conclusion, the Translator hopes that the reception
accorded in England and America to the English edition
of Dr. Neumann's work may be as favourable as that
given to the original in Germany one year ago.

LONDON : January 1898.

AUTHOE'S PEEFACE

UP to the present time, two works have been published
which treat of electrolytic methods of chemical analysis,
One of these was written by E. Smith, and has been
translated into German by Ebeling ; the other was
written by A. Classen.

Both of these works deal principally with the authors'
own methods, although a few others receive mention.

In the meantime electrolytic methods of analysis have
been adopted in many technical laboratories, and have been
accepted as valuable aids, and in some cases as useful
alternative processes, to the ordinary analytical procedure.

The methods now customarily, and even exclusively,
used in these technical laboratories for the determination of
different metals, receive in the above-named works only
subsidiary mention ; the current is given in terms of
detonating gas ; the voltage is not even mentioned. In
the present work these faults and omissions are rectified.

In the consideration of the methods of electrolytic
determination of single metals, the methods of greatest
technical importance receive the most ample treatment.
These are described in detail ; exact data regarding current,
voltage, and temperature are given, so that even the novice
will be in a position to carry them out with some degree of
success. The more important of the remaining methods
are also noticed briefly, and their relative advantages and
disadvantages are discussed.

Vlll AUTHOR'S PEEFACE

Following the next division of the work, which is
devoted to metal -separations, there comes a subdivision
containing a selection of practical examples. In this
it is shown that in the analysis of metals, alloys, and
smelting works' products, electrolytic methods of analysis
have already found acceptance, or could advantageously
be adopted.

Since the newer theories relating to electrical phe-
nomena are steadily meeting with more general acceptance,
it is certainly fitting that works on electrolysis, both
analytical and technical, should be provided with a brief
review of them.

The Author has therefore devoted the opening chapters
of the present work to such a summary. The phenomena
and laws of electrolysis are discussed and explained in the
light of the newer theories.

The most convenient forms of current-measuring and
regulating apparatus are specially described.

It has been throughout the Author's chief aim to
provide both the student and the practical chemist with a
work which should cover the whole of the ground ; one
which, while it treated fully of the theoretical side of the
subject, and gave all the necessary explanation of electro-
chemical phenomena, should still deal in an unusually full
manner with the practical aspects of these new analytical
methods, and should enable both the student and practical
chemist, by its large number of practical examples, and by
its full descriptions of the apparatus and instruments used,
to avoid those errors into which they might otherwise fall.

The numerous references to the original literature of
the subject may be regarded as a useful appendix to the
work.

DR. B. NEUMANN.

STOLBERG : September 1896.

CONTENTS

PAQB
INTRODUCTION 1

PART I

THEORY OF ELECTROLYSIS

CHAP.

I. THE PHENOMENA OF ELECTROLYSIS . . . 6

ii. FARADAY'S LAW 16

III. THE CONSTITUTION OF THE ELECTROLYTE . . 21

IV. THE MIGRATION OF THE IONS 24

V. THE CONDUCTIVITY OF THE ELECTROLYTE . . 27

VI. THE DISSOCIATION THEORY 30

VII. THE CHEMICAL AND MOLECULAR CHANGES DURING

ELECTROLYSIS 33

PART II

MEASURING AND REGULATING THE CURRENT
A. CURRENT MEASUREMENT

VIII. MEASUREMENT OF THE CURRENT STRENGTH . . . 50
IX. MEASUREMENT OF ELECTRO-MOTIVE FORCE (B.M.F) . 61

B. REGULATING THE CURRENT"

X. INCREASING THE CURRENT STRENGTH . . . . 65
XI. REDUCING THE CURRENT STRENGTH . . . .74

x CONTENTS

PART III
THE ELECTROLYTIC PROCEDURE

J'AC.K

A. INTRODUCTORY . . 79

B. DEPOSITION OF THE METALS FROM SOLUTIONS OF PURE

SALTS 91

C. SEPARATION OF THE METALS 165

D. PRACTICAL EXAMPLES 220

APPENDIX

THEORETICAL PERCENTAGE OF THE METALLIC ELEMENTS IN

CERTAIN METALLIC SALTS ....... 246

NAME INDEX 247

SUBJECT INDEX ........ 249

ELECTBOLYTIC METHODS

ANALYSIS

INTRODUCTION

IN the year 1792 Volta commenced to investigate Gal-
vani's discovery, and there resulted from his investigations
1 the voltaic pile.' Since that time a very large number
of arrangements of conductors of the first and second class
have been constructed, by means of which it has been possible
to produce an electric current with ease. It was there-
fore natural that the mode of action of the different
elements, or piles, should have received close study, and
that careful attention should have been given to the
attendant phenomena. One of the earliest observations
was, that water which had been made acid with sulphuric
acid was split up into its components oxygen and
hydrogen by the electric current. The discovery of the
decomposition of metallic salt solutions, and of the easy
separation of the metallic component, generally as a metallic
coating upon one electrode, quickly followed. We find
such depositions already technically employed at the end
of the thirties. Jacobi, who is to be regarded as the
founder of the art of electroplating, had already in 1839
prepared electrotypes of various objects, and these were

2 ELECTROLYTIC METHODS OF ANALYSIS

exhibited to the members of the St. Petersburg Academy.
Other investigators must have also devoted themselves
with zeal about this time to the study of the subject, for
in 1840 and the following years a large number of
methods were published relating to the preparation of
solutions from which one could obtain, without fail, ex-
ceptionally beautiful deposits of certain metals. For
example, in 1840 Ruolz, Elkington, and de la Rive
proposed to use potassium cyanide solutions for obtaining
deposits of gold and silver ; in 1841 the same solution
was proposed for copper and nickel, and a sodium hy-
drate solution for tin. Originally these methods were
only designed to be used for electroplating purposes ; but
since very small amounts of metals can be deposited and
detected in this way, similar methods were sought which
should render feasible the quantitative estimation of
metals, especially of poisonous ones in foods &c. (Bloxam,
Morton). These methods are, for particular determina-
tions, still in use. In 1805 Davy had pointed out in-
cidentally in his working notes that the electric current
might be used for chemical analysis, but it is Antoine
Becquerel (the elder) who must be honoured as the real
founder of analytical electro-chemistry. Becquerel pub-
lished, as far back as 1830, a practical method for
separating small amounts of lead and manganese from
other metals, the manganese being obtained as peroxide
at the anode. He showed, further, that it was possible in
a definite time to separate as peroxide all the manganese
contained in a known weight of manganese acetate. It
was not, however, until the commencement of the sixties
that electrolytic separations began to be used as aids
to, and in some cases as substitutes for, the ordinary
methods of analysis. In 1864 W. Gibbs separated electro-
lytically nickel and copper from nickel coins. In 1865
Luckow published a large number of experiments, and
showed out of what solutions it was possible to obtain
quantitative deposits of metals. In 1867 he received a

INTRODUCTION 3

prize from the ' Mansfeld'schen Ober-Berg- und Hiitten-
Direktion ' for his electrolytic method of estimating the
copper contained in the Mansfeld schists.

Since this date the employment of electrolysis both
for analytical and technical purposes has extended greatly.
While the most important facts e.g. the more or less
good or rapid deposition of metals from different solutions,
the variations in the current required, the separation of
different elements or groups of elements from the same
solution according to the current intensity or voltage, the
influence of temperature, &c. were long known, there
was lacking, until a few years ago, a theory of electrolysis
which explained clearly these phenomena.

Similarly -there was no theory by means of which it
was possible to explain clearly the changes in the energy-
creating couple. It is true that at the commencement of
this century a violent strife arose between the supporters
of the ' contact theory ' and the supporters of the ' chemical
theory' of the origin of the energy in the voltaic cell.
This strife continued into the third quarter of the century
without the supporters of either the one theory or the
other being enabled to give a clear explanation of the facts.
Such an explanation has only become possible within the
last few years, by aid of the theory of osmotic pressure, or
' osmosis.'

In the same way that this modern theory has satis-
factorily explained the origin of the energy in the voltaic
cell, the newer theories of solution and of electrolytic
dissociation, which are the result of the researches carried
on in the domain of physical chemistry during the past
twenty years, have resulted in a deeper insight into the
changes involved in the conduction of the current by
electrolytes, and have elucidated many hitherto puzzling
phenomena connected with this subject.

It will not therefore be superfluous if the opening
chapters of this work be devoted to a brief summary of
those currently accepted theories which are necessary for an

4 ELECTEOLYTIC METHODS OF ANALYSIS

understanding of the phenomena to be observed in the
conduct of electrolytic methods of analysis. 1

1 Those interested in the study of the historical development of
Electro-chemistry are referred to ' Elektrochemie, ihre Geschichte
und Lehre,' by W. Ostwald.

PABT I
THEORY OF ELECTROLYSIS

THE term * electrolysis ' is used to denote the chemical
phenomena and changes, accompanied by movement of the
particles of matter, which are produced when an electric
current is passed through a fluid conductor, i.e. a conductor
of the second class.

- Faraday, with whom originated the terms still in use,
named those bodies which, when in solution or in the
molten state, conduct the current in this manner
* electrolytes.'

The term ' electrode ' is used to denote those parts of
the conductors of the first class, carrying the current to
the electrolyte, which are in contact with it. A difference
of potential is produced by the electric current at the
electrodes, and as a result of this a movement of the ultimate
particles of matter present in the electrolyte the ' ions '
follows.

The particles, which differ fundamentally, move in
different directions ; those which move with the positive
current are called ' kations,' whilst those moving in the
contrary direction are called 'anions.' The electrode
towards which the kations drift is called the * kathode ' ;
that towards which the anions drift is called the l anode.'

The liberation of the ions at the electrodes causes
changes at the latter, which vary greatly in character.

THEOEY OF ELECTKOLYSIS

CHAPTER I

THE PHENOMENA OF ELECTROLYSIS

WHEN an electric current is passed through an electrolyte,
movements of the ions of different kinds towards the
opposite sides occur, and the products liberated at the
electrodes are consequently different.

The most simple case for consideration is that in which
the electrolyte, either in the dissolved or molten state, is
made up of only two component parts, or contains only
a base and an acid radical. For instance, if zinc or copper
chloride be electrolysed with a sufficiently strong current,
zinc or copper will be deposited at the kathode ; while the
chlorine will drift towards the anode, and, when this is of
a non-porous and to some extent chlorine-proof material
(chlorine attacks and destroys in time all electrodes), will
be liberated there as a gas. The same results are obtained
if zinc chloride or lead chloride be melted in a crucible, or
pipe-head of red clay previously warmed, and an electric
current be passed through the mass,, by means of a needle
passing down the straight stem as kathode, and of a carbon
pencil in the bowl as anode. Small spheres of molten zinc
or lead collect at the bottom of the bowl, while a portion
of the chlorine is liberated as gas at the anode. If hydro-
chloric acid be electrolysed with platinum electrodes, the
ions are hydrogen and chlorine. The hydrogen drifts as
the metals generally, towards the kathode ; and, being a gas,
can be partly retained (occluded), according to the character
of the electrode.

OF THE

UNIVERSITY

THE PHENOMENA OF

In the instances given above where the compounds
have been made up of only two components, the products
of the decomposition have been liberated directly at the
electrodes. How will the results be affected when more
complex compounds are electrolysed ?

As a general rule the passage of the current is accom-
panied by a similar division into two sets of drifting ions,
whether the constitution of the molecule be simple or
complex.

Berzelius, who supposed that salts were made up of two
parts the base (an oxide) and the acid (an anhydride) and
consequently wrote the formula of potassium sulphate, for
example, K 2 O.S0 3 , represented the school who thought that
these two component parts not only acted as such in
chemical reactions, but also drifted and separated as such
in electrolysis. In order to bring this theory into harmony
with the observed facts, he was obliged to assume, in
addition, that the electric current decomposed water and
liberated its constituent parts. The deposition of metals
from neutral salt solutions was then explained by him as
follows : The oxide of the metal drifted to the kathode,
and was there reduced by the nascent hydrogen resulting
from the decomposition of the water, and deposited as
metal.

The halogen salts would not fit into this system, and it
was partly on this account that he came to regard them as
oxygen-holding compounds.

Daniell brushed away these contradictions, and proved
that a salt is in every case to be regarded as a combination
of a metal and an acid radical. The latter may be either
a single element, as in the halogens, or a complex group of
different elements.

Hydrogen, on account of its behaviour, is to be regarded
as a metal ; the hydroxyl group of the different bases, on
the contrary, is to be regarded as an acid radical. The acids
are therefore hydrogen salts ; the bases are salts, of which
the acid radicals are the hydroxyl groups. Daniell further

8 THEOEY OF ELECTEOLYSIS

showed that potassium sulphate, for example, on electrolysis
between platinum electrodes, decomposes as other salts, into
the metal ion K and the acid ion SO 4 . These component
parts then lose their charges of electricity at the electrodes,
and secondary action upon the water follows. 1
The potassium decomposes the water :

K 2 + 2H 2 O=2KOHf H 2 ;
likewise the SO 4 ion at the anode :

SO 4 + H 2 0=H 2 SO 4 +

The final products are therefore caustic potash and
hydrogen at the kathode, an acid and oxygen at the anode.
The amounts of alkali and acid formed at the electrodes are
equivalent to the amounts of the respective gases.

That there is an actual formation of caustic potash
solution at the one electrode, and of sulphuric acid at the
other, is most simply shown by performing the electrolysis
in a V-tube containing a litmus-coloured solution of the
salt. The colour of the acid solution then changes to red,
and of the alkali solution to blue. A similar proof that K
and SO 4 are the products which drift towards the electrodes
is obtained by covering some mercury contained in a vessel
with potassium sulphate solution and by use of the mercury
as kathode with a strong current. Potassium amalgam is
formed, and this, separated from the electrolyte and treated
with water, gives visible proof of the presence of potassium,

Water shares in the carrying of the current only to a
very small degree.

The view held by Berzelius, that when potassium
sulphate was electrolysed the water also suffered con-
siderable decomposition, must be regarded as incorrect.
Daniell himself proved this, by placing in the circuit a
voltmeter containing dilute sulphuric acid, and noting that
the volumes of gases liberated in both electrolytic cells

1 The modern theory, as will be shown later, gives a simpler
explanation of the still customary conception of secondary decom-
position.

THE PHENOMENA OF ELECTEOLYSIS 9

were the same. The electrolysis of dilute sulphuric acid,
it is evident, must yield the same gases as that of potassium
sulphate namely, hydrogen and oxygen.

The electricity moves in such a way in conductors of the
second class i.e. electrolytes that the metals, the metal-
loid radicals of salts and bases, and the hydrogen of the
acids, all drift from the positive side of the cell-circuit
towards the negative ; while the acid radicals, the halogens,
and the hydroxyl groups of the basic compounds drift in
the contrary direction. No element or ion is known
which can appear both as kation and anion. The following
are to be classed as kations : The metals ; hydrogen and
the radical NH 4 ; organic substitution products of NH 4 ,
PH 4 , AsH 4 ; further, SR 3 , SeR 3 , TeR 3 , and other similar
series in which R represents hydrocarbon radicals. The
anions may be regarded as the remaining radicals of con-
ducting bodies ; as, for example, OH, Cl, Br, I, N0 2 , NO 3 ,
C10 3 , C10 4 , S0 4 , Se0 4 , P0 4 , As0 4 .

In general, one may use the following definitions : The
anion is all that which, combined with hydrogen or a
metal, forms an electrolyte ; the kation is all that which,
combined with a halogen or an acid radical, forms an
electrolyte. Oxygen, sulphur, selenium, and tellurium are
anions, but they occur chiefly in the forms OH, SH, SeH,
and TeH. The ions do not all possess the same valency ;
for instance, zinc is a dyad (Zn 11 ), bismuth a triad (Bi 111 ), while
manganese and antimony are Mn 11 and Sb m respectively.
The anions S0 4 and PO 4 are the first a dyad and the second
a triad, and are written S0 4 " and PO 4 m . Metal ions and
acid ions with variable valency are also known. The
following list of metals, in which the valency is signified
by the Roman numerals, shows examples of this : Fe 11 ,
Fe m ; Ou 1 , Cu 11 ; Hg 1 , Kg" ; Au 1 , Au m ; Sn", Sn iv . Simi-
larly the following variable anions are known : Fe m (CN) 6 ,
Fe IV (CN) 6 ; Mn'0 4 , Mn"O 4 .

It has already been shown that one may regard both
acids and bases as salts, and Hittorf has founded upon this

10 THEOEY OF ELECTEOLYSIS

view the following general definition : Electrolytes are
salts ; they break up on electrolysis into the same atoms
or atom groups which they exchange in chemical reactions.
Later it was found that one could go further and say that
all chemical reactions are exclusively reactions between
ions that is to say, elements or groups of elements can
only be detected by the customary reagents when they are
present in the ionic state. As an example of this we have
the detection of Cl in common salt or in hydrochloric acid
by means of silver nitrate.

In the chlorine substitution "products of acetic acid, or
in chloroform, no chlorine reaction is obtained with silver
nitrate, for the ions of monochloracetic acid are Na or H,
and CH 2 C1.COO. The same reasoning applies to the test
for iron in ferric chloride or sulphate of iron, as opposed
to the test for iron in potassium ferrocyanide ; for here
also the splitting up into ions is different, and is as
follows: Fe 2 C1 6 ; Fe|SO 4 ; but K 4 |Fe(CN) G . 1

When such salts as ferric chloride are electrolysed, the
constituents which migrate as ions are found to be those
which are detected by chemical reactions, and these separate
at the electrodes. When, however, potassium ferrocyanide
in which no iron can be detected by the ordinary tests,
and in which therefore iron does riot exist in the ionic
state, is electrolysed, the complex group Fe(CN) G migrates
as a simple ion towards the anode, while no deposition
of iron occurs at the kathode. Salts which contain two
different bases are usually designated double salts.

The term strictly, however, ought to be confined to those
salts which on electrolysis yield both the metallic con-
stituents at the kathode ; and salts of the class repre-
sented by potassium ferrocyanide, where the potassium
alone migrates towards the kathode and the whole of the
remaining complex group with the other metal migrates
towards the anode, should be denoted by the term ' com-

1 The bearing of these new views upon analytical chemistry is
treated of in Ostwald's ' Theoretical Chemistry.'

THE PHENOMENA OF ELECTEOLYSIS 11

plex.' (Further reference will be made to this point in
Chapter VI., upon 'Dissociation of Salts in Solution.')
The double salts of potassium cyanide, the double oxalates
and phosphates, are examples of these complex salts. One
may indeed regard these complex salts, owing to their
method of splitting up, as binary salts, of which the
anion is a complex acid radical. For example, we have
potassium ferrocyanide, K 4 |Fe(CN) 6 , the potassium salt
of hydroferrocyanic acid ; sodium platinum chloride,
Na 2 !PtCl 6 , the sodium salt of hydroplatinic acid ; and,
similarly, the cyanides K 2 Ni(CN) 4 , K Ag(CN) 2 , and the
oxalateK 3 |Cr(C 2 4 ) 3 .

It is noteworthy that, while potassium zinc sulphate,
K 2 Zn(SO 4 ) 2 , is a true double salt, a complex salt potassium
zincate is formed when caustic potash in excess is added to
its solution. This salt is to be regarded as the potassium
salt of zincic acid. Similar salts of bismuth, arsenic, and
antimony are known.

If the complex anion produced by electrolysis of these
salts did not enter into secondary reactions, the separation
of the metallic element would be as little possible by
electrolysis as its detection by chemical reactions. In
some cuses these complex groups do indeed remain practi-
cally unchanged ; in others they decompose by secondary
reactions more or less quickly. In the case of a few, this
decomposition is so rapid as for example in the double
cyanide of nickel and potassium that one is inclined to
regard them as true double salts. By such secondary
decompositions of the anion, the metallic constituent of the
group becomes an ion and migrates as a kation towards
the kathode, where its deposition occurs. It may be
remarked here that this kind of secondary deposition has
become for many metals of practical importance, not only
in chemical analysis, but also in electrotyping and electro-
plating.

From such solutions of complex salts it is possible
to obtain even compact and adherent deposits of certain

12 THEOEY OF ELECTROLYSIS

metals which separate in crystalline or dendritic form, or
which show a tendency towards formation of spongy
deposits, when their simple salts are electrolysed. It is for
this reason that silvering and gilding by ' the wet method '
that is to say, by electrolysis are always undertaken by
means of solutions of the double cyanides.

Hittorf has given the following detailed explanation of
the electrolytic decomposition of the double cyanide of
silver and potassium, when insoluble platinum electrodes
similar to those used in electrolytic analyses are em-
ployed.

The kation potassium migrates towards the kathode ;
the remaining part of the molecule, Ag(CN) 2 , migrates as
anion towards the anode, where it coats the surface of the
platinum with AgCN, and (CN) is set free as gas. The
migrating potassium anion, however, reacts secondarily with
the uiidecomposed original salt, according to the equation

K + KAg(CN) 2 =2KCN + Ag

The liberated silver migrates towards the kathode, and
is there deposited ; the newly formed potassium cyanide
re-dissolves the silver cyanide (AgCN) which covers the
platinum anode, and forms anew the complex salt ; and
this cycle of changes continues until all the silver has been
deposited at the kathode. An anode of silver is used when
silver-plating ; the anion Ag(CN) 2 in this case dissolves an
atom of silver from the anode surface, and re-forms with the
2KCN the complex salt KAg(CX) 2 .

The decomposition of the double chloride of potassium
and gold follows the same course.

The ions K and AuCl 4 are first formed ; the anion
AuCl 4 splits up into AuCLj and Cl, especially easily as the
dilution increases ; and this gold chloride (AuCl 3 ) then
breaks up into its constituent elements, gold and chlorine.

If a solution of potassium ferrocyanide, K 4 Fe(CN) G ,
slightly acidified with hydrochloric acid, be electrolysed be-
tween insoluble electrodes, Prussian blue, (Fe 2 ) 2 [F e (CN) 6 ] ;3 ,

THE PHENOMENA OF ELECTROLYSIS 13

is formed after some time, when the solution used is very
dilute.

According to Hittorf, the potassium ion migrates to-
wards the kathode and there decomposes water with libera-
tion of hydrogen :

K 4 + 4H 2 0=4KOH + H 4 .

The radical Fe(ClSr) 6 drifts towards the anode, and in
concentrated solutions that is to say, in solutions contain-
ing sufficient potassium ferrocyanide forms potassium
ferricyanide :

3K 4 Fe(CN) 6 + Fe(CN) 6 =4K 3 Fe(CN) 6 .

When, however, the solution is very dilute, the reaction
takes a different course, and Prussian blue is formed
according to the following equation :

Fe(CN) 6 + 2H 2 0=H 4 Fe(CN) 6 + 2
7H 4 Fe(CN) 6 + O 2 = 24HCN + (Fe 2 ) 2 [Fe(CN) 6 ] 3 + 2H 2

When the anions are constituted of many elements,
especially in the case of the radical groups of organic acids,
one can frequently observe that reactions occur between
the similarly constituted anions. These reactions result
generally in the formation of gaseous products, which
either escape or enter again into combination with other
ions simple or complex present in the solution. Such
reactions occur during the electrolysis of nearly all organic
acids and salts.

The decomposition of formic acid by electrolysis takes
place according to the following equations :

HCOOH=H + HCOO
HCOO + HCOO=H 2 + 2C0 2
2HCOO + H 2 0=2HCOOH +

At the anode, carbon dioxide and oxygen are evolved,
while hydrogen is evolved at the kathode. When the
alkali salts of this acid are electrolysed, hydrogen is also
evolved at the kathode in consequence of the reaction
between the liberated alkali metal and the water. When

14 THEOEY OF ELECTROLYSIS

the formic acid salts of the heavy metals are electrolysed,
the metal itself is of course deposited at the kathode.

The electrolysis of acetic acid or its salts follows a

similar course :

PTT PO

i H 2

or one may assume that a direct splitting up into the two
anions CH 3 COO occurs :

2CH 3 COOH=2CH 3 COO + H 2
2CH 3 COO=C 2 H 6 + 2CO 2

The final products at the anode are in either case
ethane and carbon dioxide.

It is possible, however, for other products to occur,
according to the concentration of the solution and the
strength of the current.

Bourgoin found only carbon monoxide and carbon
dioxide ; Bunsen found other products in addition to these ;
while Jahn, when using a low- current density, found
carbon dioxide and ethane.

When solutions of oxalic acid or its salts are decomposed
by the electric current, the acid radical of the anion falls
at once into two molecules of carbon dioxide.

r COOH 9m ITT

lCOOH =2C 2|H2

If the potassium salt of this acid be electrolysed, the
liberated carbon dioxide reacts with the potassium hydrate
formed at the kathode, and potassium bicarbonate is

formed :

2K + 2H 2 O=2KOH + H 2
2KOH + 2C0 2 =2KHCO 3

When the electrolysis is performed with the ammonium
salt, the corresponding ammonium compound is formed ; but
this immediately splits up into ammonium hydrate and
carbon dioxide.

THE PHENOMENA OF ELECTROLYSIS 15

From the heavy-metal salts the metals alone are
deposited.

The salts of tartaric acid yield on electrolysis carbon
dioxide, carbon monoxide, and oxygen as final products at
the anode, with small quantities of formic aldehyd and
formic acid.

The metal double salts of the above-named organic acids
are occasionally made use of in electrolytic methods of
analysis.

It is to be noted that only those organic compounds
which correspond to the inorganic salts in constitution are
to be regarded as true electrolytes.

16 THEOKY OF ELECTEOLYSIS

CHAPTER II

WHEN an electric current is passed through different bodies
the movement of electricity in these bodies can occur in
two different ways.

Conductors of the first class metals, alloys, carbon,
and a few other materials exhibit a heating effect which
follows the law of Joule ; but beyond this they exhibit no
change. In the conductors of the second class electro-
lytes however, chemical change is a condition of the
transfer of electricity.

In the previous chapter a large number of examples
have been given of the chemical changes which accompany
the passage of the electric current through molten or dis-
solved salts.

Michael Faraday, who was engaged with experiments
bearing upon the measurement of electrical energy,
discovered, as a result of these, in 1833, the law of 'invari-
able electrolytic action.' When any compound is decom-
posed by an electric current, the weight decomposed is found
to be proportional to the amount of current used ; and the
relative weights of the different elements or groups of
elements separated in the same time are found to be
represented by the equivalent weights of the elements. 1

Helmholtz expressed this law as follows :

The same current liberates in different electrolytes the

1 The equivalent of an element is the atomic weight divided by
the valency.

FAEADAY'S LAW 17

same number oj valency bonds, or engages a like number in
new combinations.

In general terms one may also express it thus : All
movement of electricity in an electrolyte is conditional upon
simultaneous movement of the ions, and the connection
between these is such that, with equal quantities of electricity,
chemically equivalent amounts of the different ions must be
in movement.

The Law of Faraday thus makes no direct reference
to the separation of the ions at the electrodes, but confines
itself to the movement of electricity in the electrolyte.
Faraday had himself already suggested that the conduction
of the current, and separation of the products of the
decomposition at the electrodes, were two distinct pheno-
mena.

Nevertheless, the separation of the ions at the electrodes
is the most convenient means by which to test the accuracy
of Faraday's law.

This law has up to the present survived all the tests to
which it has been submitted. If one connects in series in
the same circuit, cells containing solutions of silver-nitrate,
copper sulphate, and antimony chloride, the same quantity
of electricity must pass through each cell, and by the law
the weights of metals separated must be in the proportion of

their equivalent weights or 107'6 Ag : ^Cu : .^ Sb.

2 3

The relative proportions of the acid radicals simultaneously

o/~v pn

separated at the anodes would be NO 3 : J : 3

For a practical illustration of this law Liipke 1 recom-
mends dilute sulphuric acid, potassium -silver cyanide,
cuprous chloride solution acidified with hydrochloric acid,
copper sulphate solution acidified with nitric acid, and a
tin tetrachloride solution containing oxalate of ammonium.

1 E. Liipke, Grundzilge der Elektrochemie auf experimenteller
Basis, Berlin, Springer. Also English translation of above by
Pattison Muir.

THEORY* OF ELECTROLYSIS

Platinum foil is to be used as electrode material, or one
may use for anode a strip of the metal the solution of
which is to be electrolysed. A current obtained from 5
accumulators, which was allowed to pass through the cells
for a period of 30 minutes, gave the following results :

III

H 2 So 4 l:12

KAg(ON) a

Cu 2 01 3

CuSo 4

Sn01 4

- +

- 4-

- +

Electrode )
material J

Pt Pt

Pt Ag

Pt Cu

Pt Pt

Weight of ]
deposited }
kations )

67c.c.H= )
6-002 m.g. j

650 m.g. Ag

380 m.g. Cu

190 m.g. Cu

170 m.g. Sn

Ditto per )
1 m.g. H. )

1 mg. H.

108-2 m.g. Ag

63-6 ni.g. Cu

31-8 m.g. Cu

28-3 m.g. Su

Atomic )
weights j

107-6

63-3

117-8

Error per )
cent j

+ 6

+ 4

-4-0

These numbers enable one to obtain a very useful
insight into the course of the electrolysis ; in II and III
the metal ions, silver and copper, are univalent ; in' IV the
copper is divalent, and in V the tin is quadrivalent. Abso-
lute accuracy is not to be expected in such experiments
when complex electrolytes are used.

The use of the voltmeter, which will be described later,
rests upon the absolute truth of this law, as confirmed by
extended experiments, of the relative proportions of the
deposited weights of metal or liberated volumes of gas.

In the following Table (p. 19) are given the electro-
chemical equivalents for those elements of chief importance
to the electro-chemist. The weights given represent the
amount separated or deposited by 1 ampere (= unit
current intensity) during an interval of one second in
m.grams, or during one hour in grams.

Since, as already pointed out, a known quantity of
electricity occasions the movement or migration of equiva-
lent weights of the different ions present in the electrolyte,

FAEADAY'S LAW

Element

Symbol

Valenci

Atomic
weight

Weight separated
per ampere

m.g. per second

gr. per hour

Aluminium

III

26-90

093583

3369

Antimony

III

119-40

415387

1-4953

Arsenic .

III

74-40

258834

9318

Bismuth .

III

206-40

718055

2-5849

Cadmium

111-30

580811

2-0909

Chlorine .

35-19

367273

1-3221

Cobalt .

58-60

305800

1-1008

Cobalt .

III

58-60

203866

7339

Copper .

62-80

655434

2-3595

Copper .

62-80

327717

1-1797

Gold

III

195-70*

680830

2-4509

Hydrogen

1-000

0104368

03757

Iron

55-60

290144

1-0445

Iron

III

55-60

193429

6963

Lead

205-40

1-082300

3-8962

Magnesium

24-20

126286

4546

Manganese

54-60

284926

1-0257

Manganese

III

54-60

189950

6838

Mercury .

198-90

2-075890

7-4732

Mercury .

198-90

1-037945

3-7366

Nickel .

58-60

305800

1-1008

Nickel .

III

58-60

203866

7339

Oxygen .

15-88

082868

2983

Palladium

104-70

546369

1-9669

Platinum

193-30

504361

1-8156

Potassium

38-85

405472

1-4596

Silver .

107-13

1-118100

4-0251

Sodium .

22-87

238691

8592

Thallium j Tl

202-64

1-057370

3-8065

Tin . . Sn

117-20

611599

2-2017

Zinc

65-00

339197

1-2211

one may infer that a definite quantity of electricity moves
with each equivalent of weight. Equivalent weights of
the different ions have, that is to say, like capacities for
electrical energy, and resemble in this the atomic masses of
the elements, which, according to the law of Dulong and
Petit, have like capacities for heat. Weber and Kohlrausch
were the first who attempted to answer the question How
great is this quantity of electricity, and to express in abso-
lute units the electricity which is carried by 1 gram of
hydrogen, or the equivalent weight of any element. The

20 THEOKY OF ELECTEOLYSIS

researches of F. and W. Kohlrausch and of Lord Rayleigh
have proved that this quantity is 96537 coulombs, the
coulomb being the unit of electrical quantity. One coulomb
therefore demands for its transport a mass of any ion repre-
sented by its equivalent weight expressed in grams, and
multiplied by -000010359.

Faraday's law must not be interpreted to indicate that
like quantities of electricity demand the expenditure of
like amounts of work upon the different equivalents of
matter. The law does not touch upon the work or
energy ratios, but relates only to the one factor of elec-
trical energy measurements the quantity of current ; the
second factor the pressure or potential is unnoticed in
this law.

In concluding this chapter it will be well to note briefly
the units of electrical measurement. The unit of electrical
energy is equal to 10 7 absolute units, and is called the
Joule. The unit of potential, pressure, or electro-motive
force is the Volt. The Latimer -Clark cell at 15C. has
an E.M.F. of 1*437 volts [the temperature correction is
obtained by use of the formula -0010 (t-15], while the
Daniell cell has an E.M.F. of about 1-1 volts.

The unit of electrical quantity is the Coulomb, which
represents the quantity of electricity that by a fall of
potential = 1 volt liberates 10 7 absolute units of energy.

If one coulomb pass any cross-section of the circuit in
one second of time, the current is said to have a strength
or intensity of I Ampere. The ampere is then the unit
of current strength. If in any conductor a current of 1
ampere is produced by a fall of potential = 1 volt, the con-
ductor possesses a resistance of 1 Ohm.

The standard resistance of 1 ohm is obtained by use at
0C. of a column of mercury, 106'3 c.m. in length, and 1 sq.
m.m. in sectional area.

CHAPTER III

THE CONSTITUTION OF THE ELECTROLYTE

THE conductors of the second class, the electrolytes, must
necessarily be chemical compounds, since decomposition is
a condition of current conduction.

The converse of this is not however equally true ;
many compounds are known which do not conduct. This
ability to act as conductors for the electric current is
possessed generally by all substances in the molten state
or in aqueous solution. No pure substance is however
known, fluid at the ordinary temperature, that is a con-
ductor to any marked degree. The pure acids sulphuric
acid, hydrochloric acid, &c. which in aqueous solutions
form some of the best conductors, are non-conductors.
Organic compounds conduct only in the degree in which
they possess the constitution and characteristics of true
salts. The possibility of functioning as a conductor thus
depends upon the ability to form from the molecules of
the dissolved substance, particles of matter, which charged
with positive or negative electricity are free to move
in opposite directions. Since no substance in the mole-
cular state can become charged with positive or negative
electricity, one is obliged to assume that this property
belongs exclusively to the parts of the molecule, the ions.
The view that the electric current first causes a splitting
up of the molecule, and then makes use of the sub-molecules
for its transport, does not, however, correspond to the facts.
Such a splitting up of the molecule would demand the expen-
diture of a definite amount of work. Clausius, therefore, in

22 THEORY OF ELECTROLYSIS

1857 formulated the theory, that the current caused an ac-
celeration of the molecular movements, and that the splitting
up was a result of the collisions that ensued. The relative
proportions of the numbers of molecules and sub-molecules
remained at that time undetermined. It was not until
1887 that Arrhenius proved, from other characteristics of
the electrolyte, that in solutions of salts, strong bases, and
acids, these bodies are contained only in small part as such,
and that they are for the most part split up into their
respective ions. If the electric current were the cause of
this ionisation of the substance, those chemical substances,
the elements of which possessed the weakest chemical
affinities, would be found to be the best conductors. Ex-
periments, however, prove exactly the reverse. It may
indeed be considered somewhat strange that it should be
salts like potassium sulphate and sodium chloride, the
elements of which have the strongest affinity for each
other, that show the greatest ionisation when dissolved.
This strangeness is, however, merely the result of a con-
fusion of thought in regard to the stability, and the
chemical activity of a substance.

The metals displace hydrogen with the greatest ease
from its combinations in the mineral acids ; while in the
hydro-carbons the hydrogen is unacted on by metals. The
hydroxyl group in the alkaline hydrates is easily displaced
by an acid radical ; in the alcohols the same group remains
unattacked by acids. Thus it is the chemically inert
bodies which show the strongest chemical affinities
among their component elements. The chemically
active bodies form on the other hand the best electro-
lytes, and the relationship between these two properties
of compounds is so close that one can determine the con-
ductivity by the chemical activity, and vice versa.

The theory that free ions exist in solutions may seem
strange to those to whom it is new, for we are accustomed
to associate with the free elements other properties than
those noted in solutions.

THE CONSTITUTION OF THE

For example, in a potassium chloride solution, which
in the light of this theory contains chiefly potassium and
chlorine as ions, one can observe neither the water-de-
composing properties of the former, nor the characteristic
smell of the latter.

The explanation of this is to be found in the fact that,
although the free atoms of potassium and chlorine are
present as ions in the solution, they carry extremely
large charges of electricity, and on this account possess
chemical properties differing widely from the normal ones.
The energy charge of an ion is different from that of a free
atom, and it is this that determines the different properties
of the two. Let such a charged ion, as for example a
potassium ion, deliver up its electrical charge at the
electrode ; the properties of the normal potassium atom
at once reappear. The same is true of gaseous as well as
of metallic ions, of anions as well as kations. From the
above it follows, that when a metal salt is electrolysed and
a deposition of the metal obtained at the kathode, the
latter has occurred owing to the delivery of the electrical
charges brought by the metal ions to the electrode ; the
now electrically neutral sub-molecules of the metal being
thus able to manifest the usual metallic properties.

The acceptance of the theory, that the properties of an
atom of an element may be entirely altered by the presence
of an electric charge upon it, also explains the fact that
isomeric ions of different valency possess different properties.
The ferro- ion in divalent iron compounds, for example,
exhibits different properties as regards colour and behaviour
towards reagents, from those of the ferri- ion in ferric-salts.

A similar contrast is exhibited by the two groups
Fe(CJST) 6 in yellow and red prussiate of potash ; and the
distinctive properties of the two MnO 4 ions in manganic
and permanganic acids is another instance of the same
kind. The differences in all these cases spring from differ-
ences in the charges of energy ; the ions in each case
carrying electric charges which vary as their valency.

24 THEORY OF ELECTROLYSIS

CHAPTER IV

THE MIGRATION OF THE IONS

IN order to explain the fact that the passage of the electric
current through acidified water caused a liberation of
hydrogen and oxygen at the electrodes, different theories
were put forward, even in the early days of the science.
According to that advanced by Grotthiiss (1805) the current
made the one electrode positive, the other negative.

The electrodes then exerted a directive influence upon
the polarised molecules of water, so that the oxygen side of
the molecules faced the negative electrode, and the
hydrogen side the positive. During electrolysis, only the
two end molecules of each chain were decomposed, and the
respective oxygen or hydrogen atoms set free ; the remain-
ing hydrogen and oxygen of these molecules united with
the oxygen and hydrogen of the two neighbouring molecules,
so that combination and decomposition alternated continu-
ously in the electrolyte.

A definite electro- motive force was, according to this
theory, necessary in order to start the decomposition,
whereas experiment showed that solutions would conduct
even with the feeblest currents.

Clausius pointed out this contradiction between fact
and theory, and declared the theory to be untrustworthy.
He ought to have been forced by this reasoning into a
recognition of the absolute freedom of the ions in the
electrolyte, but he saw only half the truth, and, as noted in
the last chapter, advanced the view that the current does

THE MIGRATION OF THE IONS

not directly cause the breaking up of the molecule, but that
by its action the loosely bound constituent atoms of the
molecule are set in more rapid vibration and movement, and

o
o
o

that, as a result of this, some
molecules break up and the con-
stituent parts of these migrate
towards the electrodes. It was not
until the year 1887 that Arrhenius
published his Dissociation Theory
and finally solved the problem.

Hittorf had, indeed, in the
years 1853-1858 been engaged
upon a study of the alterations in
concentration of electrolytes at the
electrodes, and had obtained in the
course of this work a deep insight
into the subject of the migration of
the ions.

If the rate of migration of
the two ions during electrolysis
be the same, then the loss of the
liquid around the two electrodes
will be equal. This is, however,
rarely the case, and Hittorf there-
fore concluded that the ions pos-
sess different velocities. Let one
imagine an electrolyte, which con-
tains an equal number of anions
and kations (represented by the
black and white circles in Fig. 1),
divided in the middle by a porous
partition (the vertical line in Fig.
1) so that equal numbers of anions

and kations are present on each side of it. The passage
of the current will speedily disturb this equilibrium. In
Fig. 1 the row (a) represents the electrolyte before the
action of the electro-motive force ; the row (b) repre-

) 1

) J

)

26 THEORY OF ELECTROLYSIS

sents the same electrolyte after a migratory movement of
the ions.

In this movement it has been assumed that the anions
(the white circles) have moved twice as fast as the kations
(the black circles), and the horizontal lines (u) and (v)
represent the extent of the movement. Six ions have been
liberated at each electrode ; consequently six equivalents
of the electrolyte must have been decomposed and de-
stroyed.

Four of these have been lost from the left-hand side of
the partition, and the remaining two from the right-hand
side ; in other words, these losses are in the ratio of the
relative migration velocities of the anion and kation.

This system gives approximately a picture of the
migration velocities of the ions of copper sulphate ; the
S0 4 ion migrates nearly twice as fast as the Cu ion, and
covers four units of space in the time that the Cu ion
migrates through two. The quotients 2 (6= -33 and
4 [6= '66 are named by Hittorf the transport ratios for
the concerned ions. This ratio, i.e. the relative velocities
of migration, is quite independent of the working force ;
the absolute velocities are on the contrary directly pro-
portional to it. The temperature and concentration of the
solution do not materially affect the figures.

As a result of these investigations Hittorf concluded,
that in salts of which the double cyanide of silver and
potassium K.Ag(CN) 2 is a type, potassium is the positive
ion and Ag(CN). 2 the negative, and that these are the
migrating ions ; while the separation of the silver from
the anion is the result of a secondary reaction.

When a mixture of two salts is electrolysed, the ions,
if they possess similar migration velocities, as for example
chlorine and iodine, share the current in the proportion in
which they are present in the mixed electrolyte ; the
separation at the electrodes may however occur differ-
ently.

CHAPTER V

THE CONDUCTIVITY OF THE ELECTROLYTE

HITTORP had often given expression, in his papers upon the
migration of the ions, to the opinion that a deeper know-
ledge of the real nature of electrolysis would be obtained
by determinations of the specific conductivities of the
different electrolytes. No reliable method of making such
determinations was however known to him. The con-
ductivity of a body is represented by the reciprocal of its
resistance. For conductors of the first class, the resistance
is dependent upon the form, the nature, and the temperature
of the material used.

The unit of specific resistance is the ohm, or the resist-
ance of a column of mercury 106-3 c.m. in length, and
1 sq. m.m. in sectional area at OC. (The Siemens unit
of resistance, represented by a similar column 100 c.m. in
length, is still occasionally used.) In connection with
liquids, it is customary to speak of the conductivities
rather than of the resistances, and to express these in the
reciprocals of the ohm.

If one dissolves the molecular weight in grams of any
salt in 1 litre of water, and brings this solution between
two parallel electrode surfaces, placed at a fixed distance
apart, the system will be found to possess a definite re-
sistance in ohms, and corresponding to this a definite
conductivity. These constants are named the molecular
resistance, and the molecular conductivity. The molecular
conductivity of an electrolyte increases with the tempera-

28 THEOEY OF ELECTROLYSIS

ture ; metallic conductors show the reverse phenomena.
The molecular conductivity increases also with the dilution
of the electrolyte ; but in this case the number of ions in
the unit of volume, and therefore the specific conductivity
of the electrolyte, is diminished. The maximum con-
ductivity is therefore found at that point of dilution where
the second effect commences to exceed the first.

In 1880 F. Kohlrausch published a useful method for
determining the relative conductivities of electrolytes.
The principle of this method consists in the use of the
Wheatstone bridge with alternating currents in order to
avoid the errors caused by polarisation ; a telephone is also
used to replace the galvanometer.

The conductivities of liquids in comparison with the con-
ductivities of the metals are very small. For example, such
a good electrolyte as 20 per cent, hydrochloric acid solution
at 18C. possesses a conductivity only 71*4 millionths of
that of mercury at 0C. The fraction for 30 per cent, sul-

69*1

phuric acid is - , for 25 per cent, sodium chloride

1,000,000

solution " , and for 10 per cent, copper sulphate
1,000,000

solution it is only

The relative conductivities have also been determined
for solutions which contain the equivalent weight of the
salt in grams dissolved in one litre of water ; these are
named the equivalent conductivities. 1

Kohlrausch found that the equivalent conductivities of
the neutral salts were additively composed of two values,
the one depending only on the anion, the other depending
only on the kation.

1 Further information upon conductivity can be obtained from
the following works : Lehrbuch der Allgemeinen Chemie, vol. ii.,
by W. Ostwald ; Theoretische Chemie, by W. Nernst ; Grundziige der
Elektrochemie auf experimenteller Basis, by E. Liipke ; Elektro-
chemie, by M. Le Blanc. The first-named most excellent book is
that recommended for the study of the theoretical side of the subject.

THE CONDUCTIVITY OF THE ELECTROLYTE 29

These values represented in fact the relative migration
velocities of the different ions.

Finally, the absolute values of the migration velocities
of single ions have been determined.

These, with a potential drop of 1 volt per c. metre at
18C., are as follows :

Potassium '00057 c.m. ; sodium -00035 c.m. ; hydrogen
00300 c.m. ; hydroxyl -00157 c.m. ; ammonium -00055
c.m. ; silver -00046 c.m. ; chlorine -00059 c.m. The ions
of hydrogen and hydroxyl are therefore those which move
most quickly.

30 THEOEY OF ELECTROLYSIS

CHAPTER VI

THE DISSOCIATION THEORY

VAN'T HOFF has shown in his theory of solution that
Avogadro's law may be extended to dilute solutions, and
that even the laws of gas volumes formulated by Boyle and
Gay Lussac are still correct when applied to dilute salt
solutions.

In other words, dissolved salts behave as gases. From
the law of osmotic pressure Van't Hoff deduced other laws
concerning the influence exerted by the dissolved salt upon
the vapour pressure and the freezing point of the solvent.
It was found, however, that all the acids, bases, and salts
dissolved in water gave, when the normal molecular
weights were accepted as correct, too high results for the
osmotic pressure, vapour pressure, and freezing point
determinations, or the molecular weights calculated from
these results were too small. In 1887 S. Arrhenius gave
the explanation of this discrepancy between the theoretical
and observed results.

He had in an earlier paper upon the conductivity of
electrolytes given expression to the view that two kinds of
molecules active and inactive are present, and that part
only of the active molecules conduct the current. The
ratio of the active molecules to the total number of mole-
cules present he named the coefficient of activity.

The comparison of the properties of the electrolyte as
regards the depression of its freezing point, and its ability
to conduct the electric current, led him to formulate the

THE DISSOCIATION THEOKY 31

theory so fruitful in its after results of the dissociation
of all bodies dissolved in water. The discrepancy in the
freezing point determination was, in the light of this theory,
seen to be caused by the splitting up of the salt into its two
component parts on solution ; these fragments, or disso-
ciated parts, giving too high a value to the gram molecule.
The degree of dissociation of a salt on solution, i.e., the
number of decomposed molecules, as determined by the
observation of the freezing point, was found to be in very
fair agreement with the number calculated from the elec-
trical conductivity.

Since only the decomposed molecules conduct, Arrhenius
assumed that these sub-molecules or ions carried electrical
charges, even in solutions which formed no part of an
electric circuit.

According to this theory of Arrhenius, the electric
current in passing through an electrolyte does not decom-
pose the molecules ; these are already present charged
with their respective electric charges in the ionic state.

Inversely, the conduction of the current by an electro-
lyte is dependent upon the presence of free ions ; those
molecules which have not undergone dissociation take no
part in the electrolysis. The activity of the sub-molecules
does not depend alone upon their conductivity, but, as
already remarked, upon the chemical affinity of the mole-
cule. When one dilutes an electrolyte with water, the
conductivity increases up to a certain point ; at and
beyond this point of dilution, all the molecules are to be
regarded as in the dissociated state. Anhydrous liquids,
such as 100 per cent, sulphuric acid, and concentrated
hydrochloric acid, &c., &c., do not conduct, because no disso-
ciated molecules are present. Chemically pure water is
also a non-electrolyte ; the specific resistance at 18C.
being 24'75xl0 10 mercury units, or, expressed in another
way, 1^ million litres of water contain only 1 g. hydrogen,
and 17 g. hydroxyl as ions.

It is noteworthy, therefore, that good conducting

32 THEOKY OF ELECTROLYSIS

liquids are formed by the solution of acids, bases, and salts
in water. One is obliged to assume that water possesses
in a peculiar degree the property of producing dissociation
effects ; for the water molecules remain in these salt solu-
tions practically unchanged, and do not share in the con-
duction of the current. Molten bodies act as electrolytes ;
in this case dissociation would appear to be an effect of the
increase of heat.

The fact that the ions possess charges of energy, differ-
ing from those of the corresponding atoms, and as a result
of this possess different chemical properties, has already
received mention in Chapter III (p. 23).

Whence comes then this property of water to effect
dissociation ? One may perhaps assume that since solution
is generally attended by a depression in temperature, there
is an absorption of energy from without, which has some
connection with the dissociation of the salt. Up to the
present, however, no satisfactory explanation has been
given as regards the origin of the electric charges carried
by the separate ions. lonisation is certain to cause some
conversion into other forms, of the original energy of the
atoms.

CHAPTER VII

THE CHEMICAL AND MOLECULAR CHANGES DURING
ELECTROLYSIS

ELECTROLYSIS is conditional upon the passage of measur-
able quantities of electricity into the electrolyte by its
boundary surfaces.

This occasions a movement of electricity through the
electrolyte, which is intimately connected with the move-
ment of the ions. The current causes the anions to drift
towards the anode, and the kations towards the kathode.

An accumulation of negative electricity at the anode
and of positive electricity at the kathode results, which
would speedily lead to a cessation of the current if this
excess of electricity and accumulation of ions at the
two electrodes were not destroyed. At the kathode posi-
tive electricity is drawn from the kations ; at the anode
negative electricity is abstracted from the anions : and this
withdrawal of the charges from the ions is followed by
their change into neutral bodies. Electrolysis, strictly con-
sidered, therefore occurs in the voltaic cell. Of the ions
which have delivered up their electric charges, only the
metals can exist as such, and these are to be regarded there-
fore as primary products of the electrolysis. All non-
metallic ions have but a short existence, and the sub-
stances which form at the electrodes are transformation
products of ions which have lost their electric charges, i.e.
secondary products. Examples of these are the molecular
gases chlorine, C1 2 , hydrogen, H 2 , &c. &c.

34 THEOKY OF ELECTEOLYSIS

The expenditure of work in effecting electrolysis is not,
as already explained, required to split up into ions the
molecules in the electrolyte, but is needed in order to effect
the liberation of the charges of electricity from the ions
at the electrodes. The amount of work demanded for this
cannot be calculated from the heats of combination of the
individual ions, but it is as a general rule proportional to
the sum of these.

It has been customary to distinguish between the
primary and secondary phenomena of electrolysis. Ostwald
rightly points out, that it is neither advantageous nor
logical to maintain this distinction longer. The electro-
lysis of potassium sulphate (see Chapter I) yields hydro-
gen and potassium hydrate at the kathode, oxygen and
sulphuric acid at the anode. If now the separation of the
oxygen and hydrogen at the two electrodes be regarded as
a secondary phenomena, and if one assumes that, first of all,
the ions of the salt K and SO 4 have actually separated
but have immediately reacted with the surrounding water,
one is met by the difficulty that a correspondingly higher
electro-motive force would be required to effect the de-
composition of this compound. Hydrogen and oxygen
could not, if this assumption be correct, be liberated at a
lower E.M.F. Observation has, however, shown that
these gases are liberated with a much lower E.M.F. than
that postulated. The expenditure of electro-motive force
corresponds, then, not to those reactions or products which
we have been in the habit of calling primary, but to the
final products of the electrolysis.

It is, however, necessary to distinguish between those
products which conduct the current and those which
separate at the electrodes. Only in few cases are these one
and the same, as for example in the fused chlorides lead
chloride and magnesium chloride.

In most cases the ions are unable to pass into the
neutral state as regards electrical charge without under-
going an alteration in their chemical constitution. When

CHEMICAL AND MOLECULAE CHANGES 35

different substances are present, the separation is deter-
mined by the electro-motive force necessary to effect it ;
those compounds which demand the least E.M.F. for
their separation will be first obtained, this result being
entirely independent of their classification, according to
the older views, as primary or secondary products.

When the solution to be electrolysed is a mixture of
various electrolytes, the proportion in which the different
ions share in the conduction of the current is governed by
two factors, the relative numbers of the different ions and
their migration velocities. This compound expression is
altered, however, when one comes to consider the separation
of the single ions, since the different ions do not give up
their charges of electricity with equal readiness.

With a slowly increasing electro-motive force, those ions
which relinquish their charges most easily are the first to
experience the change. For example, in a solution con-
taining chloride and iodide of potassium, chlorine and
iodine as ions arrive at the anode in exactly equal propor-
tions, on account of the equality of their migration velocities
(the anode must be of a material that is not attacked by
these gases) ; but only iodine is separated at first. If one
increases the E.M.F. a point is reached at which chlorine
is also liberated ; this is about '35 volt higher than the
first. Similar considerations regulate the deposition of
metals from mixed solutions. The order in which they
are deposited is as follows : Gold, platinum, palladium, silver,
mercury, copper, hydrogen, lead, nickel, cobalt, iron,
thallium, cadmium, zinc, manganese, aluminium, mag-
nesium. Gold and platinum are most easily deposited,
while zinc and aluminium are the most difficult to deposit
of the better-known metals. The metals of the alkaline
earths and the alkali metals can only be obtained as metals
under especial conditions, and by use of a mercury kathode.
The order of the above list corresponds to that of Yolta's
series, but it is dependent upon the electrolyte.

A clear view of this subject of the progressive deposition

D 2

36 THEORY OF ELECTROLYSIS

of the metals is most easily obtained, by assuming that
every ion possesses a definite and fixed force which tends
to keep it in the ionic state ; and that this force can be
measured and expressed in terms of the E.M.F., which is
necessary to effect a separation of the ion, i.e. its transfor-
mation into the neutral condition.

It is for this reason that iodine can be separated more
easily than bromine, and the latter more easily than chlorine ;
and the same reasoning explains the ordering of the metals
in the series already given. It is seen from this series
that the noble metals and those allied to them have a
distinct tendency to pass out of the ionic state ; whilst
those at the other end of the series have the opposite
tendency, and are always striving to enter into it. Hydro-
gen occupies a position midway between these two extremes.

The order in which the metals are deposited with a
slowly increasing E.M.F. varies with the nature of the
salt used for the electrolysis. For example, if an excess of
potassium hydrate be added to a solution of the neutral
salts of zinc or tin, an increase of from -5 to '7 volt is
necessary in the E.M.F. required to effect deposition as
compared with that required for the neutral solutions.
The explanation of this lies in the fact that these metals
form respectively zincate and stannate when excess of
potassium hydrate is added to solutions of their neutral
salts, and that these complex salts yield the zinc or tin
as anion when electrolysed ; 1 while only very small
amounts of zinc and tin are present as metals in the ionic
state. Other examples of this kind are the solutions of
the double oxalates and the double cyanides, which are so
frequently used for electrolysis.

If then one has a liquid containing two or more metallic
salts in solution, it ought to be possible, in view of the
above facts, to deposit the single metals one after the other
by the use of an extremely feeble current, which is
gradually increased by means of a higher E.M.F. ; that
1 Cf. Chap. I. p. 12.

CHEMICAL AND MOLECULAE>SM2^^ 37

is to say, an analytical separation of the metals by electro-
lytic methods should be practicable.

Freudenberg 1 has proved that this is possible for a
considerable number of the metals.

If the current be increased before the whole of the
more easily separated metal has been deposited, the second
metal will take part in the deposition, and the limit will
ultimately be attained at which the two metals arrive at
the electrode, and are separated, in the ratio expressed by
their relative migration velocities. A practical application
of this phenomena occurs in the electroplating industry,
when articles are coated with brass (copper and zinc).
With feeble currents, copper alone is deposited. In order
to obtain mixed metallic deposits of the required composi-
tion, it is necessary to pay careful attention to the nature
and quantity of the salts used, and to the current density
employed. An experiment illustrative of this deposition
of alloys of the metals may be easily performed as follows :

A mixed solution of the sulphates of copper and iron
(ferro-salt) containing a little sulphuric acid is electrolysed.
With the electrodes a certain distance apart, copper alone is
deposited at the kathode ; but if they be gradually moved
nearer to each other the resistance of the electrolyte is
reduced, the current density is increased, and a white alloy
of copper and iron is deposited if the conditions be exactly
right, or a black spongy deposit may be obtained.

The term ' current density ' is used to denote the current
strength or intensity divided by the area of the immersed
part of the electrode.

The unit of area generally used for electrolytic separa-
tions in the chemical laboratory is the square decimetre
( = 100 sq. centimetres = -107642 sq. foot). Current
densities expressed in terms of this unit are denoted
as 'normal densities,' and are generally written in this
form : ' KD. 100.'

The expression ' N.D. 100 = 1*5 A ' thus signifies that
1 Zeitsch. f. phys. Chemie, 1893, 12, 197.

38 THEORY OF ELECTROLYSIS

1'5 amperes of current is used for each 100 sq. centi-
metres of the electrode surface. For technical purposes
the square metre (= 10 '76 sq. feet) is used as unit of area.
[In the calculations of current density, for technical
purposes it is necessary to note carefully whether one or
both surfaces of the electrodes will take part in the electro-
lysis. As a rule, more than two electrodes are used in
each vat, and thus both surfaces of anode or kathode come
into play. Translator's noteJ\

It is evident from the foregoing consideration that the
maintenance of a fixed current density is of great import-
ance. The influence which the current density exerts is
manifested in two directions, both the nature of the pro-
duct and the quality of the deposit being dependent upon it.
The latter influence is especially noticeable in the electro-
lysis of metallic salt solutions. As an example of the
former, the statement that, according to the current density
employed, either copper or cuprous chloride may be
obtained on electrolysing a cupric chloride solution is to be
noted. Palladium and molybdenum are obtained either as
metals, as oxides, or there may be no deposit at all, accord-
ing to the current density used for the electrolysis of their
salts.

Again, sulphuric acid can be made to yield hydrogen
peroxide, ozone, or persulphuric acid under similar varia-
tions of current density.

This influence of the current density is, however, best
illustrated by the classical example of the decomposition
of chromic chloride, as performed by Bunsen. According
to the current density, one can obtain hydrogen, chromium
trioxide, chromium sesquioxide, or metallic chromium. In
order to obtain the last, the concentration of the solution
must receive attention, as it has considerable influence upon
the result.

Since the molecules of water are dissociated to such a
slight extent, one can in most cases neglect the part played
in the electrolysis by the ions of the water ; for the limit

CHEMICAL AND MOLECULAR CHANGES 39

is quickly reached (even by very small current densities) at
which their share in the procedure ceases. Thus, if the
few hydrogen ions present in the water be caused by the
current to drift towards the kathode, a definite time must
always elapse before the original ratio between the numbers
of dissociated and non -dissociated molecules is restored,
i.e. before a further number of hydrogen ions have come
into existence.

During this intervening time the current is compelled
to make use of other kations in order to effect its passage
through the electrolyte. For example, in the electrolysis
of solutions of zinc chloride, hydrogen is liberated at the
kathode when a feeble current is employed, and Zn(OH) 2
is formed ; but a slight increase in current strength is
sufficient to cause the deposition of metallic zinc to become
the chief effect of the electrolysis. Since in this case the
question is merely one of the relative proportions of the
zinc and hydrogen ions, it is evident that a better deposi-
tion of zinc will be obtained by use of a concentrated solu-
tion than by use of a more dilute one.

These considerations will also explain why, when dif-
ferent metallic salt solutions are electrolysed by means of
insoluble electrodes, the last traces of the metal are always
most troublesome to remove from the electrolyte. It is
customary to surmount this difficulty by increasing the
current density.

Bunsen even succeeded, by the use of very high current
densities and hot saturated solutions, in obtaining smooth
metallic deposits from the chloride salts of calcium, barium,
and strontium. He, however, used an amalgamated
platinum wire as kathode, the perfectly smooth surface of
such a kathode being especially fitted to lessen the evolu-
tion of hydrogen.

Another well-known phenomena of electrolysis is also
explained by the above considerations.

If one acidifies moderately strongly a neutral salt
solution of nickel, cobalt, cadmium, or zinc, with a mineral

40 THEORY OF ELECTROLYSIS

acid (3 per cent, to 5 per cent, by volume is generally sufficient),
no deposition of metal occurs when the usual current
density (100 N.D. = 1 to 2 A) is employed, but an evolution
of hydrogen gas takes place at the kathode. The mineral
acids, when dissolved in water, undergo strong dissociation,
and thus hydrogen ions are present in such solutions in
great abundance ; the discharge potential of these hydrogen
ions is also much below that of the ions of zinc, cadmium,
&c., so that at the current density named it is impossible
to exhaust the crowds of hydrogen ions and to bring the
other metallic ions to the point of separation.

If, however, one electrolyses an acidified copper sul-
phate solution, the reverse of this occurs : copper will be
deposited, whilst the hydrogen, under similar conditions to
those obtaining above, will scarcely be visible. Hydrogen
gas will only be liberated in quantity when the current
density has been largely increased and the velocity of the
copper ions no longer suffices to carry the whole of the
current. This behaviour of different metals in acidified
solutions of their salts is made the basis of an electro-
lytic method of separation for the so-called * noble metals '
i.e. those standing above hydrogen in the list (see p. 35)
from the ' base metals,' i.e. those placed below hydrogen.

This method can also be used for analytical separations :
as, for instance, for the separation of copper and zinc in the
analysis of brass, or for the separation of copper and nickel
in mint-nickel.

The second phenomena that is closely connected with the
current density is the influence exerted by the latter upon
the character of the deposits obtained at the kathode. The
formation of spongy deposits is directly caused by the use
of unsuitable current densities.

These deposits differ from bright metallic deposits in
their dull and dark appearance and in their non-adherent
and frequently powdery nature. This latter characteristic
makes it almost impossible to wash them with liquids with-
out loss.

CHEMICAL AND MOLECULAR CHANGES 41

Such deposits are alone caused by the use of current
densities unsuited to the solution which is undergoing
electrolysis. Zinc gives by the electrolysis of different
solutions, bright, coherent, bluish-white deposits ; but from
very dilute zinc sulphate solutions spongy deposits are
nearly always obtained, even when high current densities
are employed.

One can assume that in this case the cause lies in the
arrival of considerable numbers of hydrogen ions with the
zinc ions at the kathode surface. The individual minute
bubbles of hydrogen do not detach themselves instan-
taneously from the rather rough surface of the zinc coating
on this kathode, but escape from time to time in small
masses of bubbles, and so destroy the possibility of a
perfectly smooth and regular deposition of the metal.
With concentrated solutions, on the other hand, it is
possible to obtain entirely satisfactory deposits with
current densities of from '5 to 1'5 amperes.

Mylius and Fromm have carried out investigations upon
the electrolytic separation of zinc, which have led them to
the opinion that the spongy zinc always contains either
zinc oxide or a basic zinc salt.

If this view be correct, then the chemical changes in
the solution during electrolysis must include those already
noted as occurring when zinc chloride solution is electrolysed
by a very feeble current ; hydrogen and zinc hydrate,
Zn(OH) 2 , separate at the kathode, and the latter is
mechanically enclosed in the deposit of zinc.

The metals which possess especial tendencies to form
spongy deposits are zinc, cadmium, silver, gold, and, to a
still greater degree, bismuth. This latter metal is deposited
in a black powdery form from all its solutions by almost
every current density which it is possible to employ.
Bright deposits of copper, cadmium, and zinc can be ob-
tained, by the use of weak currents, from solutions of
their salts to which ammonia and certain substances have
been added. The spongy metallic deposits have a tendency

42 THEORY OF ELECTROLYSIS

to enclose liquids and gases, so that such deposits are
useless for analytical purposes.

Another form of deposit similarly useless, though not
on account of its spongy nature, is obtained from certain
solutions of some metallic salts when a high current
density is employed. In place of a smooth adherent
deposit, one obtains a separation of needle- like or foliated
growths, which are quite as unfitted as the spongy deposits
for correct weight determinations. Such deposits are
especially striking when warm saturated stannous chloride
solution, concentrated lead acetate solution, silver nitrate
solution, and zinc chloride solution are electrolysed.

Two of these solutions are purposely used in order to
form the so-called ' trees ' of silver or lead.

Some metals as, for example, silver, bismuth, lead, &c.
possess the characteristic of being deposited from certain
solutions simultaneously as metal on the kathode, and as
peroxide on the anode. It is possible, however, by choice
of solution and maintenance of a low current density, to
obtain a deposition on the one electrode only. The in-
fluence of the current density in this matter is shown by
the example of the electrolysis of a silver nitrate solution
containing some free nitric acid. If an anode of thick
silver wire in spiral form be used, no peroxide formation
is noticeable ; but if this be exchanged for a jacket
electrode possessing sharp edges of the same metal, a
coating of black silver peroxide will be produced, especially
at the edges of the jacket electrode, where the current
density is always the greatest.

When hydrogen or hydroxyl ions are caused to migrate,
these in many cases react with the electrolyte. Thus, dur-
ing the electrolysis of nitric acid, not only hydrogen,
but nitrous acid, nitrogen, and ammonium hydrate are
formed.

The hydroxyl anions as a rule react mutually at the
anode, to form non-dissociated water molecules and
oxygen.

CHEMICAL AND MOLECULAR CHANGES 43

When sulphuric acid or chromic acid is electrolysed,
different modifications of the anode reaction may occur.

The liberated oxygen either forms ozone, O 3 , by the con-
densation of three atoms into the space of two, or it combines
with water to form hydrogen peroxide, H 2 2 , or at a
particular current density the migrating SO 4 ion undergoes
polymerisation at the anode and yields persulphuric acid,
S 2 8 .

As a rule the oxygen liberated at the anode is not poly-
merised (e.g. in solutions containing free nitric acid), but
reacts upon the dissolved salt with formation of peroxides,
as in solutions of the salts of silver, bismuth, manganese,
and lead.

According to the conditions obtaining during the
electrolysis of these salt solutions, either a deposition of
metal at the kathode with some peroxide formation at the
anode occurs, or only the peroxide is produced. This latter
form of deposition is used analytically in the case of those
metal salt solutions, from which a quantitative separation
of the metal in this form is possible. Lead and manganese
are the chief examples of metals whose separation can be
effected in this way.

When a current flows through an electrolytic cell or
through a voltaic couple, changes take place both at the
electrode surfaces and in the electrolyte ; after some time
the current steadily diminishes, often to a very considerable
degree. This phenomenon cannot be explained solely by
the change in the concentration of the electrolyte ; for an
alteration in the ' ion concentration,' represented by 1 : 10,
would only cause a difference of '06 volt, and it is the
change at the electrode surfaces which must be regarded as
the chief cause of the diminution. The term * polarisation '
is used to denote this increased resistance of the cell, and
consequent falling off of the current strength. If one
allows a current to pass through an electrolyte for some
time, using insoluble electrodes of platinum, gold, or carbon,
and then disconnects the primary current, and joins the

44 THEOEY OF ELECTEOLYSIS

electrodes by a metallic conductor, it will be found that a
current is now passing between the two electrodes in an
opposite direction to that used for the electrolysis. This
observation is most easily made by including a galvano-
meter in the external part of the circuit. This second
current is the so-called polarisation current.

If two platinum electrodes be dipped into a dilute
sulphuric acid solution, and a current with pressure of at
least 1-6 to 1-7 volt be passed through it, hydrogen will
be liberated at the kathode and oxygen at the anode.
The greater part of these liberated gases pass up the
surfaces of the electrodes in small bubbles and escape ; a
certain portion of each is absorbed by the material of the
electrodes. If the primary or polarising current be inter-
rupted after some time, a system will then exist consisting
of two platinum electrodes immersed in dilute sulphuric
acid, the one charged with oxygen, the other with hydro-
gen, gas i.e. a gas couple or battery will have been pro-
duced, the current of which will of course be contrary in
direction to the primary current. The E.M.F. of such a
system is about 1'07 volt at the normal atmospheric
pressure ; at the commencement it is, however, often some-
what higher, owing to excess pressure. The gases retained
by the electrodes form ions again, and pass into the solution.
Such polarisation currents are usually of brief duration ;
the gases disappear from the electrodes, and the equilibrium
of the system is restored. If in place of hydrogen a metal
such as zinc or copper be deposited at the kathode, the
interruption of the primary current results in the formation
of a system which yields a current corresponding to that
obtained from a voltaic couple made up of the metals zinc
and platinum, or copper and platinum, with the particular
electrolyte.

In order to obtain a clearer insight into the cause of
polarisation, one may make use of Le Blanc's conception
of * Haftintensitat.' According to this theory every ion
possesses a definite force tending to keep it in the ionic

CHEMICAL AND MOLECULAR CHANGES 45

state (cf. earlier portion of this chapter, p. 36). The
ions may, however, possess either a positive or a negative
' Haftintensitat.' The electric charges can only be with-
drawn from the first by the expenditure of work ; the
second class of ions, as they pass from the ionic into the
neutral state, produces electrical energy. To the former
class belong magnesium, aluminium, zinc, cadmium, nickel,
cobalt, and iron; to the latter, lead, hydrogen, copper,
mercury, and silver.

The anions may be similarly classified. If, then, it is
necessary to deposit zinc from its solution, the least force
with which this can be effected must exceed that repre-
sented by the ' Haftintensitat ' of zinc.

The moment this compelling force ceases to act, the
tendency of the deposited zinc to pass into the ionic state
i.e. to go into solution again becomes manifest, and
the conditions for the production of an electric current in
the opposite direction exist. The primary current for the
decomposition of a compound must therefore be at least
equal in strength to the sum of the ' Haftintensitat ' forces
which have to be overcome. It is customary to speak of
this as the ' decomposition value ' of the electrolyte. For
solutions containing the same anion, it rises with the
' Haftintensitat ' of the kation. Thus, for example, for
the sulphates copper sulphate, sulphuric acid, ferrous
sulphate, and zinc sulphate the decomposition values rise
in the order in which these compounds are named. Since
sulphuric acid possesses a decomposition value = 1'6 volt,
it follows that that of copper sulphate is below this, whilst
that of the two other sulphates is above it.

Using this conception of ' Haftintensitat ' it is possible
to present the process by which the metals are separated
by a slowly increasing electro-motive force from a mixed
electrolyte, in the following manner :

Those ions which possess the smallest * Haftintensitat '
will be first and most easily changed into neutral bodies.
The polarisation current will, however, grow in strength

THEOKY OF ELECTROLYSIS

as the ' Haf tintensitat ' of the separated ions i.e. of the
neutral bodies formed from the ions increases.

The value of the decomposition force for any compound
can be approximately calculated from the heat of forma-
tion of the electrolyte. The E.M.F. necessary for the
decomposition of the more common electrolytes ranges
from '5 up to 4 volts ; and for the same metal, varies with
the concentration of the solution and with the constitution
of the salt.

Complex salts possess, as one would have expected, a
somewhat higher decomposition value than the simple
salts. The following figures for the decomposition values
of normal solutions of various metallic salts and acids
have been obtained experimentally by Le Blanc : l

Compound

Volts

Compound

Volts

ZnS0 4

2-35

CdCl 2

1-88

ZnBr. 2

1-80

CoSO 4

1-92

NiS0 4

2-09

OoCL

1-78

NiCL,

1-85

HC1

1-31

Pb(N0 3 ) 2

1-52

H 2 S0 4

1-67

AgN0 3

HN0 3

1-69

CuS0 4

Cd(N0 3 ) 2

1-24

1-98

NaOH
NH 3

1-69
1-74

CdSO,

2-03

The following figures have been calculated from the
heat-formation data :

Compound

Volts

Compound

Volts

HgCl 2

1-30

SnCl 2

1-76

HgN0 3

Fe 2 (S0 4 ) 3
FeS0 4

1-04
1-62
2-02

SnCl 4
MnS0 4

MnCl,

1-70
2-60

2-77

AuCl 3

CuCl 2

1-36

FeCl 2

2-16

The deposition of the metal upon the kathode only
1 Zeitsclir. f. phys. Chemie, 1891, 8, 299.

CHEMICAL AND MOLECULAK CHANGES 47

requires to continue a very short time, in order to give
rise to the polarisation phenomenon ; for Oberbeck states
that an extremely thin coating of the metal suffices to
make an electrode function as though composed of the
solid metal.

PAET II

MEASURING AND REGULATING THE
CURRENT

A. CURRENT MEASUREMENT

IT is generally of great importance in electrolytic processes
that the current density, and very frequently the E.M.F.
of the current also, should be maintained within certain
fixed limits (see Chapter VII, pp. 38, 39).

These conditions can only be fulfilled by direct measure-
ment of the strength and the E.M.F. of the current employed.
The methods and apparatus for effecting the measurements
of these two quantities differ, so that it will be most con-
venient to consider them separately.

50 MEASURING AND REGULATING THE CURRENT

CHAPTER VIII

MEASUREMENT OP THE CURRENT STRENGTH

IN order to measure the current strength, different effects
of the electric current may be employed. One can make
use of the chemical or the magnetic effect produced by the
passage of the current. In the former case the apparatus
used is known as a ' voltameter,' and must not be confused
with the ' voltmeter,' the instrument by which the E.M.F.
is measured ; in the latter case the various instruments
are known as ' galvanometers ' and ' ammeters.'

According to Faraday's law (see Chapter II, pp. 16, 17),
like amounts of current cause, in equal periods of time,
equivalent amounts of simple or complex ions to drift
towards the electrodes.

Here the ions separate as neutral bodies, or cause the
separation of an equivalent amount of some loosely combined
body from the electrolyte. Simple electrolytes, the decom-
position products of which are easily determined by weight
or measurement, can be used directly for measuring the
amount of current that has been passed through them.
The electrolytes used for this purpose are solutions of
silver nitrate, copper sulphate, or dilute sulphuric acid, and
the voltameters formed with these three electrolytes are
known as the silver, copper, and detonating gas voltameters
respectively.

If one of these is to be used for measuring the amount
of current which is passing through a given cell, the

MEASUEEMENT OF THE CUEKENT STRENGTH 51

apparatus is so arranged that the resistance, the voltameter,
and the electrolytic cell follow each other in the order
named in the primary current circuit.

The Silver Voltameter, Since it is not possible for any
complications to occur in the deposition of silver from a
solution of silver nitrate, the results obtained by use of
this apparatus are
extremely accurate.
It is customary to
use a platinum cru-
cible or basin as ka-
thode, and a silver
rod as anode. It is
advisable to envelop
the latter in a small
piece of linen, 1 in
order to prevent any
small particles of
silver from falling
into the crucible or
basin. A neutral
concentrated solu-
tion of silver nitrate
is used as the electro-
lyte.

Fig. 2 is an illus-
tration of the appa-
ratus ready for use.
Since silver shows a
tendency, when strong currents are employed, to separate in
single crystals, it is necessary when such currents have to
be measured to use larger electrode surfaces, or to allow
the current to pass through the voltameter for a very short
time only.

1 Care must be taken to employ linen quite free from organic or
inorganic chemicals. Instances have occurred in which great trouble
has been caused by the chemicals contained in the materials used
for enveloping anodes, Cf. Zeits. f. Elektrochem. 4, 164.

Fra. 2. Silver Voltameter.

52 MEASURING AND REGULATING THE CURRENT

Instead of the apparatus illustrated in fig. 2, it is of
course possible to use a beaker containing the silver nitrate
solution, with a platinum cone and silver strip as kathode
and anode respectively. A current of 1 ampere in strength
separates -001118 grm. silver per second, -06708 grin, per
minute, and 4*025 grms. per hour. After the circuit has
been broken, the remaining electrolyte is poured out of the
basin, or the cone is removed from the liquid, the electrode
and its deposit of silver are washed with distilled water,
then with alcohol, dried and weighed.

The Copper Voltameter. The apparatus used is similar
to that described above. Usually one employs two sheets
of copper as electrodes, and a beaker for containing the
copper sulphate solution. Oettel recommends the following
composition for the electrolyte, in place of a concentrated
solution of copper sulphate : 15 grms. copper sulphate,
5 grms. cone, sulphuric acid, 5 c.cm. alcohol, and 100 c.cm.
water. This solution yields, with currents between -06 and
1*5 ampere, results exactly concordant with those of
the silver voltameter. With this solution the E.M.F.
required lies between '10 and -50 volt, whereas double this
E.M.F. is required with the more concentrated solution.
1 ampere separates -01966 grm. copper per minute and
1-1797 grms. copper per hour.

The Detonating Gas Voltameter. In this form of
voltameter the current decomposes dilute sulphuric acid,
and liberates hydrogen and oxygen at the two electrodes.
The apparatus is so arranged that the volume of gas
liberated at both the anode and kathode can be directly
read off. The electrodes used are of platinum. The
electrolyte is a solution of sulphuric acid of between 1-15
and 1-20 sp. gr. Many forms of this voltameter exist;
one of the most convenient is that of Kohlrausch, illus-
trated in fig. 3. The graduated eudiometer tube contains
a permanently fixed thermometer for determining the
temperature of the contained gases ; it is filled by simply
reversing the position of the apparatus. Another form of

MEASUREMENT OF THE CURRENT STRENGTH 53

detonating gas voltameter is that of Walter Neumann
(fig. 4). A movable levelling tube serves both for filling
the eudiometer and for bringing the pressure under which
the enclosed gas-volume is measured, exactly to that of
the atmosphere. 1 ampere liberates 1O44 c.cms. deto-
nating gas per minute if the measurement be made at C.
and 760 mm. pressure.
Since the electrolytic
dissociation of the

FIG. 3. Kohlrausch'sDetonat- FIG. 4. Neumann's Detonating
ing Gas Voltameter. Gas Voltameter.

electrolyte does not commence until an E.M.F. of 1*7 volt
is employed under the most favourable conditions, and may
require over 2 volts under some conditions of electrode
surface and distance apart, the use of this voltameter is
confined to those cases in which a moderately strong
current is being used.

54 MEASURING AND REGULATING THE CUKRENT

All the voltameters described are faulty, except in very
few instances, when used continuously for current mea-
surements. They also appreciably increase the resistance
of the circuit. On account of these drawbacks, they are
seldom used in electrolytic work, and measuring instru-
ments depending upon the electro-magnetic effects of the
current are much more generally employed.

The Galvano-
meter. A galva-
nometer is a
measuring instru-
ment, the mechan-
ism of which de-
pends upon the
movements of a
freely swinging
magnetic needle
under the action
of the electric
current. The

simplest form
one which can be
easily made by
the student is
that known as the
' tangent galva-
nometer ' (fig. 5).
A copper wire
bent into a circle
is placed with its

plane vertical, and also in the magnetic meridian of the
locality. At the centre of the hoop of copper wire a magnetic
needle is placed, with opportunity for free movement in the
horizontal plane. When at rest it will of course lie in the
plane of the magnetic meridian. If the free ends of the
copper wire be now connected with a cell, or other source of

FIG. 5. Tangent Galvanometer.

MEASUREMENT OF THE CURRENT STRENGTH 55

current supply, the current will cause the needle to strive
to place itself at right angles to the plane of the hoop.
The force exerted by the current upon the needle is against
that of the earth's magnetism, which seeks to hold the
needle in its original north and south position. The
needle will therefore be deviated by an angle that varies
with the current strength, and it has been found that the
tangent of this angle is directly proportional to the latter.
The angular displacement of the needle in any galvano-
meter of this form is partly dependent upon the size of the
instrument ; and the calibration is best performed empiri-
cally by comparing these deviations with the current strength
as ascertained by means of a voltameter placed in the same
circuit. The current values are then written on the scale
card of the galvanometer, so that a direct reading of the
current strength is possible.

Since the angular deviation of the needle is inexact
between 60 and 90, it is necessary to place the instru-
ment in a ' shunt circuit ' that is, a secondary circuit
through which only a part of the primary current is pass-
ing when strong currents have to be measured. If a
conductor be divided at any point in the outer circuit, and
two or more paths be opened to the current, a portion of
it will pass along each of these. The relationship between
these different portions of the main current is expressed
. by the reciprocals of the resistances of the separate wires.
If the resistance of one wire be made ten times as great
as that of the other, only one tenth of the current will
pass along the first wire. It is thus possible to arrange
for any desired fraction of the main current to pass through
the galvanometer, by placing known resistances in the
secondary circuit containing it.

Very delicate and sensitive instruments containing
many coils of wire may be used in this manner for current
measurements, as, for instance, the much-used horizontal
galvanometer illustrated in fig. 6.

56 MEASURING AND REGULATING THE CURRENT

This galvanometer requires previous calibration by aid
of a voltameter.

The Torsion Galvanometer. The torsion galvanometer
shown in fig. 7 is much used for measuring current
strengths in electrolytic experimental work, and is both
accurate and convenient. This galvanometer consists of a
* bell magnet ' hung by a spiral spring between two
' multiplicators ' containing a very large number of coils of
wire. When the magnet has been forced from its original
position by the action of a current
passing through these coils, it can

FIG. 6. Horizontal Galvanometer.

FIG. 7. Torsion Galva-
nometer.

the

be brought back to its former position by turning
spiral spring.

The torsion of this spring indicated by the circular
angle through which the pointer attached to the upper part
of the spring is moved in order to effect this is propor-
tional to the deviation of the magnet, and is therefore a
measure of the strength of the passing current. The
resistance of the wire coils of such a galvanometer is
generally made to equal exactly 1 ohm, and each degree on
the scale of angular measurement to equal "001 ampere.
Since this degree of sensitiveness is generally much greater

MEASUREMENT OF THE CURRENT STRENGTH 57

than is necessary for technical purposes, and since the scale
is only divided up to 180, and the measurement of currents
above '17 ampere is therefore impossible, it is customary
to use shunt resistances made out of ' manganine ' equal
to -J, T T y , and - 9 *g- ohm with this galvanometer. One of
these is inserted in the shunt circuit round the torsion
galvanometer, and the frac-
tion of the current in the main
circuit which passes through
the latter may therefore be
reduced to ^Q, 5 1 G , or -j-i^ at
will. Each degree of the scale
by which the movement of the
pointer is measured then indi-
cates '01, -05, or -10 ampere,
and the maximum current,
the measurement of which
is possible with these three
different shunt resistances, is
1*7, 8 -5, and 17 amperes re-
spectively. Stronger currents
than the latter may also be
measured, by the application
of the same principle with
this form of galvanometer.

Ammeters for technical
purposes. Instruments have
been constructed for technical
use which permit currents
of varying strength to be
measured with sufficient ac-
curacy ; these have been named ' amperemeters ' or 'am-
meters.'

The principle of the construction of these depends
upon the observed fact that when a current is allowed to
pass through a spiral of wire a so-called solenoid an at-
tractive force will be exerted upon a short bar of soft iron

FIG.

}. Kohlrausch's Spring
Galvanometer.

58 MEASURING AND REGULATING THE CURRENT

placed near to one of its ends, or a force acting towards the
periphery upon a piece of similar iron placed eccentrically

FIG. 9. Ammeter.

within it. These instruments are calibrated empirically.
They are sufficiently sensitive for ordinary work, but the
divisions of the scale are usually very close together at the

lower part of the scale,
so that accurate read-
ing for weak currents
is rendered somewhat
difficult. The spring
galvanometer designed
by Kohlrausch (tig. 8)
is of the type first
described.

The current pass-
ing through a vertical
solenoid causes it to
draw within it a small
cylinder of soft sheet
iron suspended from
a spiral spring. A
FIG. 10. Ammeter. pointer is attached to

MEASUREMENT OF THE CURRENT STRENGTH 59

the cylinder. The tension of the spring ultimately counter
balances the attractive force exerted by the solenoid, and a
position of equilibrium is attained.

The elongation of the spiral spring is indicated upon
a scale by the pointer, and is a measure of the current
strength. For electrolytic purposes, an instrument of this
kind with a range of measurement of from '50 to 5*0
amperes is sufficient. Two other forms of this type of
ammeter are shown in figs. 9 and 10. In these the lineal

FIG. 11. Ammeter.

movement of the iron cylinder within the solenoid is con-
verted into the circular movement of the indicator pointer
by means of a lever.

Other forms of ammeter are based upon the second
action of solenoids noted above. These possess a
horizontally placed solenoid, within which a short bar of
soft iron capable of movement about the axis of the
solenoid is set ; one end of this bar being attached to a
pointer moving over a circular scale (fig. 11).

When no current is passing, the counterpoise weight is

60 MEASURING AND REGULATING THE CURRENT

at the lowest point, and the index finger is at zero ; hence
the name ' gravity control ammeters.'

When a current passes through the solenoid, the small
bar of iron is attracted to the periphery of the coil. The
stronger the current the greater is the displacement of the
iron from its original position, unti] equilibrium is attained

FIG. 12. Current strength measurement.

between the rotary force exercised by the current, and the
force of gravity.

It is necessary to note that all measurements of current
strength are made by insertion of the measuring instrument

FIG. 13. E.M.F. measurements.

in the main circuit, whereas measurements of E.M.F. are
made by placing the instrument in a shunt circuit.

The diagrams (figs. 12 and 13) indicate the different
arrangement in the two cases. A = an ammeter ; F= a
voltmeter ; W = a resistance of known value.

CHAPTER IX

MEASUREMENT OF ELECTRO-MOTIVE FORCE (E.M.F.)

THE determination of the E.M.F. by the method of com-
parison with that of a standard cell is too inconvenient
for technical use, which demands that the measurement
should be easily and quickly made at any necessary
place.

Instruments have therefore been designed for this pur-
pose similar to those used for technical measurements of
current strength. Galvanometers may even be directly used,
under certain conditions, for carrying out measurements of
E.M.F. Tf a conductor be divided into two or more dis-
tinct parts, the relative strength of currents in each of
these is inversely proportional to the resistance. If one
inserts a branch or loop circuit, in the main circuit between
any two points, a and b (see fig. 13), and places in this
branch or shunt circuit a galvanometer of known resist-
ance, the fraction of the main current passing through the
galvanometer will be the smaller, the greater the resist-
ance of the wire and galvanometer forming the shunt
circuit. If the galvanometer be a high-resistance one, or
if a high -resistance coil be inserted before it in the shunt
circuit, it may be used directly for E.M.F. measurements.

According to Ohm's law, 1 ampere x 1 ohm = 1 volt,
and since the deviation of the needle of the galvano-
meter measures the former, and the resistance of the
galvanometer and wire of the shunt circuit is known,
it follows that the angular displacement of the needle is a

62 MEASURING AND REGULATING THE CURRENT

measure of the E.M.F., and that the scale of the galvano-
meter may be marked in volts. The torsion galvanometer
illustrated in fig. 7 (see p. 56) is especially adapted for
using in this way when inserted in a shunt circuit, with a
suitable resistance coil in front of it. The internal resist-
ance of the galvanometer is = 1 ohm, and the resistance
box attached to this instrument contains resistances of
9, 99, and 999 ohms.' The total resistance of the shunt
circuit may thus be made equal to 1 ohm, 10, 100, or 1,000
ohms at will. If a deviation of the needle of 1 indicates
a current of "001 ampere, it follows, from Ohm's law, that

FIG. 14. Voltmeter. FIG. 15 Voltmeter.

with the galvanometer alone in the shunt circuit, the
E.M.F. represented by the deviation is -001 x 1-0 = '001
volt. If the resistance coils equal to 99 and 999 ohms be
successively introduced into the shunt circuit, the total
resistances will be raised to 100 and 1000 ohms, and 1
deviation will now equal '001 x 100 = *10 volt or
001 x 1000 = 1-0 volt, according to the resistance used.
It is therefore possible, with the aid of the three resistance
coils named, to measure E.M.F.s lying between ! 001 and
17 volts by means of the torsion galvanometer. The
galvanometer is connected with the wires of the shunt

MEASUREMENT OF ELECTRO-MOTIVE FORCE 63

circuit by means of its corresponding binding screws, the
greatest resistance of the three attached to the galvano-
meter always being first put into the circuit with it in
order to guard against the danger from excessive currents.
When a suitable deviation of the needle has been obtained
(in some cases the internal resistance of the galvanometer
alone will suffice to keep the deviation within the limits
required), the torsion screw is turned until the needle is
again brought back to its original position, and the E.M.F.
is calculated from the angu-
lar measurement given by
the indicator finger of the
torsion screw. In exactly
the same manner as that in
which the torsion galvano-
meter may be changed into
a voltmeter, the different

forms of technical ammeters

, . FIG. 16. Voltmeter,

may be converted into in-
struments for measuring E.M.F.s by increase of their
resistance. Technical voltmeters closely resemble am-
meters in appearance, and differ in construction from
these only in the greater number of coils which they possess,
and in their higher resistance. Illustrations of some of
the usual forms of voltmeter are given in figs. 14-16 ;
others resemble in their outward form the ammeters of
which illustrations appear on pp. 58, 59. Voltmeters must
always be inserted in a shunt circuit (see fig. 13), and
not in the main circuit.

64 MEASURING AND REGULATING THE CURKENT

B. REGULATING THE CURRENT

IT is necessary now to turn to a consideration of the
methods and apparatus by which the regulation of the
current is effected. In Chapter VII the influence
of the E.M.F and density of the electric current upon
electrolytic phenomena in certain cases was discussed, and
in Chapters VIII and IX the instruments by which
these two electrical quantities are measured were described.

The question now arises How must one proceed in
order to obtain, the required current and E.M.F. from
the different sources of electrical energy, or how can one
attain in the electrolytic cell the exact current conditions
that are desired ?

The answer to this question is obtained by a considera-
tion of Ohm's law. The current strength or intensity
depends, in the first place, upon the electro-motive force
created by the source of electrical energy. The greater
this force in any complete electrical circuit, the greater is
the intensity of the current that will pass through the
circuit under otherwise exactly equal conditions. All con-
ductors metallic and non -metallic, solid and fluid offer,
however, a definite resistance to the passage of the current,
and the amount of current passing any given point in
conductors of fixed sectional area in a unit of time for
example, 1 second varies inversely to the resistance. The
greater therefore the total resistance of any complete
circuit, the smaller is the current which passes through it.
From this it follows that the intensity or strength of the

REGULATING THE CURRENT 65

current in any complete circuit is equal to the E.M.F.
divided by the sum of the resistances which it contains.
If one employs the usual units of current measurement
(see Chapter II, p. 20), one may state this law in the

m ' i > 1 volt
following manner : 1 ampere =

1 ohm

Ohm's law therefore shows how it is possible to attain
the desired current density, or current strength, in any
electrolytic cell which offers a known resistance to the
passage of the current, and for which it is necessary to use
a definite E.M.F. to effect the electrolytic decomposition.

66 MEASURING AND KEGULATING THE CUKEENT

CHAPTER X

INCREASING THE CURRENT STRENGTH

IT is possible to increase the current strength in any
circuit by increasing the E.M.F., or, when feasible, by
diminishing the total resistance of the circuit. A diminu-
tion of current strength is effected in the contrary manner
the E.M.F. is diminished or the resistance is increased.
As regards the first, both methods are not always applicable.
The increase of the E.M.F. has definite limits fixed by the
type and construction of the source of energy employed.
The dynamo produces an E.M.F. the upper limit of which
is dependent upon the const ruction , and is speedily reached.
The E.M.F. produced by the thermo-battery is likewise
limited, and depends apart from small variations upon
the heat applied and the size of the type used, that is,
upon the number of single thermo-couples it contains.
With galvanic batteries and accumulators the maximum
E.M.F. that can be obtained is similarly fixed, in these
cases being dependent upon the nature of the elec-
trodes and of the electrolytes. In order to obtain from
galvanic cells a greater E.M.F. than can be obtained
from the single galvanic couple, a number of the single
elements are connected together to form a battery. The
manner in which the connections of single elements to
form batteries are made is of importance. If the con-
necting or coupling r-as, for example, of five Daniell ele-
ments be so carried out that the copper of one cell is
connected to the zinc of the next, the arrangement is called
' series coupling,' and is diagrammaticallly represented by
fig. 17. The current in this case passes from the first through

INCREASING THE CURRENT STRENGTH

the second, and so on, and the E.M.F. of the series is equal
to five times that of a single Daniell cell. Each cell possesses,
however, a definite internal resistance, and the total internal
resistance of this
series will be five
times that of the
single cell. The cur-
rent intensity is not
increased in the
slightest degree by
this method of coup-
ling cells together,
for the current
strength is practi-
cally the same with
the five cells in series
as with the single
cell. If, however,
the resistance of the
external circuit is
very large as com-
pared with the in-
ternal resistance of
the five cells forming
the battery, the cur-
rent strength ob-
tained with the series
arrangement of the
cells will be nearly
five times as great
as it would have
been with the single
cell. In order to
obtain, therefore, large currents when the external resist-
ance is great, the cells are coupled in series. The other
arrangement of cell coupling is known as c parallel coupling,'
and is used when the resistance of the external circuit is

68 MEASURING AND REGULATING THE CURRENT

small. By this term is understood the arrangement in
which similar poles of the successive cells are connected
together. Fig. 18 shows this method of coupling applied
to five Daniell cells. All the copper poles are connected
together, likewise all the zinc poles ; and the combined
copper cylinders act as one large cylinder five times their size
would behave if placed in the same electrolyte, opposite to an
equally enlarged cylinder of zinc. The sectional area of the
liquid between the electrodes is, by the 'parallel coupling'
of the five cells, increased to five times that of the single cell,
and the internal resistance of the five cells is thus reduced
to one-fifth of that of any one of them alone. The current
strength obtained by this ' parallel coupling ' of the cells is
therefore equal to the E.M.F. of a Daniell cell divided by
one-fifth of the internal resistance of this form of primary
cell. If the resistance of the external circuit be high, the
current strength obtained will differ but slightly from that
of a single Daniell cell ; for in this case the E.M.F. of one
Daniell element is divided by the whole external resistance.
These considerations have led to the formation of the rule
given above, that 'parallel coupling should be used only
when the resistance of the external circuit is low.'

In carrying out the various electrolytic decompositions
required in analytical chemistry, the resistance of the
electrolytic cell will be found to vary between '40 and 2*5
ohms, according to the nature and temperature of the
electrolyte. The resistance of the conducting wire can be
neglected. In those cases in which the resistance of the
external part of the circuit including the electrolytic cell
is approximately equal to the internal resistance of the
primary cell, it will be found most advantageous to couple
the cells both in series and in parallel, and the arrange-
ment thus obtained is known as * series parallel coupling.'
Figs. 19 A, B, c, D are diagrammatic representations of this
type of coupling, and are self-explanatory.

The arrangement shown in fig. 19 A gives an E.M.F.
equal to two Daniell cells, with an internal resistance

OF THB

UNIVERSITY

INCREASING THE CURRENT

equal to one half that of a single cell. The arrangements
shown in figs. 19 B, c, and D yield E.M.F.s equal to two,
three, and four times the E.M.F. of a single Daniell cell
respectively ; while the internal resistance is equal to that
of one Daniell cell in figs. 19 B and 19 c, and equal to that
of two Daniell cells in the arrangement shown in fig. 19 D.
The choice of the coupling arrangement to be used in
any particular case is governed by the resistance of the
external portion of the circuit. As a general rule it will

C D

FIG. 19. A, B, c, D, Diagrams illustrating ' Series Parallel Coupling.'

be found advisable to couple the cells in such a manner that
the internal resistance of the battery is approximately
equal to the total resistance of the external circuit.
Secondary cells or accumulators are used in a similar
manner to that just described for primary cells and
batteries. In this case, however, the method of parallel
coupling is in most instances unnecessary, since accumula-
tors are very seldom used containing only single plates of
spongy lead and lead peroxide. As a rule a secondary cell
contains a large number of these positive and negative

70 MEASURING AND REGULATING THE CURRENT

plates arranged alternately. All the negative plates are
connected together, and similarly all the positive plates.
Such a secondary cell therefore represents, and acts as,
a battery of single elements coupled in parallel.

It sometimes happens that many electrolytic decomposi-
tions have to be proceeded with simultaneously, and in
such cases parallel coupling of the smaller types of accu-
mulator may be necessary. For electrolytic purposes an
E.M.F. of from 4 to 6 volts is usually sufficient. When
accumulators are used as the source of electrical energy,
three or four cells coupled in series are therefore required.
If the only source of electrical energy for charging the
accumulators be a thermo-battery, it is necessary to employ
accumulator cells of the smaller pattern, and in the use of
these for electrolytic separations cases can occur in which
* series parallel coupling ' is necessary. It is most con-
venient in such cases to construct a special switchboard
which for four accumulator cells would have the form
shown in fig. 20. Small pieces of brass of the shapes
shown in the diagrams of fig. 20 are screwed upon a base-
board. Each of these is connected to the wire proceeding
from a pole of an accumulator cell ; the upper row of brass
plates being connected to the positive, the lower row to the
negative poles, in the order indicated by the numbers in the
diagram. The round openings between the separate pieces of
brass serve for the reception of accurately ground-in metal
plugs, which complete the connection between neighbouring
pieces, and consequently also between definite electrodes
of the battery cells. The arrangement of the plugs in these
openings makes it possible to couple the four cells in series
or in parallel with the expenditure of a minimum amount
of time and trouble.

In fig. 20 A the four cells are coupled in parallel, and
the E.M.F. produced is only equal to 2 volts.

In the arrangement of the plugs shown in fig. 20 B, cells
Nos. 1 and 2 and also cells Nos. 3 and 4 are coupled in
parallel, but the plug in the middle serves to couple each of

INCREASING THE CURRENT STRENGTH

these pairs in series, so that the total E.M.F. obtained is
4 volts. Fig. 20 C represents an arrangement by which an
E.M.F. of 6 volts is obtained ; cells Nos. 1, 2, and 3 are
coupled in series, Nos. 3 and 4 in parallel. This arrange-
ment cannot be recommended for practical purposes, as
the accumulator cells would be unequally discharged.

It is better that the discharge of the cells should occur
equally, and that the recharging should likewise be regular,
so that the recharging of the cells may be completed at one
and the same time.

1 +2 *3 *

_JL^jL^JL^ I

-i -2 -3 ~ t

A
f-1 +2 +3 -f <t

MJLJ B

-^ni ^ ir^ni 1 i

-J -2-3 -4 -l -Z -3 -n

C D

FIG. 20. A, B, c, D, Diagrams of a Switchboard for Accumulators.

By the arrangement of the plugs shown in iig. 20 D all
four cells are coupled in series. A useful form of accumu-
lator battery of this kind is shown in fig. 21. The battery
contains four accumulator cells, each of eight ampere hours'
capacity. The charging can be effected by means of a
thermo-battery. The four cells are enclosed in a wooden
chest, upon which a switchboard similar to that described
above is screwed.

When charging the cells by aid of a thermo-battery, it
is necessary to adopt the parallel coupling shown in fig.
20 A. This switchboard may also be used for larger
patterns of accumulator cells, if it be necessary to use these

72 MEASURING AND REGULATING THE CURRENT

in connection with one or two of the smaller cells. In
order to effect this combination, holes are bored in the
metal plates, and plugs are inserted which make the con-
nection with the poles of the larger cell.

In the larger patterns of Gulcher's thermo-battery there
are sixty-six thermal elements coupled in series. An alter-
ation of the manner of coupling is in this case not possible,
nor is it necessary in the majority of electrolytic separations ;
for with an E.M.F. of 4 volts and a current strength of

about 2 amperes
(this varies accord-
ing to the resist-
ance), nearly all
the electrolytic ana-
lyses can be carried
out. The E.M.F.
and with it the
current strength
obtained from such
a thermo - battery
can be diminished,
if the wire connec-
tion be made not
as customarily at
the terminal bind-
ing screw, but at
one of the cooling
plates attached to the negative elements. Should a case
occur in which the E.M.F. or current strength yielded by the
thermo-battery is insufficient to effect the desired electro-
lysis, one can use the thermo-battery and accumulators
together, coupled in parallel or in series. l

As already noted at the commencement of this chapter,

a second method of increasing the current strength in any

circuit is^ that dependent upon a decrease of the total

resistance. This is made up of the internal resistance of

1 Elbs, Chem. Zeitg. 1893, 17, 66, 97.

FIG. 21. Accumulator Battery with Switch-
board.

INCREASING THE CURRENT STRENGTH 73

the source of electrical energy and of the resistance of the
external circuit, the latter of which equals the resistance of
the metallic and fluid conductors, and in addition that pro-
duced by polarisation.

The reduction of the internal resistance of the source of
electrical energy is only feasible with galvanic elements. It
is effected by coupling a large number of the cells in
parallel that is, by greatly increasing the superficial area of
the electrodes.

In the case of accumulators at least, as regards the
larger patterns this has already been carried out, and the
internal resistance of these is extremely small. In the case
of dynamos and thermo-batteries the internal resistance
cannot be reduced at will in this manner. The resistance
of the external circuit may be diminished in various ways.
It is known that the resistance of the metallic conductors
or * leads,' apart from the specific resistance of the metal
used, is directly proportional to their length, and inversely
proportional to their sectional area. For copper wire
conductors it is customary to allow 2 or 3 amperes per
sq. mm. ; and since in electrolytic experiments currents
of more than 2 amperes are comparatively rarely used, it
follows that for this class of work copper wire 1 sq. mm. in
sectional area is ample. The resistance offered by the
electrolytic cell may be diminished, as in the case of the
battery cell, by increase of the superficial area of the
electrodes. A second method is to increase the conduc-
tivity of the electrolyte. 1 This method, in most cases of
electrolytic deposition, cannot be made use of, since a
definite composition and degree of concentration of the
solution have to be maintained in order to effect the desired
separations. As a general rule it is not advantageous in
electrolytic analysis to utilise the electrical energy in the
most economical manner.

1 [In the majority of in stances heating an electrolyte increases its
conductivity, and this method of reducing the resistance of the
electrolytic cell is very often used. Translator's note.']

74 MEASURING AND REGULATING THE CURRENT

CHAPTER XI

REDUCING THE CURRENT STRENGTH

THE reduction of the current strength is of course effected
by means exactly the reverse of those adopted in order to
increase it. The reduction of the E.M.F. of the source of
electrical energy cannot be effected in the case of primary
or secondary cells ; and it is only in special cases as, for
example, in separations based upon the different ' decom-
position values ' of metallic salts that need for a reduction
of the E.M.F. exists. A reduced E.M.F. can be obtained
from a thermo-battery when one reduces the number of
single elements concerned in the production of the current.
The manner in which this is achieved has already been
described (see p. 72). The E.M.F. produced by dynamos
can be reduced by inserting resistances in the shunt circuit
that excites the magnets. The strength of the field is thus
reduced and the E.M.F. diminished.

In spite of this arrangement the E.M.F. produced by
dynamos is usually too high for many purposes, and the
E.M.F. in the circuit is still further reduced by inserting
resistances in it made from wire of high specific resistance,
or by using only a fraction of the main current for the
electrolysis. The alloys used for these resistances are
the following: 'German silver,' ' Rheotan,' ' Nickel in,'
' Manganin/ arid ' Konstantan.' Of these the alloys of
manganese and copper, and of nickel, manganese, and
copper, are the most satisfactory, as their resistance does
not vary much with the temperature. These resistance

REDUCING THE CURRENT STRENGTH 75

coils of wire are employed with all forms of current
producer, they are extremely convenient in use, and
they enable one to obtain the desired current strength
within extremely narrow limits. In order to reduce the
current from any source of electrical energy to the desired
strength, one might stretch out a length of wire made from
one of these poorly conducting alloys, and by means of a
sliding contact take the current from any desired point

FIG. 22. Resistance Box with Plug Contacts.

upon it. Such a simple arrangement is in most cases, how-
ever, not sufficient.

The customary form is shown in fig. 22. The insulated
wire is wound into rolls or coils, the ends of these are
soldered to brass connections arranged in order upon a
wooden or slate base, and the well-insulated coils- are
enclosed in a box having this base as its cover. The brass
pieces that represent the terminals of the coils are so

76 MEASURING AND REGULATING THE CURRENT

arranged that by the use of plugs they can be put in or
out of circuit as desired. The resistance of the coils is
expressed in ohms. They are arranged in the box in
order of value, in a similar manner to that found in a set
of weights, so that any resistance between 1 and 100 ohms
can be produced. If all the plugs be inserted as shown in
the illustration, the current passes entirely through the
pieces of brass upon the cover of the box, and does not
travel by any of the coils. Another form of resistance box

FIG. 23. Resistance Box with Mercury Contacts.

is shown in fig. 23. In this case the ends of the coils are
brought into small depressions, or cups, filled with mercury
when the box is in use, and bent pieces of stout copper
wire are used instead of plugs, to place the coils out of
circuit.

One can make a similar variable resistance of less
range by forming spirals of * Manganin ' or ' Nickelin '
wire of different thicknesses, and by fixing these upon
a frame, as shown in fig. 24. The ends of the wire

REDUCING THE CURRENT STRENGTH

are soldered to stout copper wires which dip into mercury
contact cups. If small binding screws be used with this
resistance, the whole or part of any division of it may be
cut out by binding two neighbouring wires together.

Since it is not necessary in the greater number of elec-
trolytic separations to use resistances of any definite and
fixed value, one frequently finds resistances in use which
have not been standardised, and in which the contacts are
made not by means of plugs or of mercury cups, but by
means of the so-called ' sliding contacts.' Figs. 25 and 26

FIG. 24. Frame Resistance.

are illustrations of this type of resistance ; many other forms
of it exist.

The wire for the separate coils can be roughly measured
according to the resistance data supplied by the manufac-
turer, and the arrangement of the coils upon the frame
does not follow any order of value. When very strong
currents are being employed it is preferable to use strips
of thin sheets of the different alloys, in place of wire, in
resistance frame.

If one desires to employ the second method for the pro-
duction of feeble currents that in which only portion of
the main current is utilised the following arrangement is

78 MEASURING AND REGULATING THE CURRENT

adopted. A strip of sheet brass, or a strip of one of the
above-named alloys, is fastened in zigzag manner upon a
small board, and the bends of the strip of brass or other
alloy are soldered to brass pins or to the usual form of bind-

FIG. 25. Adjustable Resistance. FIG. 26. Adjustable Resistance.

ing screws, so that the current may be led from any of
these points at will. This instrument is inserted in the
main current circuit, and by use of these numerous contacts
any desired fraction of the main current is switched off
through a shunt circuit containing the electrolytic cell.

PAET III
THE ELECTROLYTIC PROCEDURE

A. INTRODUCTORY

IN order that the current produced by any source of elec-
trical energy may exert its dissociating influence upon a
salt solution, it is necessary that suitable conductors should
be chosen whereby the current may be led to and from the
liquid. These conductors or 'electrodes' may be formed
of different metals or of carbon, but for analytical work
only those of platinum are employed. Platinised sheets of
other metal have not proved serviceable j and although
gold has proved suitable in certain instances, yet platinum
is to be preferred.

The electrode material must not only be proof against
the acids used to dissolve the deposited metal, when the
electrolysis has been completed and the deposit weighed,
but must be also proof against the action of the anions
liberated during the electrolysis.

For example, gold would be useless as an electrode
material for the electrolysis of chloride or alkali-sulphide
solutions. Even platinum is slowly attacked by chlorine,
and on this account the electrolysis of solutions of chlorides
is undertaken in as few cases as possible. If a solution of a
zinc salt solution be electrolysed with a platinum kathode, a
black deposit will remain upon the latter especially notice-
able at that part of the electrode which cut the level of the
electrolyte when the zinc has been dissolved off'.

80 THE ELECTEOLYTIC PROCEDURE

This deposit is insoluble in acids. According to Vort-
mann and others, it consists of finely divided platinum, and
its removal from the electrode is difficult and is harmful to
the latter.

In order to avoid this cause of injury to the platinum
electrode, it is customary when zinc is to be deposited to
coat the electrode previously with silver, copper, or tin.
The surface of the electrodes is generally smooth, but it
has been found that in certain cases it is more advan-
tageous to use electrodes that have become roughened by
frequent use, or that have been artificially made dull and
dead by means of the sand-blast.

Peroxide deposits and also metallic deposits of anti-
mony adhere better to such a roughened surface. The
strength of the sheet metal used for the electrodes should
not be too low ; the metal ought to be sufficiently thick to
resist any mechanical strains to which it may be subjected.
Electrodes made from an alloy of platinum and iridium are
found to resist both the chemical action and mechanical
wear and tear of use, better than those made of platinum
alone.

As regards the form of the electrodes, great differences
may exist, and practically all the possible forms are in actual
use. They may be divided into two broad classes. Under
the one come all those electrodes used in pairs for the
electrolysis of liquids contained in beakers or other non-
conducting vessels. Under the other are grouped those
forms in which one electrode acts as a basin or vessel for
holding the electrolyte.

Neither class possesses decisive advantages in all cases ;
but each is found to be especially convenient in particular
separations.

The 'basin electrode' is used in two forms. That
recommended by Classen is shown in fig. 27, and has no

HP.

The form shown in fig. 28 is recommended by N. v.
Klobukow. It possesses a spherical cup -shaped bottom, per-

INTRODUCTOEY 81

pendicular sides, and a lip. The latter form may be slightly
more convenient for use when it is necessary to calculate
the electrode surface covered by a definite volume of the
liquid, but apart from this it has no advantage over the
former. It is of great convenience to have marks upon
the inner surface of the walls of the basin which denote
the superficial area covered by the liquid when standing at
any particular height in the basin ; since this simplifies the
work when carrying out electrolytic depositions by means
of definite current densities.

The basin shown in fig. 27 is about 9 cm. in diameter
and about 4 cm. in depth ; its capacity is between 200 and
250 c.cms. liquid.

FIG. 27. Basin for Electrolysis. FIG. 28. Basin for Electrolysis.

The other electrode for use with this basin can have
many forms. The form shown in fig. 29 corresponds best
to the shape of the basin, and secures a uniform current
density ; it suffers from the disadvantage that, in spite
of the openings in its sides, the volume of liquid enclosed
within it passes but slightly into circulation. The ' saucer
electrode,' shown in fig. 30, possesses a round hole at its
deepest part ; while the ' disc electrode,' shown in fig. 31,
is pierced with many round openings. These two forms
are more satisfactory than the first, since they hinder the
circulation less, and the holes permit the escape of the
gases liberated at their under surface.

The separate parts of these electrodes should be riveted
together ; if they should be soldered even with gold, the

THE ELECTROLYTIC PROCEDURE

solder will be speedily dissolved away when they are used as
anodes.

FIG. 29.
Basin Electrode.

FIG. 30.
Saucer Electrode.

FIG. 31.
Disc Electrode.

Turning to the other group of
electrodes, many forms are found to
have been proposed and used. The
simplest arrangement is that of two
sheets of platinum opposed to one
another in the vessel containing the
electrolyte ; but this is seldom adopted.
In some cases a fork-shaped anode
is used with a single sheet of metal
as kathode, in order to obtain a
uniform coating upon each side of tho
latter. The forms most in use, how-
ever, are a cylindrical or conical sheet
electrode, enclosing a similarly shaped

FIG. 32^The Mans- S P iral f thick P latinum wire ' One of

feld Electrodes. the oldest types of this arrangement

is the so-called Mansfeld electrode,

shown in fig. 32. This consists of a closed platinum
cylinder, and of a platinum wire spiral used within the

INTRODUCTORY

cylinder. The first practical attempts to effect electrolytic
determinations of metals were made with this form of elec-
trode.

If one decides to make use of this form, it will be
advisable to cut the cylinder through in the direction o.
its axis, and also to bore some holes at other points in it,
in order to facilitate the circulation of the electrolyte,
and also to allow the deposition to occur to some extent
upon the outer side of the cylinder. The conical jacket
electrode shown in fig. 33 is also a form much used, and

FIG. 33. FIG. 34. FIG. 35.

FIGS. 33-35. Cone-shaped and Spiral Electrodes.

in this case the electrode is better when provided with
openings to promote the circulation of the electrolyte.
Figs. 34 and 35 show the forms of spiral used with this
jacket sheet electrode. The form shown in fig. 36 is,
however, to be preferred if a uniform current density at
all parts of the surface of the cone is required. The cone
is generally used about 8 cm. in height, and about 6 cm.
in diameter at its base. If these jacket electrodes be pro-
vided with an opening parallel to their axis, they possess

84 THE ELECTROLYTIC PEOCEDUEE

the advantage that when the electrolysis is completed
they can be lifted directly out of the solution. The gas
bubbles that are given off from the lower parts of the wire
electrode also aid the thorough mixing of the electrolyte
during the electrolysis. The disadvantage of these forms
lies in the difficulty of obtaining a uniform current density
at all parts of the sheet electrode, and on this account
these electrodes cannot be used in performing some of the
more particular electrolytic depositions.

As holder for the electrodes, a stand having a heavy
cast-iron base into which a strong glass rod is fixed will be
found to answer best. Upon this glass rod slides an arm
of brass, copper, or aluminium, capable of being fixed at
various heights, and bearing two binding screws, one for
connection with the current supply, the other for connec-
tion with the electrode. If the jacket electrode is to be
employed, either two such stands are used, or a compound
holder such as that illustrated in fig. 36 is used, in which
the conducting parts a and b are separated by an insulating
piece x.

If the basin electrode be used, one of the arms must be
bent into a ring, and this is provided on its upper side with
three platinum points, which make the electrical contact
between the ring and the basin. When the column of the
stand is of glass, both arms may be fixed upon it, as shown
in fig. 37.

Another form suitable for use with the basin electrode
consists of a turned wooden foot having a thick metal wire
bent into a circle fixed upon it. The basin rests firmly
upon this, while a bent metal arm, fixed at one side of this
wooden base, serves to hold the disc electrode over the
middle point of the basin. This form of holder has not
come much into use, one of its disadvantages being that it
is impossible to heat the electrolyte during the electrolysis
by means of a small burner placed beneath the basin.

In order to avoid loss of the liquid during the electrolysis,
the basin or beaker must be covered with a large clock-

INTRODUCTOKY

When the former is used to hold the electrolyte,
this glass will answer if it have only a hole through its
centre ; but when a beaker is used, it is necessary to have
a narrow slot in the glass cover, extending from the cir-
cumference to the centre.

It is especially necessary in electrolytic work to see that
the electrodes are perfectly clean ; with dirty electrodes it
is impossible to obtain a uniform and adherent deposit.
Films of grease, which can be produced merely by passing

FIG. 36. Stand for Electrolysis. FIG. 37. Stand for Electrolysis.

the fingers over the electrode surface, are especially detri-
mental.

If an electrode that has been soiled in this way
cannot be freed from its impurity by heating to redness,
or by treatment with acids, it will be necessary to heat it
with fused acid potassium sulphate or with borax, or to
clean it by mechanical means with fine sea-sand. The
methods first named are least harmful to the electrode, and
are to be preferred, when effective. The electrode when
perfectly clean is well washed, and is dried by. heating.

In order to carry out the electrolysis, the prepared

86 THE ELECTEOLYTIC PROCEDURE

solution of the metallic salt containing the necessary
additions of other chemicals is placed in the platinum
basin, or in the beaker designed for use with one of the
sheet electrodes. For electrolytic analyses, the volume of
liquid should lie between 150 c.cms. and 200 c.cms. ; the
volume of liquid to be dealt with must therefore be brought
within these limits by concentration or dilution, as the
necessities of the case demand. If the jacket electrode,
either in cylinder or cone form, is being used, the distance
apart of the two electrodes will be alike at every point if
care has been taken to centre the inner electrode properly
when fixing it in the holder. Care must also be given
to guard against any displacement or contact of the two
electrodes when placing them in, or when taking them out
of, the liquid in the beaker. When the basin electrode is
used, attention must be paid not only to the central position
of the disc electrode, but also to its height above the bottom
of the basin. This should lie between 1'5 and 2*0 cms. in all
cases. The clock-glass used for covering the vessel in which
the electrolysis is performed must be laid on at the com-
mencement. When a metallic deposit is to be obtained,
the basin or the sheet electrode is used as kathode ; but
when a deposit of the metal as peroxide is desired, the
current direction is reversed, and the basin or sheet electrode
is used as anode.

The current density given in the directions for the
different depositions in the following pages is always
calculated upon the superficial areas of these electrodes,
and not upon those of the wire ones. When the current
connections are first made for such an electrolytic cell as
that described above, a very large resistance is always used
in the circuit ; and the amperes having been calculated
that will yield the desired current density with the electrode
surface that is immersed, the resistance is reduced until
this current strength is obtained. Since the interior sur-
face area of the form of platinum basin customarily used
(see fig. 27), having a diameter of about 90 mm. and a

INTRODUCTORY 87

depth of about 40 mm., is only determined with difficulty
mathematically, one uses the following data for the current
density calculation : 125 c.cms. liquid cover 100 sq. centi-
metres, and 180 c.cms. cover 150 sq. centimetres of the
interior surface of the basin.

These ratios are sufficiently exact for use in most cases,
since the maintenance of the current density within more
accurate limits is unnecessary.

The concentration of the electrolyte undergoes change
during the electrolysis, and in most instances this occa-
sions an increase in the resistance it offers to the passage
of the current. If the external conditions remain the
same i.e. if the E.M.F. of the battery or dynamo, and
resistance of the external circuit, remain unaltered the
current strength, and therefore the current density at the
electrodes of the electrolytic cell, will be diminished. In
order to maintain the current density at the kathode (when
peroxide depositions are in progress at the anode) at a
definite value, many measurements must be made during
the course of the electrolysis, and the strength of the
current must be increased proportionately to the growth of
the resistance of the electrolytic cell, by diminishing the
resistance that has been inserted in the external circuit.
The E.M.F. measured at the terminals of the electrolytic
cell is always a guide to the resistance of the cell, and by fre-
quent measurements one can observe the increase in this as
the electrolysis proceeds. As already mentioned, it is now
customary to calculate the current density that is, the
strength of the current in amperes per unit of surface
upon a superficial area of 100 sq. centimetres.

This ' normal density,' as it is called, js used both for
the kathode, and for the anode when peroxide deposits are
being obtained. For electrodes of other size the calcula-
tion is simple. In the following special part of this work
all the current strengths given are calculated for a super-
ficial area of 100 sq. centimetres, even when this is not
particularly noted.

88 THE ELECTROLYTIC PROCEDURE

As regards temperature, in most cases the electrolysis
can be conveniently carried out at the normal indoor
temperature of 20 C. (68 F.) The electrolyte frequently
increases in temperature when strong currents are employed,
but it is often advisable to heat the liquid by the aid of some
external source of heat, in order to reduce the resistance it
offers to the current, and to lessen the -time necessary for
the complete deposition of the metal. A high temperature-
is also often requisite in order to obtain a metallic deposit
possessing the desired physical characteristics. If then it
be desired to electrolyse a warm or hot solution, a burner
with a small easily regulated flame is placed under the
basin or beaker containing the liquid. One may use for
this purpose either an ordinary Bunsen burner from which
the vertical tube has been removed by unscrewing, or one
may make use of a small Bunsen burner especially designed
for this purpose, possessing a horizontal mixing tube turned
upwards at its end. In order to obtain an equal and
regular temperature, a piece of asbestos paper or board is
loosely placed under the basin or other vessel, so that the
latter is heated by the intervening air rather than by
direct contact with the hot asbestos. In the majority
of instances it is sufficient to lead strong currents 1-0
to 1 '5 amperes through the warmed electrolyte ; the tem-
perature then remains about the same. When this is not
feasible the temperature of the electrolyte will fall from
60 C. to about 40 C. during the electrolysis, if no ex-
ternal heating be used. Such a fall in temperature ought
not to prove detrimental to the results obtained with
any really trustworthy method, and therefore, t in the
conduct of technical electrolytic analyses, the use of a
burner for maintaining the temperature has been dis-
continued.

The completion of the deposition and end of the electro-
lysis can be determined in many ways. When the deposit
is a coloured one as, for example, that of copper or of the
peroxides the observation is carried out by adding a little

INTRODUCTORY 89

distilled water to the electrolyte, and thus covering a fresh
portion of the kathode surface with the liquid. This newly
covered surface is examined after an interval of five or ten
minutes ; if no trace of a deposit be visible, the electrolysis
is completed. When the basin electrode has been used,
instead of adding water to the electrolyte, a small strip of
clean platinum foil is hung over the edge of the basin, so
that it dips into the liquid.

When the jacket electrode is used, it is extremely easy
to change the first electrode for a second one free from any
deposit. These methods are, however, not applicable in the
case of those metals the deposits of which differ little from
that of the platinum in colour as, for example, the deposits
of cobalt, nickel, iron, and silver. In these cases the test
is usually made by withdrawing a small portion of the
electrolyte by means of a small pipette, and by applying to
this small test portion some delicate chemical test. If no
reaction for the concerned metal is obtained, one concludes
that the last traces have already been removed from the
electrolyte by the action of the current.

When the electrolysis is completed, the circuit is broken,
the electrodes are freed from the binding screws, and the
liquid is poured out of the basin, or the jacket electrode is
lifted out of the liquid in the beaker. The portion of the
liquid that clings to the deposit is washed away with dis-
tilled water, and the water is then itself removed by rinsing
the deposit with strong alcohol. The final drying is accom-
plished either over the open flame, upon a heated asbestos
or iron plate, in the air-bath at 100 C., or in a desiccator,
according to the nature and character of the deposit. The
alcohol is used to prevent, as far as possible, the oxidation
of the moist metallic coating.

In many electrolytic depositions the breaking of the
circuit leads to a re-solution of the deposited metal by the
electrolyte, and in these cases it is necessary to displace
and wash out the electrolyte before breaking the circuit.
This is accomplished by hanging over the side of the vessel

90 THE ELECTEOLYTIC PROCEDURE

a small siphon filled with water, and by adding distilled
water to the vessel as the original liquid is carried away
by this siphon. A conducting medium thus remains be-
tween the two electrodes, the current continues to pass in
the original direction, and only after suitable dilution of
the original electrolyte is the circuit broken, and the elec-
trode with its deposit removed for further treatment in the
usual way. When the jacket electrode has been employed,
one may avoid using this displacement method of removal
of the electrolyte by a quick withdrawal of the electrode
from the electrolyte. The former method ought to be used
for deposits of copper from nitric acid solutions, of antimony
from sodium sulphide solutions, of lead peroxide from nitric
acid solutions, and of other metals ; but one may even in
these cases substitute for it a rapid emptying of the basin
without introducing any serious error into the results.
The weight of metal which can pass into solution again
during the few seconds required for pouring off the electro-
lyte and diluting the remainder, varies with the metal, the
solution, arid the duration of the operation ; but experiments
have shown that it is, as a rule, between -0004 and '001
grm. The electrode bearing the dry deposit must never
be weighed until it has cooled down to the temperature of
the balance room.

/ Since a repetition of any experiment is only possible
' when data concerning the concentration and temperature
of the electrolyte, the strength and electro-motive force of
the current, and the superficial area of the electrodes have
been kept, it is advisable even from the commencement of
the experiments to record all these details, as well as the
observations made during the progress of the experiment,
and the results obtained, in some such form as the follow-
ing :

Experiment. (Example Copper, or separation of
copper and lead.)

Electrolyte. (1 grm. CuS0 4 ; 5 c.cms. HNO 3 ; 150
c.cms. H 2 O.)

INTRODUCTORY

Source of Energy. (Two accumulator cells in series.)
Electrodes, (Basin jacket electrode or foil.)

OBSERVATIONS AND MEASUREMENTS

Electrode
area in
sq. centi-
metres

Elec-
trodes ;
distance
apart

Volume
of the
electro-
lyte

Tem-
perature
of the
electro-
lyte

Time of
current
measure-
ment

Current
strength
in
amperes

E.M.F. at
the elec-
trodes in
volts

Resist-
ance of
the cell
in ohms '

Results

Current density (per 100 Character of the deposit

sq. cms.) Used for analysis

E.M.F. Found

Temperature Difference

Duration Remarks

grm.
grm.
grm.

DEPOSITION OF THE METALS FROM
SOLUTIONS OF PURE SALTS

In the following division of this work a number of dif-
ferent methods are given under the headings of the indivi-
dual metals, by which the separation of the concerned metal
can be effected.

These will be found to include not only electrolytic
methods of a particular class, but also practically all the
methods in actual use, whoever may have been their
authors.

Those methods, which are easily carried out, and which,
in some cases by variation in the conditions, can be made
to yield reliable results, have received especial attention ;
since such methods have been adopted in some technical
laboratories as substitutes for the older analytical pro-
cesses.

In all cases the necessary information concerning

E.M.F.

Resistance

current

92 THE ELECTROLYTIC PROCEDURE

current density, concentration, temperature, and voltage
has been given. A very full list of references to original
papers will facilitate the consultation of these when neces-
sary, and will enable the reader to obtain a comprehensive
survey of the methods proposed up to the present date. It
will also serve to check the very common rediscovery of old
methods by new workers in this branch of science.

Those separations which are of technical importance
have been indicated, so that the choice of methods for prac-
tice by the novice has been simplified as far as possible.

COPPER

Copper was the first metal of which proof was given
that it could be deposited quantitatively by electrolysis.
Electrolysis was thus shown to be applicable to analytical
purposes.

Copper is distinguished from other metals by the ease
with which it can be deposited from acid solutions, and by
the character of the deposit so obtained. This is nearly
always bright red in colour, and of a metallic lustre. The
position of copper below hydrogen in the series of metals
given in Chapter VII (p. 35) signifies that the metal can
be deposited without difficulty, especially from solutions
containing free acid. In such solutions the decomposition
value of the salt of copper is comparatively low, and the
deposition can be effected with a very small expenditure
of energy.

An E.M.F. of 1*8 volts suffices to separate copper from
solutions containing nitric acid ; from solutions containing
ammonia a voltage rather lower than this is sufficient ;
while from solutions containing ammonium oxalate only
1 -5 to 1 -6 volt is required.

Solutions of copper sulphate or copper nitrate containing
free nitric acid are especially suited for laboratory electro-
lysis ; l but the amount of free acid present must not be

1 Zeitschr.f. anal, Chem. 19, 1.

OF THK

UNIVERSITY

DEPOSITION FROM PURE S^K^^^^T 93

allowed to exceed 8 to 10 per cent. 1 For the carrying out
of such an electrolysis a weighed amount of copper sul-
phate, generally about 1 grin., is dissolved in water, the
solution is diluted to about 150 c.cms., and from 3 to
5 per cent, by volume of cone, nitric acid (sp. gr. 1*40) is
added to it. The solution is heated to 50 or 60 C. in
the basin or in the beaker in which the electrolysis is
to be performed, and a current of about 1 ampere
is passed through it. The E.M.F. should be from 2 to
2J volts.

The reddish deposit of metallic copper upon the kathode
can be noticed immediately the circuit is completed. The
current is allowed to continue until the blue copper solution
appears to have lost all its colour ; this result should be
obtained in the course of about two hours. Before the
circuit is broken, however, it is necessary to prove that the
whole of the copper has been deposited. The complete
separation of the copper contained in 1 grm. of copper
sulphate, which contains theoretically 25'33 per cent. Cu,
should require, with the strength of current named above,
from two to three hours. In order to test whether in a par-
ticular case traces of copper are still present in the solu-
tion, sufficient water is added to the electrolyte to cause
the immersion of a clean portion of the kathode. As the
reddish colour of the deposited copper is very distinct
against the platinum kathode, a ready means of checking
the completion of the deposition is afforded.

After the addition of water to the electrolyte, the current
is allowed to continue for ten or fifteen minutes, and the
freshly immersed surface of the kathode is then examined
for a thin coating of copper. A more delicate method of de-
termining when the electrolysis is complete consists in the
withdrawal of a very small quantity of the electrolyte by
means of a glass tube which has been drawn out at one end
to a fine jet, and in the testing of this by chemical methods.
The two reactions made use of for this purpose are those
1 Berg- u. Hiitten-Zcit. 21, 220.

94 THE ELECTROLYTIC PROCEDURE

with sulphuretted hydrogen and potassium ferrocyanide ;
the test with ammonia is not sufficiently sensitive for such
small amounts of copper. The liquid that has been with-
drawn is treated with a few drops of sodium sulphide
solution, after addition of acid if it be a neutral or alkaline
solution of copper; the presence of traces of copper is
indicated by a distinct brown colour of the solution. The
use of sulphuretted hydrogen gas is less convenient. In
order to apply the potassium ferrocyanide test, a few drops
of the solution of this salt are placed in each of two test
tubes, the liquid in each is acidified with a little hydrochloric
acid, and the solution withdrawn from the electrolyte is
added to the contents of one of these test tubes. On
viewing the tubes against a white background, the presence
of a very small amount of the reddish ferrocyanide of
copper is easily detected. One can also mix finely ground
potassium ferrocyanide with a drop or two of hydrochloric
acid upon a porcelain tile, and allow the solution which is
to be tested to fall in drops into the middle of the mixture ;
a reddish coloration will appear at the edges of the mixed
solutions if traces of copper be present.

When it has been proved that the whole of the copper
has been removed from the solution, it is simply necessary,
if a jacket electrode has been used, to remove this from the
holder, to lift it quickly out of the solution, and to wash it
under a running stream of water in order to free it from
the adherent acid liquid. When a basin has been used as
negative electrode, this would demand some little time, and
it is necessary to proceed differently if all danger of the
acid liquid acting upon the deposit of copper is to be
avoided. In this case the copper must be washed before
the current is discontinued. This is effected by using a
siphon to remove the electrolyte from the basin, while
water is allowed to flow in. In this way the acid in the
solution becomes diluted to a point at which action upon
the deposit is impossible. The contents of the basin are
then emptied out, it is rinsed a few times with water, and

DEPOSITION FROM PURE SALT SOLUTIONS 95

lastly with strong alcohol, and quickly dried in an air bath
or by the naked flame.

The solvent action of the remaining electrolyte upon
the deposited metal is, it must be admitted, not very
great, since a portion of the nitric acid present at the
commencement of the electrolysis will have been converted
into ammonia. Experiments made with a copper sulphate
solution to which 8 per cent, of nitric acid had been added,
which was electrolysed for an hour with a current of
1 ampere, showed that '0004 grm. of the deposited copper
was dissolved in each case by the remaining solution in a
quarter of a minute from a surface of 100 sq. centimetres.
An expert analyst would never require such an interval
of time for disconnecting and emptying the basin ; and,
further, the loss of '0004 grm. would be unimportant in its
effects upon the results of technical analyses.

The proposal of Riidorff to add sodium acetate to the
electrolyte, instead of breaking the circuit in order to carry
out the washing, is likewise a useful one.

The deposit of copper obtained from a solution con-
taining free nitric acid possesses a bright red colour, and is
of crystalline structure. If the current be passed for a
considerable time through such a solution, a portion of the
nitric acid will be itself decomposed with formation of
ammonia ; this ammonia neutralises a further portion of
the free acid, so that the amount of free acid present
diminishes as the electrolysis proceeds.

The separation of copper from its solution containing
free nitric acid can also be effected by allowing a weak
current to act for a longer period of time. Thus, the
solution of which details have been given above may be
left overnight with a current of from '2 to '3 ampere
passing through it ; the deposition will be complete by the
next morning, and no heating of the solution prior to the
commencement of the electrolysis will have been necessary.

In this case it is, however, requisite to increase the
amount of free nitric acid added to the solution ; an

96 THE ELECTROLYTIC PROCEDURE

addition of 10 c.cms. for a volume of 150 c.cms. of the
solution suffices. In the presence of insufficient free
acid, and of increase of the ammonia contents of the
solution, the copper is deposited in a brown spongy form,
which adheres so slightly to the electrode that, even on
washing, portions become loosened and are lost. Such bad-
coloured spongy deposits are not adapted for weighing,
and the results obtained with them are inaccurate.

The fact that it is possible to obtain satisfactory
deposits of copper from the solutions containing free nitric
acid, with current densities up to and over 3 amperes, is
worthy of notice.

In the same manner that copper may be separated
from solutions of its sulphate after addition of nitric
acid, it is possible to obtain useful deposits from solutions
containing free sulphuric acid. 1 The amount of acid
present must, however, not exceed 8 to 10 per cent.

In order to carry out this method, 1 grm. of copper sul-
phate is dissolved in about 150 c.cms. of water, 2 to 3 c.cms.
of cone, sulphuric acid (or a corresponding amount of dilute
acid) are added, and the solution is then electrolysed at the
normal temperature with a current density of 1 ampere. In
the course of one and a half or two hours the whole of the
copper will be separated as a red deposit. When the
amount of copper is considerable, this deposit is never so
brilliant in colour as the deposit obtained from nitric acid
solutions, but it nevertheless yields exact results.

The E.M.F. required for the electrolysis of sulphuric
acid solutions is from 2 '5 to 3 volts.

It is possible by warming the solution bo lessen the
time required for this electrolysis. This diminishes the
resistance of the electrolyte, and if the current con-
ditions remain unaltered in the external circuit, the
current will consequently increase, and with it the amount
of deposit, in a definite time. When using solutions con-

1 Luckow, Dingl. polyt. Jour. 1865, 177 ; Gibbs, Zcitschr. f. anal.
Chem. 3, 334.

DEPOSITION FROM PUEE SALT SOLUTIONS 97

taining free sulphuric acid in the proportion described
above, it is not wise to allow the current density to exceed
1'5 amperes per 100 sq. centimetres, because with higher cur-
rent densities, in spite of the free acid present, the deposit
has a tendency to separate in a less compact and some-
what spongy form. In order to gauge the end of the
electrolysis, the same reactions are made use of as with
nitric acid solutions.

The same precautions with regard to stopping the
current and washing the deposit must also be observed.
The deposition of copper from a sulphuric acid solution
was the first example of the use of electrolysis for technical
analysis.

Gibbs, in the year 1864, determined the copper in
copper-nickel coinage by this method, and in 1865 Luckow
made use of this same process for the determination of
copper in the metal used for the fireboxes of locomo-
tives.

One would expect that copper could also be obtained
as a beautiful deposit from the neutral sulphate solution,
since in this case the electrolyte gradually becomes acid
owing to the formation of sulphuric acid at the anode.
This assumption is found to be correct ; but the resistance
of the neutral electrolyte is so great that an E.M.F. of 6
volts is requisite at the commencement in order to obtain
a current strength of '5 ampere. Heating this electrolyte
does not much increase its conductivity.

For practical work, it is therefore better to lessen the
resistance by the addition of a few cubic centimetres of
nitric or sulphuric acid.

Luckow and Drossbach have made attempts to elec-
trolyse copper sulphate solutions to which excess of ammo-
nia had been added ; but bright deposits could only be ob-
tained when very feeble currents were employed. Oettel 1 and
MacCay 2 found, however, that the conditions were much

1 Chem. Zeitg. 1894, 879 ; Zeitschr.f. EkUtrocliem. 1894, 142.

2 Chem. Zeitg. 1890, 509,

98 THE ELECTROLYTIC PROCEDURE

improved by the addition of definite amounts of ammonium
nitrate to the solution.

In order to carry out this electrolysis 1 grm. of copper sul-
phate and 4 grms. of ammonium nitrate should be dissolved
in water, and ammonium hydrate should be added until it is
present in slight excess. The deep blue solution is diluted
until it occupies about 150c.cms., and it is then electrolysed
at the normal temperature with a current of from -1 to -3
ampere. The E.M.F at the commencement of the electro-
lysis is about 2 volts ; during its course, however, it rises to
about 3 volts. At the end of a period of from six to seven
hours the deposition is complete, the copper being obtained
as a brilliant metallic coat. If the amount of ammonium
nitrate be decreased, or if that of ammonium hydrate be
increased, there is some danger of the copper separating in
the brown and spongy form. This method can be employed
for effecting copper separations during the night.

While copper can be separated in bright metallic form
from solutions of the sulphate and nitrate containing free
sulphuric or nitric acids, experiments with cupric chloride
and hydrochloric acid have shown that the solution of this
salt is not suited for analytical purposes, since the copper
has a tendency to separate from it in the spongy form.

It may be noted here that it is necessary to avoid as
far as possible the use of chlorides for electrolytic work if
metal electrodes are to be employed, as the free chlorine
produced by the electrolysis has always some action upon
them.

Kiidorff ] has nevertheless found that a useful deposit
can be obtained from solutions of the chloride to which
ammonium nitrate and excess of ammonium hydrate have
been added. To obtain a deposit from such a solution, -5
to 1 '0 grm. of cupric chloride and 4 to 5 grms. of ammo-
nium nitrate are dissolved in 100 to 125 c.cms. of water, and
25 to 30 c.cms. of ammonium hydrate are added. This solu-
tion is then electrolysed at the normal temperature with a
1 Berichte, 21, 3050.

DEPOSITION FROM PUEE SALT SOLUTIONS 99

current density of about 1 ampere ; the E.M.F. required is
from 3-3 to 3-6 volts.

To effect the separation of the copper contained in 1 grm.
of cupric chloride, from two and a half to three hours are
necessary. During the electrolysis the temperature of the
solution is found to increase slightly, and it is therefore
better to carry out the deposition at a temperature of
50 C. by means of a feebler current. The deposit, which is
at first a brilliant red, becomes later dull. In spite of this
change in its character, it is, however, not spongy, and
it is well suited to analytical requirements.

The employment of copper salt solutions to which an
excess of potassium cyanide had been added dates back
very many years, since in 1840 Ruolz had suggested the
use of this form of solution for electrotyping purposes.
Later it was suggested by Luckow l and Moore 2 for ana-
lytical work.

The solution is prepared by dissolving 1 grm. of copper
sulphate in a small amount of water, and by adding suffi-
cient potassium cyanide solution 3 to this, to cause the
re-solution of the precipitate of cupric cyanide which is at
first formed.

The solution is then made up to 125 to 150 c.cms. by the
addition of water. The double cyanide of potassium and
copper which is present in this solution requires at least
2-2 volts for its decomposition. If a current of 1 ampere
be passed through it at the normal temperature, an E.M.F.
of from 5-2 to 5 -8 volts will be found at the terminals ;
if, however, the electrolyte be heated to 60 C., the same
current density at the kathode can be attained with an
E.M.F. of 4-2 volts. The copper contained in 1 grm of
copper sulphate will be completely deposited in an hour and
a half under these conditions. The deposit obtained from

1 Zeitschr. f. anal Chem. 19, 1.

2 Chem. News, 1886, 53, 209.

3 The purer the potassium cyanide the better the results. The
commercial article is too impure to be used for this purpose.

100 THE ELECTROLYTIC PROCEDURE

cyanide solutions of copper does not possess the crystalline
structure characteristic of the deposits obtained from nitric
acid solutions, but is a pale rose-coloured homogeneous
coating.

The whole of the copper will be deposited from a solution
heated to 60 C. in two hours, if a current of \ ampere
should be used in place of the 1 ampere named above.

This method of deposition from cyanide solutions is also
adapted for use with still feebler currents ; and it is also
useful when circumstances prevent close and constant
attention to the course of the electrolysis.

The use of the double oxalate salt of copper and
ammonia for electrolytic separations of the copper has been
discussed by Classen and von Reiss l and by Classen and
Bongartz. 2 It has been found that better results are
obtained by use of this salt in solutions made acid with
oxalic acid than in the originally used neutral solutions.

In order to carry out this electrolysis, 1 grm. of copper
sulphate and 4 grms. of ammonium oxalate are dissolved
separately in water. On mixing these two solutions, the
precipitate which first forms is seen to redissolve. The
solution is heated to between 50 and 60 C., and a current of
l to 1 ampere intensity is passed through it, after first adding
some few cubic centimetres of a saturated solution of oxalic
acid, sufficient to cause the electrolyte to give a distinctly
acid reaction with litmus paper. In the absence of this
addition of oxalic acid, the deposit obtained quickly changes
to a brown and spongy form ; whilst if too great an excess of
oxalic acid be used, an insoluble form of copper oxalate
separates at the kathode. It is on this account advisable
to add the oxalic acid in small amounts only, from time to
time, during the electrolysis. The oxalic acid is decomposed
by the current with formation of carbon dioxide. A current
of 1 ampere will deposit the copper contained in 1 grm. of
copper sulphate from this electrolyte at a temperature of
50 to 60 C. in two hours, the E.M.F. necessary being from
1 Berichte, 14, 1627. * Ibid. 21, 2898.

DEPOSITION FROM PUEE SALT SOLUTIONS 101

2-8 to 3'2 volts ; while with a current of half this intensity
the deposition would occupy two and a half hours, and the
E.M.F. required would fall to between 2'5 and 2-8 volts. This
method gives bright metallic deposits (the contrary assertions
on this point are incorrect) when exact attention is paid to
the necessary precautions, especially to those concerning the
addition of the acid ; that is to say, continuous and close
attention is required in order to obtain good results by its
use. Since, however, there are other methods which are
simpler and more conveniently carried out, which always
yield reliable results, this method with double oxalates
cannot be recommended.

Smith l has proposed the use of solutions containing
phosphate of soda and free phosphoric acid.

Heydenreich 2 has, however, pointed out that the deposi-
tion from such solutions occupies a very long time (seventeen
hours), and that the deposits are not bright. The E.M.F. re-
quired is from 2 '4 to 3 volts. Brand 3 has examined into the
effects produced by the use of the alkaline pyrophosphates.
The deposits obtained are fawn-coloured and dull, and the
time required is excessive.

Other solutions that have been recommended are those
containing sodium acetate with free acetic acid, 4 and the
tartrates of the alkali metals and of ammonium. 5

The latter methods offer no advantages over those first
described. The methods which are actually employed in
technical laboratories as substitutes for the ordinary
gravimetric or volumetric processes of analysis arej
exclusively those in which free nitric or sulphuric acid
used. When separations of metals have to be effected, th(
methods with potassium cyanide and excess of ammonia i
also used.

1 Amer. Chem. Jour. 12, 329.

2 Berichte, 1896, 1585 ; Zeitschr. f. Elektrochem. 1896, 151.
8 Zeitschr. f. anal. Cliem. 28, 581.

4 Warwick, in Zeitschr. f. anorg. Chem. 1, 285.

5 Smith and others, in Jour. Anal, and Appl. Chem. 5, 488 ;
7, 189, 252.

102 THE ELECTROLYTIC PROCEDURE

With regard to the accuracy of the electrolytic
methods for copper determination, it is proved that these
yield results equal to those of the best purely chemical
processes of analysis. Long practical experience is not
required in order to obtain accuracy with these electrolytic
methods.

It is customary to obtain the weight of the deposits to
the fourth decimal place, and a difference of "001 grm.
between successive determinations may be regarded as the
maximum of the deviation that ought to occur.

IRON

Iron, in contrast to copper and the noble metals, belongs
to that group of metals which cannot be separated from
moderately acid solutions by the current or E.M.F. which
it is customary to have at one's disposal for analytical
purposes.

Since iron is precipitated from its solutions as hydroxide
by ammonia and the alkaline hydrates, the use of these
reagents for preparing the iron solution for electrolysis is
also excluded. The choice is therefore restricted to the
neutral salts, the double salts, and some complex substances.
It has, however, been proved that complete deposition is
not possible when neutral salts are electrolysed, and these
are therefore unsuited for analytical purposes.

One of the best methods for the electrolytic separa-
tion of iron is that suggested by Parrodi and Mascazzini ]
and by Classen and von Reiss. 2 This depends upon the
decomposition of the double oxalate of iron and am-
monium.

In order to carry out this method, 1 grm. of ferrous
sulphate or of ferrous ammonium sulphate is dissolved in a
small quantity of water, and at the same time 5 to 6 grms. of
ammonium oxalate are dissolved in about TOO c.cms. of water
with the aid of gentle heat. The two solutions are mixed

1 Gazz. Chem. Ital. 1879, B. 8.

2 Berichte, 1881, 14, 1622, 2771.

DEPOSITION FROM PURE SALT SOLUTIONS 103

by pouring the iron salt solution into the other, and the
mixture is stirred until the first-formed precipitate has re-
dissolved. If one proceeds in the reverse manner a ferrous
oxalate will be precipitated, the re-solution of which is
difficult.

The clear solution, which should measure about 150 c.cms.,
is now electrolysed with a current density of from 1 '0 to 1 '5
amperes, and with an E.M.F. of from 3 -5 to 4 '5 volts. The
electrolysis can either be performed at the normal tempera-
ture or at 50 C. ; in the latter case the rate of deposition
is increased. The length of time necessary to effect the
deposition of the iron contained in the amount of salt named
above will be about four hours with a 1 -ampere current,
and between two and a half and three hours with a current
of from \\ up to 2 amperes.

It is also possible to carry out this electrolysis by means
of a weak current of from -3 to '5 ampere strength, and in this
case the separation can be effected during the night. It is
necessary, however, when the lower current strength is
adopted, to increase the amount of ammonium oxalate in
the solution ; and it is also necessary to increase the current
to a strength of at least 1 ampere at the end of the electro-
lysis, in order to effect the removal of the last traces of
iron from the electrolyte. The ammonium oxalate is
decomposed by the current, carbon dioxide is liberated
at the anode, ammonium carbonate is formed in the
solution, and the electrolyte becomes alkaline with separa-
tion of flakes of ferric hydrate. By use of a large amount
of the ammonium oxalate salt, one can to a very large
extent avoid this result.

If, however, in spite of this increase, ferric hydrate should
be formed, it is possible to bring it into solution again by the
addition of a small amount of oxalic acid to the electrolyte.
Since the decomposition of the ammonium oxalate occurs
more rapidly in hot solutions, it is always preferable to
carry out the electrolysis in cold solutions, with current
densities of from 1 to 1 -5 amperes.

104 THE ELECTKOLYTIC PEOCEDUEE

In order to ascertain if the whole of the iron has been
removed from the solution, a few drops are withdrawn by
means of a small pipette, and to this, after acidifying with
hydrochloric acid, a small portion of sulphocyanide of
potassium solution is added. A red coloration of the
mixture denotes the presence of iron, and occurs with
extremely small amounts of this metal. If it be found
that all the iron is deposited, the liquid is poured out of
the basin, or the jacket electrode is raised out of the solu-
tion, the deposit and its support are rinsed several times
with water, then with alcohol, and are finally dried in the
air bath at 100 C.

The deposit of iron should be of a bright steel- grey
colour.

Ferric salts may be treated in an exactly similar way,
but in the case of these it is unnecessary to take any pre-
cautions in mixing the iron and ammonium oxalate solu-
tions. The difference in colour between the complex double
oxalates of iron and the simple salts of iron are worthy of
notice. While the latter give for ferrous salts green
solutions, and for ferric salts solutions reddish brown in
colour, the ferrous double oxalate yields a red and the
ferric double oxalate a green solution.

When the ferric salts are electrolysed as double
oxalates, the colour of the solutions changes from green to
red, and the red gradually fades away to a complete
absence of any colour whatever. Ferric potassium sul-
phate (iron alum), Fe 2 (SO 4 ) 3 .K 2 SO 4 + 24H 2 O, or ferric
potassium oxalate, Fe 2 (C 2 O 4 ) <J 3K 2 C 2 O4 + 6H. 2 O, may be
used as ferric salts, or one may use the ordinary
hydrated ferric chloride. The chlorides may in fact be
safely used when employing this method of electio-
lysis of double oxalates ; but nitrates must be avoided,
since their use almost invariably occasions a separation of
ferric hydrate. If therefore the nitrate salts are to be
analysed, it is best to convert them into sulphates by
heating with excess of sulphuric acid. The chief portion

DEPOSITION FROM PURE SALT SOLUTIONS 105

of the excess must be removed by evaporation, and the
remainder is then neutralised by ammonia.

For the electrolysis of iron, besides the double oxalate
salts, solutions containing citrates and tartrates of the
alkali metals have been recommended and used by Smith. 1
A solution containing 1 grm. of ferrous sulphate is treated
with 2 to 3 grms. of ammonium citrate and a small amount of
citric acid, and is then electrolysed with a current of from '7
to 1 ampere in density. 2 Tt is possible to obtain bright de-
posits of iron in this way, but on dissolving the deposit in
dilute sulphuric acid, particles of carbon will be detected ;
and this is especially the case when high current densities
have been employed for the deposition of the iron. As a
consequence of this, the results obtained are too high.

The separation of iron from this solution also occurs
very slowly, six to seven hours being necessary to deposit
the iron contained in 1 grm. of ferrous sulphate.

Solutions containing the tartrates of the alkali metals
behave similarly. For example, a complete separation of
the iron is possible from an ammonium tartrate solution, 3
but the deposit contains carbon. Luckow has recom-
mended the ammonium fluoride double salt. 4

Moore has proposed solutions containing sodium phos-
phate, r> Brand solutions containing pyrophosphates of the
alkali metals, 6 for use in obtaining deposits of iron. These
solutions demand a very high E.M.F., and much time for their
decomposition ; and the deposits obtained are not very good.

[Nicholson and Avery have suggested an improvement
of Classen's oxalate method by the addition of sodium
borate to the ferrous ammonium oxalate solution. 7
Translator's note.]

1 Amer. Ghent. Jour. 10, 330.

2 v. Miiller and Kiliani, Lehrbuch der Analyse.

3 Jour. Anal, and Appl. Chem. 1891, 5, 488.

4 Zeitschr. f. anal. Chem. 19, 1.

5 Chem. News, 1886, 53, 209.

* Zeitschr. f. anal. Chem. 28, 581.
' Jour. Amer. Chem. Soc. 18. 654.

106 THE ELECTROLYTIC PROCEDURE

The method which depends upon the use of the double
oxalate salts is the only simple and safe one to employ for
the analytical electrolysis of solutions of iron. This method
has been used in determining the atomic weight of iron.
For technical purposes the electrolytic determination of iron
is of little importance, since it is unlikely to be used in
place of the volumetric method with permanganate of
potash. It may, however, be employed in standardising
such solutions of permanganate.

NICKEL

Nickel, like iron, is one of that group of metals which are
not separated by the customary currents of from 1 to 2
amperes from strongly acid solutions. The separation
from neutral salt solutions is only incomplete. Luckow,
however, states l that this drawback is avoided by the
addition of a small quantity of acetic acid, and Riche
states <2 that the same effect is produced by other acids.
The method proposed by Gibbs, 3 and by Fresenius and
Bergmann, 4 depends upon the use of a solution containing
free ammonia and ammonium sulphate, and this has proved
to be the most convenient and neat. Either the sulphate
or chloride of nickel may be employed. A solution of
1 grm. of nickel sulphate in a little water is prepared, and to
this a solution of from 5 to 10 grms. of ammonium sulphate is
added, together with 30 to 40 c.cms. of ammonium hydrate.
This solution is electrolysed at the normal temperature,
with a current density of '5 to 1*5 amperes and an E.M.F. of
2-8 to 3-3 volts. The deposition will be complete in about
two hours. If the solution be heated to 50 to 60 C. the
deposition can be effected in fifty to sixty minutes, by use
of a current density of 1'5 amperes and an E.M.F. of 3'4 to
3-8 volts. The deposit obtained is bright and shining, and
resembles in appearance rolled platinum ; and it is to some
extent proof against the action of dilute acids.

1 Zeitschr.f. anal Chem. 1880, 19, 1. 2 Ibid. 21, 11(5.

3 Ibid. 1864, 3, 334. 4 Ibid. 1880, 19, 320.

DEPOSITION FEOM PUKE SALT SOLUTIONS 107

This method can be employed for solutions containing a
larger amount of nickel than that given above. Care must,
however, be given to the maintenance of an excess of
ammonium hydrate in such cases, as when this is not
present, a coating of nickel, bad in colour, is obtained, and
a crusting of black nickel oxide forms upon the anode. Too
great an excess of ammonia retards the deposition. The
metal separated from these solutions is silver-grey in colour,
and adheres firmly to the kathode. Winkler l states that
large amounts of nickel may be deposited in this way. The
last traces of nickel, as in the case of iron, are difficult to
separate from the solution. On this account it is not
advisable to work with currents of less than 1 ampere in
density, or, in case such weaker currents have been used, it is
necessary to increase the density to 1 ampere towards the end
of the electrolysis, and to allow this current to pass through
the solution for a period of from fifteen to thirty minutes.

In order to satisfy oneself that all the nickel has been
deposited, a few drops of electrolyte are tested by means of
an ammonium sulphide or sodium sulphide solution. The
formation of a brown colouring in the mixture is proof of
the presence of nickel. Potassium sulphocarbonate may
also be employed for this purpose ; in this case the presence
of nickel causes a rose-red coloration to appear. When it
has been proved that all the nickel is deposited, the removal
of the electrolyte and the washing of the deposit are effected
without breaking the circuit in the manner described under
Copper. The electrode and its deposit are then washed
with water and alcohol, and dried at 100 C. As already
remarked, the metallic coating of nickel obtained in this
way possesses the noteworthy property of being but slowly
attacked by sulphuric or hydrochloric acids. On this account
it is best to use nitric acid for removal of the deposit from
the electrode. This method for the separation of nickel
always gives reliable and good results. The nitrate salt
interferes with the satisfactory course of the electrolysis,
1 Zeitschr. /. anorg. Chem. 1894, 8.

108 THE ELECTEOLYTIC PROCEDURE

so that it is necessary, when this salt is to be analysed,
to convert it into the sulphate by means of sulphuric acid.

It has frequently been asserted that the presence of
chlorides as, for example, ammonium chloride has also a
disadvantageous influence upon the separation of nickel.
Oettel has contradicted this, 1 and has shown that useful
deposits of nickel may be obtained from chloride solutions,
when attention is given to the following points. The nickel
chloride solution must be strongly alkaline ; at least 10 per
cent, of ammonium hydrate (sp. gr. *92) being required
in the solution to prevent the separation of the black nickel
oxide at the anode.

In addition to this, at least sufficient ammonium chloride
must be present to form the double chloride of nickel and
ammonium ; a larger amount is not injurious, but such an
addition of ammonium chloride only compensates for a
shortness of ammonium hydrate when high current densities
are employed. Insufficiency of ammonium hydrate in-
creases the time required for the separation, and also the
danger that the nickel may be partly separated as oxide.
In order to carry out an electrolysis by this method, 1 grm.
nickel chloride and 2 to 4 grms. ammonium chloride are
dissolved in 100 c.cms. water, 40 c.cms. ammonium hydrate
are added, and the solution is electrolysed at the normal
temperature with a current density of -5 ampere. The
deposition of the nickel is complete in four to five hours.

Larger amounts of nickel can also be obtained as firmly
adhering deposits by use of this solution. Thus 1 grm.
nickel may be completely deposited under the above con-
ditions in six to seven hours, or with a current density of '1 1
ampere in fourteen hours. When using a flat kathode of sheet
metal, Oettel recommends the use of a fork-shaped anode,
in order to obtain an equal current density upon the two
sides of the kathode, which is fixed in the centre of the
anode. Although this arrangement was originally sug-
gested for nickel deposition, it is one which can be recom-
1 Zeitschr.f. Elektrocliem. 1894, 1, 194.

DEPOSITION FROM PURE SALT SOLUTIONS 109

mended for adoption in all cases in which a metal has to be
deposited upon a flat electrode. Nitrates exert a disturbing
influence upon the course of this electrolysis.

Good deposits of nickel are also easily obtained by use
of solutions of the double oxalates of nickel and am-
monium, recommended by Classen and v. Reiss, 1 and also
by Classen. 2

The solution is prepared similarly to that of iron, by
dissolving 1 grm. nickel sulphate in water, adding a solu-
tion of 5 to 6 grms. ammonium oxalate, and by diluting this
mixture until it measures 150 c.cms.

The solution is then electrolysed with a current density
of 1 ampere. If the solution be heated to 50 or 60 C. the
E.M.F. required will be 2'8to 3'3 volts, and the deposition
will occupy about four hours ; if the separation be effected at
the normal temperature, the E.M.F. will be increased to
between 3'5 and 4*2 volts, the time to five or six hours.
Equally good deposits can be obtained by the employment of
weaker currents ; but it will be necessary to increase these
to 1 ampere or higher towards the end of the electrolysis, in
order to effect the separation of the last traces of the metal.

The nickel obtained in this way is bright and steel- grey
in colour, with a reddish tinge. By this method it is un-
necessary to wash out the electrolyte before breaking the
circuit.

In the same manner that solutions of the neutral nickel
salts to which oxalates of the alkali metals have been
added are used for obtaining useful deposits of nickel,
solutions of neutral nickel salts containing tartrates,
citrates, and acetates of the alkali metals have been
proposed by Luckow, 3 Wrightson, 4 Ohl, 5 Schweder, 6 and
Smith and Muhr. 7 Good metallic deposits are obtained ;

1 Beriche, 1881, 14, 1622.

2 Zeitschr.f. Elektrochem. 1894, 1, 280 ; Berichte, 27, 2072.
8 Dingl.polyt. Jour. 117, 225.

4 Zeitschr. f. anal. Chem. 15, 300.

5 Ibid. 18, 523. 6 Ibid. 16, 344.
Jour. Appl. Chem. 1891, 5, 488 ; 1893, 7, 189.

110 THE ELECTROLYTIC PROCEDURE

but when using the tartrates and citrates, there is the same
tendency, observed with iron, for carbon to be deposited
with the rnetal. Solutions containing an excess of
potassium cyanide may also be used for nickel deposition,
according to Ohl, 1 Schweder, 2 Wrightson, 3 and Luckow. 4
The author has obtained, however, under the most varying
condition, only dark-coloured non-adherent deposits from
this solution. Von Foregger 5 states that useful deposits of
nickel may be obtained from solutions of nickel salts to
which ammonium carbonate has been added. To prepare
such a solution 1 grm. nickel sulphate and about 15 grms,
ammonium carbonate are dissolved in 150 c.cms. water.
This is heated to 50 or 60 C., and a current of from
1-0 to 1-5 ampere indensity is used to decompose it, with an
E.M.F. of 3-5 to 4-0 volts.

The complete separation of the nickel in the usual
bright metallic form will be effected in 1^ hrs. Useful
deposits of nickel can be obtained from solutions that have
merely received an addition of 10 c.cms. of ammonium
hydrate, on electrolysing them at the normal temperature
with currents of from *1 to '5 ampere in density.

Similar results can be obtained from solutions containing
an excess of pyrophosphoric acid and ammonium carbonate.
A solution of nickel sulphate is treated with 25 c.cms.
ammonium hydrate and 25 c.cms. of a saturated solution of
sodium pyrophosphate ; this is then electrolysed, either at
the normal temperature or after heating. With a current
of from '3 to '5 ampere the deposition will occupy sixteen
hours, with -5 to *8 ampere, nine hours.

Stronger currents may also be employed.

The deposits obtained are good, but the deposition
takes place too slowly, and the solution has a tendency to
permit nickel oxide to separate at the anode.

[Nicholson and Avery have recommended the use of

1 Zeitschr.f. anal Chem. 17, 215.

2 Ibid. 16, 344. 3 Ibid. 15, 300.

4 Ibid. 19, 1. 5 Dissertation, 1896, Berne.

DEPOSITION FROM PURE SALT SOLUTIONS 111

solutions containing the metal as sulphate, with addition
of ammonium oxalate and sodium borate an improvement
of Classen's oxalate method. 1 Translator's note.]

The method which finds frequent and exclusive employ-
ment in technical laboratories is that first described, de-
pending upon the use of ammonium hydrate and ammonium
sulphate. Nevertheless, the methods in which ammonium
chloride, ammonium oxalate, and ammonium carbonate are
used may occasionally be employed with advantage.

COBALT

Cobalt so closely resembles nickel in most of its
properties, that those electrolytic methods which are found
useful for separating nickel are also applicable for cobalt. In
most cases the proposals made by various experimenters for
the deposition of nickel cover at the same time that of cobalt.

To carry out an electrolytic determination of cobalt,
1 grm. cobalt sulphate and 5 grms. ammonium sulphate are
dissolved in 100 to 120 c.cms. water, and 30 to 40 c.cms.
ammonium hydrate are added. This solution is then elec-
trolysed with a current density of from -5 to 1 -5 ampere,
either at the normal temperature or at 50 to 60 C.

The E.M.F. and time required for this deposition are
practically the same as in the case of nickel ; and
ammonium sulphide or potassium sulphocarbonate is like-
wise used to determine the completion of the deposition.
Nitrates produce a disturbing effect upon the electrolysis.
The remarks made under ' Nickel ' regarding the use of the
chloride with ammonium chloride are also correct for
cobalt ; but as cobalt is in general more difficult to separate
from its solutions than nickel, it is necessary to use a
weight of ammonium chloride equal at least to four times
that of the cobalt present ; and the ammonium hydrate
added must equal one -fifth of the total volume of the liquid.
The electrolysis requires less time the greater the pro-
portion of ammonium hydrate present in the electrolyte.
1 Jour. Amer. CJiem. Soc. 18, 654.

OF THE -.

GTT'-V tt

112 THE ELECTROLYTIC PROCEDURE

The solution is therefore made up as follows : 1 grm.
cobalt chloride and 5 grms. ammonium chloride are dis-
solved in water, and the solution, after addition of 30 c.cms.
ammonium hydrate, is made up to 150 c.cms. It is then
electrolysed with a current density of 1*5 amperes. The
time required to complete the separation is five to six hours ;
and as a general rule it is more difficult to remove the last
traces of cobalt from the solution than those of nickel.

The method with solutions of the double oxalates gives
equally good results with salts of cobalt as with those of
nickel. The colour of the deposit of cobalt is slightly dif-
ferent to that of the deposit of nickel ; but the metallic coat
is equally bright. The separation from a solution containing
1 grm. cobalt sulphate and 5 to 6 grms. ammonium oxalate
in 150 c.cms. water is complete in four hours, when the
electrolysis is conducted at a temperature of 50 to 60 C. ;
at the normal temperature six to seven hours are requisite
to complete the deposition. The E.M.F. in the former
case is from 3'2 to 3'8 volts ; in the latter, 3-8 to 4-2 volts.

When small currents have been used to effect the
separation, an increase of the current strength is absolutely
necessary in order to remove the last traces of the metal
from the solution.

The method with ammonium carbonate has also been
suggested by von Foregger for use with cobalt salts. l

Quantitatively exact results are obtained, but the metal
deposit is not so bright as that obtained when using the
first method described. The solution is prepared by dis-
solving 1 grm. cobalt sulphate and 15 grms. ammonium
carbonate in water, adding a few cubic centimetres of am-
monium hydrate, and making up to a volume of 150 c.cms.
by addition of water. This solution is then heated to 50
or 60 C., and is electrolysed with a current density of
1 ampere. The E.M.F. required is from 3-7 to 3-9 volts ; the
time varies between 2^ and 3J hours.

From solutions containing an excess of potassium
1 Dissertation, 1896, Berne.

DEPOSITION FROM PUKE SALT SOLUTIONS 113

cyanide a quantitative separation of cobalt is no more
possible than in the case of nickel, in many cases cobalt
oxide separating at the anode.

The method with the pyrophosphates of the alkali
metals gives the same results as with nickel.

A cobalt solution containing 3 grms. sodium pyrophos-
phate and 100 grms. ammonium hydrate requires seven
hours for the separation of '1 to -2 grm. of the metal, and is
therefore unfitted for practical use.

The method with ammonium sulphate and ammonium
hydrate is that which alone finds employment in actual
practice ; though, as regards the other methods, the remarks
made upon this point under ' Nickel J are also true for
cobalt.

If a solution contains both nickel and cobalt, all the
methods described above will lead to a simultaneous
separation of the two metals at the kathode.

ZINC,

Zinc belongs in accordance with its position in the
voltaic series of metals to the same group as iron, nickel,
and cobalt ; that is to say, the quantitative separation of
zinc from solutions containing more than a very small
percentage of free acid is not possible by the ordinary
current intensity.

In many respects zinc behaves similarly to the metals
whose separation has been already described, but it differs
from these in its tendency to separate in the spongy form.
Such spongy deposits are obtained especially by the electro-
lysis of neutral salt solutions ; and in order to avoid this
objectionable feature Reinhardt and Ihle have recommended
the addition of a small amount of acetic acid to the
electrolyte, 1 while Luckow (I.e.), Parrodi and Mascazzini, 2

1 Jour. f. prakt. Chem. 24, 195.

2 Gazz. chim. Ital. 1877, iv. v. 222 ; Berichte, 10, 1098.

114 THE ELECTEOLYTIC PEOCEDUEE

Riche, 1 Millot, 2 Reinhardt and Ihle, 3 and Riidorff 4 have
recommended the addition of sodium acetate for the same
purpose. Neither of these additions, however, effectually
prevents the occurrence of spongy deposits. It is necessary
to note here that the deposition of zinc upon the usual
platinum electrodes leads to unpleasant results ; for on dis-
solving the dried and weighed deposit of zinc a black
powdery coat remains, as a rule beneath the whole of the
deposit but at least about the edges of the electrode,
which is neither soluble in hot hydrochloric acid nor in hot
nitric acid.

Vortmann states that this black coat consists of finely
divided platinum, 5 and that a mere mechanical rubbing
with sand will remove it. This is of course an objection-
able treatment for the platinum electrode. On this
account it is customary to coat the electrode which is to
serve as kathode with some other metal before its use for
zinc depositions. Copper, silver, or tin is most generally
used for this purpose.

The procedure described under 'Copper,' in which
nitric acid is used in the electrolyte, is especially suitable
for obtaining such a coating of copper. The current is
only allowed to pass for a few minutes ; the electrode is
then washed, dried, and weighed. This coating of other
metal, used to protect the platinum of the electrode, may
lead to incorrect results during the after electrolysis of the
zinc salt, owing to minute drops of the electrolyte being
carried or spirted, by the bubbles of gas which escape from
the liquid, on to the portion of the coating which is not
immersed. The copper may in this way be oxidised ; the
other metals may even be dissolved. To guard against this
source of error, the coating upon the inner surface of the
basin, or upon the jacket electrode, is made only very slightly
higher than the level to which the electrolyte will reach.

1 Compt. rend. 85, 226 ; Zeitschr. f. anal. Chem. 17, 218.

2 Bull, de la Soc. Chim. 1882, 37, 339.

3 Jour. f. prakt. Chem. 24, 195.

4 Zeitschr. f. angew. Chem. 1892, 179. 5 Berichte, 24, 2753.

DEPOSITION FKOM PUKE SALT SOLUTIONS 115

This is most simply attained by measuring the volume
of the zinc solution of which the electrolysis is to be made,
and by using a volume of silver or copper solution 5 to 10 c.c.
greater.

In order to avoid the trouble involved in preparing
these coatings, nickel basins or jacket electrodes have been
used. These are of course subject to the action of the
acid when the zinc is dissolved.

The zinc deposit obtained from any solution is only
fitted for analytical work when it is of a pale greyish-blue
colour, and is firmly adherent to the kathode.

Dark deposits are to be regarded with distrust, while
deposits which are partly or wholly spongy in character
must be rejected. At the commencement of the electro-
lysis most solutions yield a deposit of the desired character,
but it is from a few only that absolutely trustworthy
deposits can be obtained when the electrolysis lasts for a
considerable period of time.

One of these latter is the solution of the double cyanide
of zinc and potassium, recommended by Luckow, 1 Beilstein
and Jawein, 2 and Millot. 3

The solution for electrolysis is prepared by dissolving
1 grm. zinc sulphate in a little water, and by adding to
this a solution of pure potassium cyanide in small portions
at a time, until the precipitate of zinc cyanide, which first
forms, has dissolved in the excess of potassium cyanide.

If other salts of zinc containing free acid be used, it is
necessary to neutralise this acid with sodium hydrate, or
to make the solution slightly alkaline before adding the
potassium cyanide.

The clear colourless solution of the double cyanide is
made up to 150 c.cms., and may be electrolysed with either
strong or weak currents, at the normal or at a higher
temperature. Under all conditions and without any
attention, it yields on electrolysis, homogeneous, pale blue

1 Zeitschr. f. anal Chem. 19, 1. 2 Berichte, 12, 446.

8 Bull, de la Soc. Chim. 1882, 37, 339.

116 THE ELECTROLYTIC PROCEDUEE

deposits of zinc which are firmly adherent to the
kathode.

At the normal temperature an E.M.F. of 5-8 volts
will be required for a current of *5-ampere density, but
this E.M.F. will fall during the electrolysis owing to the
heating of the electrolyte by the ^current. The deposition
will occupy between two and two and a half hours.

If the electrolysis be conducted at 50 C. } the E.M.F.
required is reduced to 5 volts and the time to two hours
for the above current density. It is possible to obtain
equally good deposits when using a current density of
1 ampere ; in this case, at a temperature of 50 "* to 60 C.,
all the zinc is deposited in from an hour and a half
to an hour and three-quarters with an E.M.F of from
5 to 5-2 volts.

This method with the double cyanide of potassium and
zinc is also adapted for obtaining depositions during the
night with weak currents.

The recognition of the complete deposition of the zinc
is attained by use of potassium ferrocyanide solution.

The presence of zinc is proved by the formation of a
white insoluble precipitate, insoluble in hydrochloric acid,
or a white cloudiness of ferrocyanide of zinc. In order to
apply this test, the small test-portion of the electrolyte
the withdrawal of which demands care, lest any should be
sucked into the mouth is treated with a few drops of hydro-
chloric acid, and after warming is mixed with potassium
ferrocyanide.

For some solutions which are already alkaline, or the
test-portion of which has been made so by the addition of
ammonia, sodium or ammonium sulphide solution may be
used to determine the presence of zinc. In. this case white
zinc sulphide is formed.

When the deposition has been proved to be complete,
the cone is removed from the solution, or the basin is
washed out before breaking the circuit, and the deposit of
zinc is washed with water and alcohol and dried in the air-

DEPOSITION FROM PURE SALT SOLUTIONS 117

bath at 100 to 1 10 C., as previously described. The deposit
should be of a pale greyish-blue colour.

Reinhardt and Ihle l and Classen and v. Reiss 2 have
recommended the use of the double oxalates for obtaining
useful deposits of zinc. In order to prepare such a solution
1 grm. zinc sulphate is dissolved in water, and to this is
added a solution containing 4 grms. ammonium oxalate.
The precipitate of zinc oxalate which first forms dissolves
in the excess of ammonium oxalate, which is present in
much greater amount than that requisite to form the
double salt. This neutral solution may be electrolysed at
the normal temperature with a current density of '5 ampere.
An E.M.F. of from 3-8 to 4-1 volts will be required, and
the deposition will be completed in about four hours.

With smaller amounts of zinc, and current densities
which do not exceed '5 ampere, it is possible to obtain
bright deposits ; but spongy deposits under these circum
stances may also occur.

Similar results are obtained by use of the potassium
zinc oxalate salt recommended by Reinhardt and Ihle, ' and
by v. Miller and Kiliani. 3 This solution is prepared by
dissolving 1 grm. zinc sulphate, 4 grms. potassium oxalate,
and 3 grms. potassium sulphate ; the best results are ob-
tained when it is electrolysed at the normal temperature
with a current density of *3 ampere. The E M.F. required
is from 3 -9 to 4-2 volts, and the time from three to four
hours. Classen has shown that in order to obtain with
certainty reliable deposits of zinc from the solutions of the
double oxalates, it is absolutely necessary to maintain the
electrolyte acid during the electrolysis, by means of the
addition of small amounts of free organic acids. 4 Either
oxalic, tartaric, or lactic acid may be used for this pur-
pose. It is most convenient to use a 5 per cent, solution
of tartaric acid, which is less readily decomposed than
oxalic acid ; 1 to 2 c cms. of this solution are added to the

1 Jour. /. prakt. Chem. 24, 195. 2 Berichte, 1881, 14, 1630.
3 Quant. Analyse. * Zeitschr.f. Elektrochem. 1894, 1, 280.

118 THE ELECTEOLYTIC PROCEDURE

ammonium zinc oxalate solution prepared as described
above, at the commencement of the electrolysis.

During the deposition of the zinc small amounts continue
to be added, and tests with litmus paper are made in order
to have proof that the electrolyte has been kept acid. Too
great an excess of acid delays the deposition. By use of
this method dense and bright deposits of zinc can be ob-
tained ; but the electrolysis demands some attention during
its course. The ammonium zinc oxalate solution is best
electrolysed at a temperature of 50 to 60 C. with '5 ampere ;
the E.M.F. required will be from 3-5 to 4-0 volts, and the
time two hours. When 1 ampere is used, the E.M.F. rises
to between 4'7 and 4'8 volts, and the time is reduced to
an hour and a half. The deposit obtained is pale bluish-
grey in colour. If the electrolyte has not been kept acid
during the electrolysis, grey zinc sponge is nearly always
formed.

When tartaric acid has been used as recommended
above, in order to maintain the electrolyte in the acid state
the test with potassium ferrocyanide is not applicable. It
is necessary to wash the electrolyte from the basin or cone
before breaking the circuit, when using this method with
the double oxalates.

Jordis has shown that good deposits can be obtained
wnen ammonium lactate and free lactic acid are used in
place of ammonium oxalate and oxalic or tartaric acid. 1
The solution is prepared by dissolving 1 grm. zinc sulphate,
2 grms. ammonium sulphate, and 6 grms. ammonium
lactate in 150 c.cms. water, and by adding to this solution
10 drops of lactic acid.

The electrolysis may be carried out with current densi-
ties varying between -5 and 1 ampere ; in the latter case,
with solutions heated to 50 to 60 C., the deposition
occupies an hour and a half, while the E.M.F. required
is from 3 '8 to 4 '5 volts.

The zinc deposit obtained is pale blue. It would be
1 Zeitschr. f. Elektrochem. 1895, 2, 656.

DEPOSITION FROM PURE SALT SOLUTIONS 119

erroneous to draw the conclusion, from the similarity which
in other respects exists between zinc, and cobalt or nickel,
that the solution of the double sulphate of ammonium and
zinc, to which excess of ammonium hydrate has been added,
will yield good deposits of zinc. A solution containing
1 grin, zinc sulphate and 6 grms. ammonium sulphate, with
a small addition of ammonium hydrate, will give bright de-
posits under certain conditions with weak currents. From
neutral solutions of zinc ammonium sulphate it is possible
to separate all the zinc contained in 1 grm. zinc sulphate
with a current density of from -3 to -5 ampere in an hour,
as a rule, in a useful form ; but occasionally the deposit is
spongy. The E.M.F. required varies between 3 and 4 volts.

The presence of chlorides or of ammonium hydrate in the
electrolyte increases, as a general rule, the tendency to form
spongy deposits.

The use of zinc sulphate solutions to which sodium or
ammonium acetate and free acetic or citric acid have
been added has been proposed by Riche, 1 Rudorff, 2 and
Parrodi and Mascazzini. 3 1 grm. zinc sulphate and 3 grms.
sodium acetate are dissolved in water in order to prepare
such a solution, and 20 drops of acetic acid are added.

In order to electrolyse this solution at the normal tem-
perature with a current of -5 ampere, an E.M.F. of from
5 -9 to 6 '3 volts is required.

The deposit, which is at first bright, becomes later
spongy in character ; if a current of only '2 to '3 ampere
be, however, employed, a useful deposit can be obtained ; the
time required will be about eight hours. If such a solution
be heated to 50 or 60 C., and a current of -5 ampere be
again used, a good deposit of zinc, bluish- white in colour,
can be obtained. The E.M.F. required will in this case be
reduced to about 5 volts, and the time required will be
between three-quarters and one hour.

1 Compt. rend. 85, 226 ; Zeitschr. f. anal Chem. 17, 208.

2 Zeitschr. f. angew. Chem. 1892, 179.

3 Gaza. chim. Ital. 1877, iv. v. 222 ; Berichte, 10, 1098.

120 THE ELECTROLYTIC PROCEDURE

Insufficiency of acetic acid increases the tendency to
form a spongy deposit ; it should therefore be added in
small portions at a time during the electrolysis. Excess of
this acid delays the deposition. It is necessary to wash
without breaking the circuit, when using this method. This
electrolysis yields precisely similar results when ammonium
acetate is used, in place of sodium acetate, with the zinc
sulphate solution. In order to prepare such a solution
1 grm. zinc sulphate is dissolved in water, and ammonium
hydrate is added until the precipitate which first forms
has redissolved. Acetic acid is now added until a feebly
acid reaction is produced. From this solution, heated to
50 or 60 C., the zinc can be wholly deposited in about an
hour by use of a current of '5 ampere. An E.M.F. of from
3*5 to 4 volts is required. The deposit is bright and firmly
adherent to the electrode.

Yortmann has recommended the use of zinc solutions
containing tartrates of the alkali metals. 1 The solution of
zinc sulphate is prepared in this case for electrolysis by
adding 5 to 6 grms. sodium potassium tartrate and 2 to 2^
grms. caustic soda, and it is then electrolysed at the normal
temperature with a current of from -40 to -70 ampere in
density. The complete deposition requires from two to
three hours ' } the character of the deposit obtained is satis-
factory.

If an excess of sodium hydrate solution be added to a
solution of a zinc salt, a solution of sodium zincate is
formed, which has also been recommended for electrolysis
by Millot 2 and Kiliani and v. Foregger. 3 This solution
does not give bright deposits in all cases.

Satisfactory results may be obtained if solutions of 1
grm. zinc sulphate and from 2'5 to 4'0 grms. caustic soda
are mixed, and, after dilution and heating to 50 to 60 C.,
are electrolysed with currents of from '70 to 1 *5 amperes

1 Monats.f. Chem. 1893, 14, 546.

2 Bull de la Soc. CUm. 1882, 37, 339.

3 Dissertation, 1896, Berne.

DEPOSITION FROM PURE SALT SOLUTIONS 121

in density. The E.M.F. required is from 3'9 to 4'5 volts ;
complete deposition is effected in two hours. Increase in
the amount of caustic soda improves the character of the
deposit.

Solutions containing sodium or ammonium chloride give,
under certain conditions, good deposits ; but on account of
the unpleasant nature of the gas liberated at the anode
chlorine they are but little used. Little use is also made
of a solution which contains only 10 c.cms. ammonium
hydrate in addition to the zinc salt.

In this latter case a current of from '10 to '30 ampere
in density requires five to six hours to complete the de
position of the zinc.

Other additions that have been suggested are am-
monium phosphate, by Moore, 1 and sodium pyrophosphate
and ammonium carbonate, by Brand. 2 Solutions prepared
with these salts seldom give good deposits. [Nicholson and
A very recommend the use of zinc solutions containing the
metal as sulphate with the addition of formic acid in
excess, and sodium hydrate. 3 Translator's noteJ]

The traces of zinc that remain in the electrolyte
towards the end of the electrolysis of zinc salt solutions are
very difficult to remove. In this respect zinc resembles
iron and nickel. In order to effect their deposition, it is
necessary towards the end of the electrolysis to increase
the current density if this be feasible, or otherwise to allow
the current to continue to pass through the electrolyte for
a fairly long period of time.

In technical laboratories the electrolytic methods of
determining zinc are little used ; this is especially true of
those laboratories in which a large number of zinc estima-
tions have to be made concurrently. This does not signify,
however, that in particular instances the electrolytic
methods are not the most suitable. When these methods

1 Chem. News, 1886, 53, 209.

2 Zeitschr. f. anal. Chem. 28, 581.
8 Jour. Amer. Chem. Soc. 18, 654.

122 THE ELECTROLYTIC PROCEDURE

are employed, that first described with the double cyanide
of zinc and potassium is the most to be recommended,
since it yields with certainty good deposits, and permits the
use of fairly strong currents.

CADMIUM

This metal is closely related to zinc, not only in its
electrical but in its chemical and other properties. Like
iron, nickel, and zinc, it cannot be deposited from strongly
acid solutions, and it especially resembles the latter metal
in the tendency that it exhibits, even more strongly than
zinc, to separate in a spongy or loose form. The presence
of from 1J to 2 per cent, mineral acid in the solution com-
pletely stops the deposition. Bright silvery white metallic
deposits can only be obtained with regularity from com-
paratively few solutions ; and the amount of the metal
which can be separated in this form is, further, very
limited.

For these reasons it is especially necessary, in the
electrolysis of cadmium salt solutions, to take great care
that the electrodes are perfectly clean, and also that the
form of the electrodes used for carrying out the electrolysis
is such that the current density will be practically the
same at all points of the kathode surface. The electrodes
are most satisfactorily cleaned by boiling with acids or by
immersion in fused potassium bisulphate.

One of the best deposits of cadmium is obtained by the
electrolysis of the double cyanide of cadmium and potas-
sium, as recommended by Beilstein and Jawein l and by
Wallace and Smith. 2

This electrolysis is carried out in the same manner as
that of the corresponding salt of zinc. '50 grm. cadmium
sulphate is dissolved in water, and to this solution pure
potassium cyanide solution is added until the first-formed
precipitate of cadmium cyanide has redissolved. The solu-
tion is then diluted to 150 c.cms., and is electrolysed at the

1 Berichte, 1879, 12, 446. - Ibid. 1892, 25, 779.

DEPOSITION FEOM PURE SALT SOLUTIONS 123

normal temperature with a current of '50 ampere. The
E.M.F. required will be from 4*75 to 5-0 volts, and the
time from six to seven hours. The presence of an excess of
potassium cyanide in the solution is advisable, in order to
lessen the tendency to form a spongy deposit.

In order to test whether the deposition is complete, sul-
phuretted hydrogen is used with a feebly acid test-portion of
the electrolyte. A yellow precipitate or a yellow colouring of
the solution indicates the presence of cadmium. When the
electrolyte contains cyanides, as in the above case, it is first
necessary to destroy these by boiling the test-portion with
excess of dilute sulphuric acid (under a draught-hood) ;
the solution is then nearly neutralised, and sulphuretted
hydrogen gas is passed through it.

Good deposits may also be sometimes obtained by use
of neutral salt solutions ; but it has been found that the
deposits are denser and show less tendency to a spongy
formation if some free acid be added to the liquid during
the electrolysis. Luckow l and Smith 2 have recommended
sulphuric, nitric, or acetic acid for this purpose ; while
Warwick 3 has recommended formic acid.

The solution of '30 grm. cadmium sulphate in 150 c.cms.
water receives an addition of from 1 to 2 c.cms. of dilute sul-
phuric acid, and after heating to 70 or 80C. is electrolysed
with a current of from '60 to 1-0 ampere. The E.M.F.
required to produce this current varies between 2 '5 and 5'0
volts according to the amount of free acid present the
deposition demands three hours. A silver- white deposit is
obtained.

Heydenreich 4 has stated that a solution of -30 grm. cad-
mium sulphate containing free acetic acid yields, with cur-
rents of from '10 to '40 ampere in density, a bright deposit
of a crystalline lamellar character, and not particularly

1 Zeitschr. f. anal. Chem. 19, 1.

2 Chem. Jour. 10, 330.

3 Zeitschr. f. anorg. Chem. 1, 285.

4 Zeitschr. f. Elektrochem. 1896, 3, 151.

124 THE ELECTROLYTIC PROCEDURE

adherent to the kathode. The E.M.F. required is from 4 -5
to 7'5 volts.

Solutions made alkaline with ammonium hydrate can-
not be recommended for use ; the metal separates in the
spongy form, even after addition of ammonium sulphate.

Classen and v. Reiss } found that good results could be
obtained by use of the double oxalate salt of cadmium and
ammonium. The whole of the metal can be deposited from
a solution containing '30 grm. cadmium sulphate and from
8 to 10 grms. ammonium oxalate, with a current of '60
ampere, in about two hours.

At a temperature of 50 to 70 C., the E.M.F. required
is from 2'7 to 3'4 volts. The deposit is bright and firmly
adherent to the electrode.

The addition of a little free acid to the electrolyte is
found (as in the case of zinc) to increase the density of the
deposited metal, and to lessen the tendency to sponge
formation even at higher current densities. In order to
carry out the electrolysis in this way, the solution of the
double salt, as described above, is again prepared, and
during the electrolysis of the hot solution a few cubic
centimetres of an oxalic acid solution are added from time
to time, so that the electrolyte is kept slightly acid.

Tartaric acid, which is more stable than oxalic acid,
may be used with equally good results.

The current density can, under these conditions, be
allowed to vary between '50 and 1'5 ampere without any
danger to the character of the deposit. The E.M.F.
required for a current of from "60 to '70 ampere at 70 C.
is between 2'7 and 3-2 volts, and the time about three hours
and a half ; while for 1 ampere the E.M.F. is between 2*75
and 3-3 volts, and the duration of the electrolysis about three
hours. When tartaric acid has been used to acidify the elec-
trolyte, the E.M.F. required is slightly increased, being from
3-0 to 3'4 volts ; while the deposition takes place rather more
slowly, and on this account less close attention is necessary.
1 Berichte, 1881, 14, 1622.

DEPOSITION FKOM PUKE SALT SOLUTIONS 125

V. Miller and Kiliani have recommended the use of
solutions containing sodium acetate and free acetic acid
for cadmium depositions. 1

Such a solution is prepared by dissolving "50 grm.
cadmium sulphate and 3*0 grms. sodium acetate in water
and mixing the solutions. A little free acetic acid is
added to the mixture, and after diluting and heating to
50 C. it is electrolysed with a current of from "20 to '70
ampere. The complete separation of the metal from this
solution requires between five and eight hours. The current
densities which may be safely employed in this method are
too small for practical work \ and there is the further dis-
advantage, noticed by the authors, that the deposit has a
tendency to pass into the spongy form.

Moore has found that solutions of cadmium to which
sodium phosphate and phosphoric acid have been added,
yield non-metallic deposits unfitted for quantitative work. 2
If, however, the cadmium be precipitated from its solution
by means of a solution of sodium pyrophosphate, and the
precipitate be dissolved in an excess of ammonium hydrate,
the electrolysis of this solution with currents of from '10
to '30 ampere will yield useful deposits.

Warwick has recommended the use of the double salt of
cadmium and sodium or potassium formate ; 3 while Smith
and Moore have suggested the double tartrate. 4

These solutions, however, like those that have just
received notice, are not adapted for the requirements of
practical work.

Solutions of cadmium salts containing free ammonium
hydrate give in nearly every case spongy deposits.
Deposits which are not firmly adherent, and of a dead
silver- white colour, cannot be trusted to yield exact results,

The determination of cadmium is not of frequent

1 LcJirbuch dcr Analyse.

* Chem. News, 1886, 53, 209.

3 Zeitschr. f. anorg. Chem. 1, 285.

1 Jour. Anal, and Appl. Chem. 1893, 7, 189.

126 THE ELECTEOLYTIC PKOCEDUKE

occurrence in technical laboratories ; and up to the present
the electrolytic methods have not been used, because the
deposition has not been satisfactorily complete and the
amount of the metal which could be obtained in compact
and adherent form was too small. There is, however, no
longer cause for the exclusion of the electrolytic methods
for determining cadmium from the technical laboratory,
as the result obtained by use of the potassium cyanide
method, and also those obtained with the solutions con-
taining free sulphuric acid, or with the acidified solution
of the double oxalates, are perfectly reliable, and the
methods are easily carried out.

LEAD

Lead belongs to a group of metals, of which manganese,
silver, bismuth, and thallium are the other chief members.
These differ from the metals which have so far received
attention, in their property of separating from many
solutions in a non-metallic form.

This separation occurs as peroxide, or at least as a
higher oxide, at the anode. Frequently the metal
separates in the metallic form at the kathode concurrently
with its deposition as peroxide at the anode.

Manganese and lead, however, differ from the other
metals of the group in the ease with which it is possible to
obtain the deposition of all the metal present in the electro-
lyte, as peroxide at the anode. The methods proposed
for obtaining the quantitative separation of lead as the
metal are numberless. Some of these yield unsatisfactory
results, owing to the deposits of lead occurring not as
uniform firmly adnerent coats, but as growths of needle-
ike or lamellar structure, which extend out toward
the anode and cause short circuiting in the electrolytic
cell.

Other solutions e.g. those of the highly complex salts
which yield the lead as homogeneous and dense deposits
at the kathode, are nevertheless unfitted for use in the

DEPOSITION FROM PURE SALT SOLUTIONS 127

quantitative determination of this metal, because after
washing with water and alcohol some oxidation of the
lead coating occurs during the after drying, whether this
be conducted in the air-bath or in the desiccator. It
is found impossible to prevent this oxidation, and its
occurrence of course leads to incorrect results.

Solutions of the neutral lead salts yield deposits of
lead and lead peroxide. Deposits of metallic lead alone
may be obtained from the following : neutral lead acetate
solution, proposed by Luckow l and Kiliani ; 2 solutions
containing free acetic acid, proposed by Yortmann ; 3
solutions containing an addition of saturated sodium
chloride, suggested by Kiliani 2 and Becquerel ; 4 solutions
to which have been added excess of sodium hydrate,
recommended by Weil, 5 Kiliani, 2 Schiff, 6 Schucht, 7 and
Parrodi and Mascazzini ; 8 solutions containing an addition
of tartrates or acetates of the alkali metals or of ammonium
oxalate, proposed by Classen and v. Reiss ; 9 solutions to
which pyrophosphatea of the alkali metals have been
added, as suggested by Brand ; 10 and, in addition to these,
all solutions which suffer decomposition by means of easily
oxidised (reducing) bodies. In spite, however, of the com-
plete separation of the lead which is possible with the
above solutions, they are not in use for the quantitative
determination of lead.

The separation of lead as peroxide is quite as easily
effected as the separation as metal ; and this can rank with
the very best electrolytic methods in regard to its con-
venience and accuracy. Luckow pointed out, so long ago as
1865, that lead could be completely separated as peroxide

1 Zeitschr. f. anal. Chem. 19, 1.

2 Berg- u. Hiitten-Zeitg. 1883, 285.

3 Berichte, 24, 2758.

1 Compt. rend. 1854, No. 26; Dingl. polyt. Jour. 1854, 213.
5 Tommasi, Electrochemie. 6 Berichte, 10, 1098.

7 Zeitschr. f. anal. Chem. 1883, 22, 287.

8 Ibid. 16, 469. 9 Berichte, 14, 1627.
10 Zeitschr. f. anal. Chem. 28, 581.

128 THE ELECTROLYTIC PROCEDURE

from solutions containing free nitric acid, 1 if there were at
least 10 per cent, by volume of the free acid present in he
electrolyte. 2 In order to carry out such a separation, I grm.
lead nitrate is dissolved in a little water, from 20 to
30 c.cms. nitric acid are added, and the mixture is diluted
to 150 c.cms. The cell connections are then made, care
being taken that the electrode of greatest surface area i.e.
the basin or the jacket electrode is used as anode. It is
advantageous to employ a dulled, or at least a much-used
electrode for the separation of lead as peroxide, as the
deposit adheres more firmly to such than to a new and
perfectly smooth surface. The current densities employed
in this separation may rise to 2 amperes without injury
to the deposit. With a current of '50 ampere, at the normal
temperature, an E.M.F. of from 2'Oto 2'4 volts is requisite ;
and between two and two and a half hours suffice to effect the
complete separation of the lead as peroxide. If the solu-
tion be heated to 50 or 60 C., and a current density of 1*5
amperes be employed, the E.M.F. required will be from 2'1
to 2 '5 volts, and the time will be reduced to about an hour.
A higher temperature is not to be recommended, since the
adherence of the deposit to the electrode is unfavourably
affected by temperatures exceeding 60 C. If the amount
of nitric acid present has been too small, part of the lead
will be found to have separated as metal at the kathode.
The deposit of peroxide is golden-yellow or reddish in
colour when only small amounts of lead are present in the
solution ; the deposit is, however, dark brown or black, even
from the commencement, when larger amounts are present.
The deposit of lead peroxide obtained in this way is not
represented by the formula PbO 2 , but contains water. The
deposit cannot be reduced to the anhydrous condition by
drying at the usual temperature in the air-bath ; to effect
this it is necessary to dry at 180 to 200 C. The weight of
anhydrous peroxide found, multiplied by '866, will give the

1 Dingl. polyt. Jour. 1865, 177, 178.

2 Zeitschr.f. anal. Chem. 19, 1.

DEPOSITION FKOM PURE SALT SOLUTIONS 129

weight 'of metallic lead present in the salt used for the elec-
trolysis. The deposition of the lead as peroxide cannot
be effected from solutions containing chlorides.

The deposit is redissolved if the electrolysis be allowed
to continue for too great a period of time ; but in all
probability this only occurs when the amount of free acid
present is insufficient.

In order to test whether all the lead has been deposited
from the solution, the reaction with sodium sulphide or
sulphuretted hydrogen gas may be used.

The test with potassium bichromate is, however, more
sensitive and less troublesome. The test-portion of the
electrolyte is neutralised with ammonium hydrate, acidi-
fied with acetic acid, and then treated with a solution of
potassium bichromate. Mere traces of lead cause a cloudi-
ness, or a precipitation of yellow lead chromate.

When the electrolysis is completed, the acid liquid must
be washed out of the basin, if this has been used as anode,
before breaking the circuit.

The deposits of the metals which have hitherto been
dealt with are easily removed from the electrodes by means
of nitric acid. This method is useless for deposits of lead
peroxide. In order to effect the removal of these, one may
either use the dilute nitric acid solution to which oxalic
acid or potassium nitrite has been added, which has then
been heated, or one may use the same dilute nitric acid
solution with a strip of copper or zinc to form the second
element of a galvanic couple.

The latter is the simpler plan, and results in the rapid
solution of the deposited peroxide.

This method of determining lead as peroxide is fre-
quently used in technical laboratories ; it is not only sim-
pler and more accurate than the gravimetric methods of
determination, but it offers the further advantage that a
separation of lead from other metals is at the same time
effected. This employment of the method will, however,
receive a fuller notice under ' Separations' in Part III, C.

130 THE ELECTROLYTIC PROCEDURE

MANGANESE

This metal, which resembles iron very closely in its
chemical properties, behaves on electrolysis very differently
from iron, and much more resembles lead. As with the
latter metal, so manganese may be separated from certain
of its solutions by the current, in the form of metal ; from
others, as metal and peroxide ; while from others it may
be obtained in the form of peroxide alone.

To obtain deposits of the metal, Moore 1 and Smith and
Frankel 2 have recommended the use of solutions to which
potassium sulphocyanide has been added ; for both metal
and peroxide, Warwick has suggested the use of the
acetate ; 3 but the same deposits can be obtained at times
from neutral salt solutions or solutions containing a small
excess of nitric acid, the acid in the latter being converted
into ammonia by the action of the current.

Since metallic manganese decomposes water, these
proposals are of no value for the quantitative determination
of the metal, for the after washing and weighing of the
separated metal is quite impossible.

To obtain deposits of pure peroxide, Luckow has re-
commended neutral salt solutions 4 ; Riidorff 5 and Riche 6
have suggested neutral salt solutions to which dilute
sulphuric acid has been added ; Luckow, 7 Classen and von
Reiss, 8 Riche, 6 and Schucht 9 have proposed the same with
nitric acid in place of sulphuric acid ; Becquerel 10 and
Classen 1 1 the same, with acetic acid in place of the mineral
acids ; while Classen and von Reiss 8 have recommended the

1 Chem. News, 1886, 53, 209.

2 Chem. Zeitg. Eep. 1889, 13, 257.

Zeitschr. f. anorg. Chem. 1, 285.
4 Zeitschr. f. anal. Chem. 19, 1.

Zeitschr. f. angew. Chem. 1892, 3, 197.

6 Compt. rend. 85, 226. 7 Zeitschr. f. anal Chem. 8, 24.

8 Berichte, 14, 1626. 9 Zeitschr. f. anal Chem. 22, 492.

10 Anal Chim. phys. 1830, 43, 380.

11 Zeitschr. f. Elektrochem. 1894, 1, 280.

DEPOSITION FROM PURE SALT SOLUTIONS 131

double oxalate of potassium and manganese, and Brand l has
suggested the double pyrophosphate salt. Though it is
possible to obtain from all of these solutions especially
from those containing free acid deposits of manganese
peroxide, their use suffers from the disadvantage that only
very small amounts can be obtained in adherent form at
the anode ; about '15 grm. calculated as metal. To carry
out the electrolysis of one of these solutions, one may
dissolve '30 grm. manganese nitrate in water, and to this
solution add 2 c.cms. nitric acid. The mixture is then
diluted to 150 c.cms., and is electrolysed at a temperature
of 50 to 60 C. with a current density of '30 ampere. The
connections must be so made that the basin or the jacket
electrode functions as anode. In this case, as in that of
lead, a dulled electrode surface is most suitable for the
reception of the deposit. The E.M.F. required will be from
3'0 to 3 - 5 volts ; the deposition will demand about two hours.
If the amount of free acid present should exceed 3 per
cent., no peroxide is formed ; permanganic acid will be
produced instead. During the electrolysis the nitric acid
will be decomposed and partly converted into ammonia ;
on this account an addition of nitric acid must be made
during its course. The peroxide will be found not to
adhere very well to the electrode. In place of the nitrate,
one may use "30 grm. manganese sulphate, the solution of
which has been acidified with 10 drops of concentrated
sulphuric acid. At a temperature of 60 to 70 C., a current
of from '40 to '60 ampere will suffice to deposit all the man-
ganese from this solution in three and a half or four hours.
The E.M.F. required will be 4 volts. In this case there is no
necessity to add sulphuric acid during the course of the
electrolysis. This method gives better results than that
with nitric acid, but the deposit in this case is still un-
satisfactory as regards its adherence to the electrode.

Latterly acetic acid has been again recommended for
employment in place of the above two acids. In order to
1 Zeitschr.f. anal. Chem. 23, 581.

132 THE ELECTROLYTIC PROCEDURE

carry out an electrolysis with this acid, '30 grm. manganese
sulphate is dissolved in about 125 c.cms. water, and 25 c.cms.
60 per cent, acetic acid is added. The acid solution is
heated to 50 or 60 C., and is electrolysed with a current
of -30 ampere. The E.M.F. required under these con-
ditions will be from 4*3 to 4-9 volts, and the whole of the
manganese will be separated as peroxide in from two to
two and a half hours. The deposit is no better as regards
adherence to the electrode than that obtained from the
preceding solution.

The same results are obtained by use of a solution con-
taining excess of sodium pyrophosphate and free ammonium
hydrate. All the manganese will be deposited in about
two hours with an E.M.F. of 4*1 volts and a current of '30
ampere, but in this case the deposit is just as liable to
part from the electrode as in the previous examples.
Additions of free tartaric, oxalic, milk, or phosphoric acids
delay the deposition of the peroxide.

In order to obtain adherent deposits of larger amounts
of manganese peroxide, Engels has recently suggested a
method of aiding the separation by the addition of other
chemicals. 1

To a solution of 1 grm. manganese sulphate in water, a
solution of 10 grms. ammonium acetate and of 1*5 to 2 grms.
chrome alum is added ; the mixture is made up to 150
c.cms., and after heating to 80 C. it is electrolysed with a
current of from '50 to *60 ampere density. Under these
conditions the E.M.F. will be from 2-8 to 3'1 volts ; if the
current density be increased to 1-0 ampere, the E.M.F. will
rise to between 3'7 and 4-1 volts. The deposition will
require from an hour and a quarter to an hour and a half.

The addition of the chrome-alum solution gives to the
deposit of manganese peroxide at the anode a physical
character differing from that observed in deposits from
acid solution. The chief distinction is that, even in
comparatively large amounts, it is firmly adherent to the
1 Zeitschr.f. Elektrochem. 1895, 2, 410.

DEPOSITION FROM PURE SALT SOLUTIONS 133

anode. For still larger amounts of manganese than that
named above, the addition of chrome alum must also be
increased. Alcohol may be used as a substitute for
chrome alum. To prepare such a solution, -50 grm. man-
ganese sulphate and 10 grms. ammonium acetate are dis-
solved in water, the mixture is diluted to about 140 c.cms.,
and from 5 to 10 c.cms. alcohol are added.

The solution is heated to 70 to 80 C. and is electro-
lysed with a current density of 1 ampere.

The E.M.F. required under these conditions will be
from 4-0 to 4*2 volts, and the time about an hour and a
quarter.

In order to ascertain if all the manganese has been
separated from the solution, the best and most sensitive
test is that with lead peroxide. The reaction with am-
monium sulphide is not applicable. The small test- sample
of the electrolyte is heated with lead peroxide and a few
drops of concentrated nitric acid. A purple coloration,
due to the formation of permanganic acid, will be produced
if manganese be present in the solution in even the
smallest amounts.

The brown or blackish-brown deposit of manganese per-
oxide which has been obtained by these various methods
upon the anode is no better fitted for direct weighing after
drying than the deposit of lead peroxide, since it also
separates in a hydrated form.

Riidorff has stated that if the deposit of manganese
peroxide be first dried over sulphuric acid and then at
60 C., it will be found to possess the constant composition
represented by the formula Mn0 2 -f H 2 O.

Groger has, however, proved by the iodine method that
the constitution of the deposit dried under these conditions
is only approximately represented by this formula. 1 Clas-
sen has shown that, if the peroxide be converted into the
lower oxide (Mn 3 O 4 ) by ignition, a compound of constant
composition will be obtained, the weight of which multiplied
1 Zeitschr. f. angew. Chem. 1895, 253.

134 THE ELECTEOLYTIC PEOCEDUEE

by -720 will yield the weight of metallic manganese pre-
sent in the electrolyte.

The electrolytic method for the determination of man-
ganese, which until very recently suffered under the
disadvantage that only very small amounts of manganese
could be obtained as an adherent deposit of the peroxide,
was naturally not fitted to compete with the gravimetric or
volumetric processes for determining this metal.

The two electrolytic methods last described remove this
disadvantage, it is true ; but the volumetric process for
manganese determination is so simple that one can hardly
expect these improved electrolytic methods to replace it in
technical laboratories.

SILVER

Silver is classed as one of the noble metals, and there-
fore one can prophesy that it will be possible to deposit
it from solutions containing free acid. This prophecy is
found to be correct ; but the deposition of silver from such
solutions is, from a practical point of view, attended by
several objectionable features.

In the first place, silver, under certain conditions,
separates concurrently as metal at the kathode and as per-
oxide at the anode. A further difficulty is caused by the
character of the metallic deposit, which is compact, smooth,
and bright only in exceptional cases, unless extremely
feeble currents have been employed in electrolysing these
acid solutions.

The neutral salts yield flocculent bulky deposits of a
brown colour even with the feeblest currents and most
dilute solutions. Luckow states that similar deposits are
obtained from solutions to which ammonium hydrate and
ammonium carbonate have been added, but in this case silver
peroxide is deposited at the same time at the anode.

If free nitric acid be added to a solution of silver
nitrate, the electrolysis of this mixture will yield, at times,
adherent and bright deposits of the metal ; but quite as

DEPOSITION FROM PURE SALT SOLUTIONS 135

frequently greyish-brown non- adherent deposits will be
obtained, with peroxide formation at the anode. The
addition of lactic or tartaric acid prevents the peroxide
separation.

Fresenius and Bergmaim 1 have shown, however, that
even with this addition it is only possible to obtain useful
deposits from this solution with any degree of certainty,
when using very feeble currents and a very dilute elec-
trolyte.

On this account the time required to complete the
separation of the metal is very great. In order to carry
out such an electrolysis, a maximum of '50 grin, silver
nitrate or silver sulphate is dissolved in water, and after
addition of 5 to 6 c.cms. nitric acid the mixture is diluted
to 125 or 150 c.cms., and is electrolysed at a temperature of
50 to 60 C. with a current density of "04 to '05 ampere.
The separation will demand four to five hours.

The electrolysis with this solution may be carried out at
the normal temperature, if a current density of from *10 to
20 ampere be not exceeded. The E.M.R required in this
case will be about 2 volts. Higher current densities than
these, or insufficiency of nitric acid, cause the formation
of peroxide and of non-adherent deposits of the metal.

The well-known reaction with chlorides is made use of
to ascertain the completion of the electrolysis of the silver
salt. The acid liquid must be washed out of the basin before
breaking the circuit ; the deposit of silver must be dried at
100 C. The colour of deposits of silver obtained from
nitric acid solutions is white with a metallic lustre, and
much resembles that of platinum when the electrolysis has
been successfully carried out. Deposits that are a light
greyish-brown in colour are untrustworthy.

The method proposed by Luckow, 2 in which the silver
is deposited from the double cyanide salt of silver and
potassium, is much to be preferred to that described above.

1 Zeitschr. f. anal. Chem. 19, 316.
- Ibid. 1.

136 THE ELECTROLYTIC PROCEDURE

The solution for this electrolysis is prepared by dissolv-
ing amounts not exceeding 1 grm. in weight of silver nitrate
or silver sulphate in water, and by adding to this solution a
freshly prepared solution of pure potassium cyanide until
the precipitate of Ag(CN) 2 , which first formed, has dissolved
in the excess of the potassium cyanide. Rather more than
the exact amount necessary to obtain a clear solution is
added. From 2 to 3 grms. solid potassium cyanide will be
requisite. The solution is then diluted to 150 c.cms. It is
advisable to make use of the purest potassium cyanide that
can be obtained, since the use of the impure commercial
product leads to a less satisfactory deposit of silver at the
kathode. The current density employed with this solu-
tion may rise to 1 ampere without injury to the character
of the deposit. If feeble currents of from -20 to '30
ampere be employed to effect the deposition at the normal
temperature, the E.M.F. required will be between 3*3 and 3-5
volts, and the complete separation will demand four to five
hours ; if the currents be increased to '50 or '60 ampere,
an E.M.F. of from 4*0 to 4*6 volts will be requisite, and the
time will be reduced to from two to two and a half hours.
The electrolysis may also be carried out with a heated elec-
trolyte, without any danger to the character of the deposit.
Using a current density of 1 ampere and an E.M.F. of
5 '8 volts, a solution containing '50 grm. of the silver salt
heated to 60 C. will have all the silver deposited as a dead-
white coating upon the kathode in half an hour. The E.M.F.
required for such a heated solution when the current is
reduced to -60 ampere is only 4'8 volts. This potassium
cyanide method may also be used with extremely feeble
currents from '10 to '20 ampere at the normal tempera-
ture, and on this account it may be employed for performing
the electrolysis at night. The E.M.F. required in this case
is 3-3 volts.

The deposit obtained from the double cyanide solution
is of a dead silver- white colour, and therefore differs in
this respect from that obtained from the acid solutions.

DEPOSITION FKOM PUEE SALT

The deposit appears to be partly crystalline in structure,
but in spite of this it adheres firmly to the electrode. The
use of roughened or well-used electrodes as kathodes is
found to be advantageous. The dead -white appearance of
the deposit is at times found to give place to patches of
brown. This indicates that at these spots the current
density has been too great, a result that can easily arise
with certain forms of electrodes. When a basin electrode
has been employed, the liquid must be washed out before
breaking the circuit. This and the remaining washing and
drying operations are carried out exactly as described for
copper and the other metals of this group.

Krutwig has recommended an ammoniacal solution of
silver containing ammonium sulphate, for obtaining deposits
of this metal. 1 In order to prepare such a solution '50 grm.
silver nitrate or silver sulphate is dissolved in water,
and to this solution 25 c.cms. ammonium hydrate and a
solution of 6 grms. ammonium sulphate are added. This
mixture is heated, and, according to v. Miller and Kiliani, 2
should be electrolysed with a current density of from -02 to
'05 ampere. The E.M.F. required will be about 2*5 volts.
The results obtained by use of this method are uncertain.
Stronger current densities result always in the separation
of the silver in a loose flocculent form, greyish brown in
colour, at the kathode, a result which cannot be avoided by
lessening the amount of ammonium hydrate. The deposit
of metal also encloses ammonium sulphate, and for the
removal of this a very careful washing with water is
required. The method therefore cannot be recommended.

Other proposals emanate from Brand and from Smith.
The former has suggested the use of an ammoniacal sodium
pyrophosphate solution, 3 the latter the use of an ammo-
niacal phosphate solution. 4 Neither of these methods
yields satisfactory results.

Luckow has shown that in the case of silver it is

1 Berichte, 15, 1267. '-' Lehrbiich der Analyse.

3 Berichte, 28, 581. ' Amer. Chem. Jour. 181)0, 12, 329.

138 THE ELECTKOLYTIC PEOCEDUEE

possible to directly decompose insoluble salts such as the
chloride, bromide, and iodide by electrolysis, if these are
first covered with dilute acetic or sulphuric acid. l In order
to effect this decomposition, the finely ground insoluble
salt is placed at the bottom of a beaker in which a cone
electrode functions as kathode. On making the necessary
current connections, it will be found that the powder gra-
dually disappears from the bottom of the vessel. If the
basin electrode be used, this must be connected to act
as anode ; the deposit of silver will then be found upon
the smaller disc. . If strong currents be used, similar ob-
jectionable features will occur in relation to the deposition
of the silver as those noted in the case of the nitric acid
solution. It may therefore be regarded as more advan-
tageous to bring the insoluble halogen salt of silver into
solution by aid of potassium cyanide, and then to decom-
pose this double cyanide solution in the way already
described.

From the statements concerning the usefulness of the
different methods that have already been made in the
descriptions of them, it will have been gathered that only
one method fulfils the requirements demanded in a
satisfactory and reliable process for the electrolytic separa-
tion of silver namely, the potassium cyanide method.

Since in technical work it is extremely rarely that pure
solutions of silver are at one's disposal for analysis, it is not
at all probable that the electrolytic method will displace
the usual gravimetric or volumetric processes of analysis.

MERCURY

This metal, which differs from all others in its property
of being fluid at the normal temperature, electrolytically
considered, exhibits likewise distinctive characteristics. The
separation of mercury occurs in the form of tiny spherical
globules, which nevertheless adhere comparatively firmly
to the electrode surface.

1 Zeitschr. f. anal. Chem. 19, 1.

DEPOSITION FKOM PUKE SALT SOLUTIONS 139

These tiny spheres of the metal increase in size, and
ultimately run together to form small drops, when larger
amounts of mercury are deposited.

In the case of some of the solutions used for effecting
mercury depositions, the first form of the deposit lasts for
a longer period than in the case of other solutions, as, for
example, acid solutions ; and it is therefore possible with
these solutions to separate a larger amount of mercury as a
uniform coating of the metal upon the electrode.

A spongy formation of the deposit is, in the case of this
metal, perfectly impossible ; and it therefore follows that
most solutions of mercury yield, on electrolysis, deposits
which are perfectly satisfactory in character.

On account, however, of the tendency of the fluid metal
to collect into drops upon the bottom of the basin in which
the electrolysis is conducted, there is a practical limit to
the amount of mercury which may be separated. This
limit is about 2 grms. metal ; above this, the washing and
drying of the deposits is difficult or impossible. The jacket
electrodes are not so well adapted for this electrolysis as
the basin electrode. When the deposition is completed
the electrolyte is washed out before breaking the circuit,
the deposit upon the basin is washed several times with
water, and, the alcohol wash being omitted, it is finally
dried in the desiccator over sulphuric acid. If the metal
should have collected into larger drops, and so have rendered
it difficult to pour away the wash-water without loss of mer-
cury, it will be found most easy and safe to remove the re-
maining portions of the wash-water by means of filter paper.

In the case of mercury the use of alcohol must be
avoided, since it produces a grey dull skin upon the surface
of the metal.

A basin having a roughened or at least dulled inner
surface is recommended for this electrolysis.

Luckow 1 and Smith and Knerr 2 state that the neutral

1 Zeitschr.f. anal. Chem. 19, 1.

2 Amer. Cliem. Jour. 8, 206.

140 THE ELECTEOLYTIC PKOCEDUKE

salts the chlorides, sulphates, and nitrates in either the
mercurous or mercuric form, permit complete separation of
the metal ; but these neutral solutions are such bad con-
ductors that it is more advantageous to add to them 1 or 2
per cent, sulphuric or nitric acid, as recommended by Clarke, 1
Riidorfi^ Classen and Ludwig, 3 and Smith and Mover. 4
In order to prepare such a solution '50 grm. mercuric
chloride is dissolved in water, 1 to 2 c.cms. sulphuric acid are
added, and the mixture is then diluted to the usual volume.
This is now electrolysed at the normal temperature with a
current of from '60 to 1 '0 ampere. The E.M.F. required will
vary from 3'5 to 5 - volts, according to the amount of acid
present. The separation will be complete in two to two
and a half hours. The metal will be found in tiny bright and
silvery spherical globules, which, if the quantities named
above have been used, will adhere firmly to the walls of the
basin, although here and there they may have run somewhat
together to form larger spheres. In order to test whether the
electrolysis of the mercury salt is complete, sulphuretted
hydrogen gas is passed through the small test-portion of the
electrolyte, or a few drops of ammonium sulphide are added
to the same. The presence of mercury is marked by a
brownish tint in the mixture. The washing and drying of
the mercury is carried out as already described.

Similar results are obtained when nitric acid is used
in place of sulphuric acid. In this case the solution is
prepared by adding 3 c.cms. nitric acid to '50 grm. mercuric
chloride dissolved in water, and by diluting the mixture to
150 c.cms. This solution may be electrolysed with a current
density of 1 ampere at the normal temperature.

An E.M.F. of 3'6 to 4-0 volts will be required ; the time
will be between two and a half and three hours. The character
of the deposit resembles that obtained with sulphuric acid.

1 Amer. Jour, of Sc. and Art, 16, 200.

2 Zeitschr. f. angew. Chem. 1894, 388.

3 Berichte, 19, 324.

' Jour. Anal, and Appl. Chem. 1893, 7, 252.

DEPOSITION FEOM PURE SALT SOLUTIONS 141

Classen 1 and de la Escosura 2 have pointed out that
similar results can be obtained by use of a mercuric
chloride solution containing hydrochloric acid or sodium
chloride. This solution, however, on electrolysis produces
chlorine, and therefore its employment cannot be re-
commended.

It was noted under Silver that the deposits obtained
from the cyanide solution were dead white, as opposed to
the metallic lustre of those obtained from the acid
solutions. Smith and Frankel, 3 Smith and Cauley, 4 and
Smith and Wallace 5 have noted that mercury behaves in a
similar manner. If *50 grm. mercuric chloride be dissolved
in water, and 3 grm. pure potassium cyanide be added, a
clear solution will be formed. This may be diluted to
ISOc.cms. and electrolysed at the normal temperature with
a current density of from '50 to 1 -0 ampere. The deposit of
mercury obtained will closely resemble the dead-white silver
deposit described under Silver, and only few of the tiny
globules of the metal which compose it will run together.

The E.M.F. required will be between 5-5 and 6'0 volts,
and about an hour will suffice to deposit the whole of the
mercury. The results are equally satisfactory when the elec-
trolysis is performed at the normal temperature with a current
density of only -02 ampere ; in this case the complete separa-
tion of the metal from the solution demands twelve hours.
The cyanide solution may also be heated to 60 C. without
any injury to the character of the deposits, and under
these conditions the separation will be completed in a
shorter time than any mentioned above.

Mercury is often present as the sulphide in solutions
obtained in the course of analysis, and it is therefore of
some convenience that this metal can be quantitatively
separated by electrolysis from a sodium sulphide solution.

1 Electrolyse (Text-book).

2 Revista Minera, 1886 (Madrid).

3 Jour. Franklin Tnst. 127, 469.

4 Jour. Anal, and Appl. Chem. 1891, 5, 489.

5 Berichte, 1892, 779.

142 THE ELECTROLYTIC PROCEDURE

This method has been described by de la Escosura, Smith, 1
and Vortmann. 2

In order to carry out such a determination, '50 grm.
mercuric chloride is dissolved in water, and sulphuretted
hydrogen gas is passed through the solution until all the
mercury is precipitated. From 40 to 50 c.cms. of a saturated
solution of sodium sulphide and a small portion of sodium
hydrate are now added, and the clear solution which is thus
obtained is diluted to 150 c.cms. At the normal temperature
an E.M.F. of 3'5 to 4*0 volts is required to produce a current
density of 1 ampere : if heated to 50 to 60 C. an E.M.F. of
3 volts suffices for this ; in either case the separation of the
mercury occurs satisfactorily, and the time required is about
an hour. If a sufficiency of sodium sulphide has been used,
sulphur will separate at the anode ; if the reverse has been
the case, a dark-coloured deposit of mercuric sulphide
mixed with sulphur will be obtained there. Similar
deposits are obtained with antimony and tin, of which
further mention will be made later. The amount of sodium
sulphide named above is ample to hinder this result.
In order to judge when the separation is completed, the
small test- sample of the electrolyte is treated with a few
drops of acid. A brown coloration of the liquid indicates
the presence of mercury. The reaction with sulphuretted
hydrogen cannot be employed in the case of the above
solution. The character of the deposit of mercury at the
kathode closely resembles that of the deposit obtained from
the nitric acid solution.

In addition to the solutions that have already received
notice, Schmucker has recommended solutions containing
ammonium tartrate, 3 whilst Vortmann 4 and Classen 5 have
recommended the addition of ammonium oxalate. The use
of the former solution is attended by the usual disadvantages

Jour. Anal, and Appl. Chem. 1891, 5, 202.

Chem. Zeitg. 1881, 390.

Zeitschr. f. anorg. Chem. 5, 206.

Berichte, 24, 2750.

Zeitschr. f. Elektrochem. 1894, 1, 280.

DEPOSITION FROM PUEE SALT SOLUTIONS 143

of the tartrate methods ; the latter gives useful results. A
solution containing '50 grm. mercuric chloride is mixed with
one containing between 4 and 5 grms. ammonium oxalate,
and after dilution to 150 c.cms. the mixture is electrolysed
at the normal temperature with a current density of
1 ampere. The E.M.F. required will be between 4 and 4-6
volts ; the time from one and a half to two hours.

Solutions containing sodium pyrophosphate and am-
monium hydrate or ammonium carbonate only yield satis-
factory results with the mercuric salts. With a current
of '20 ampere the deposition occupies five hours.

Insoluble compounds of mercury may be directly
decomposed by the electrolytic method, as in the case of
the similar compounds of silver. An indication of the
possibility of this is given in the observation that may be
made in the course of some of the preceding experiments.
In the case of many solutions of mercuric chloride, this
latter is converted by the action of the current into
insoluble mercurous chloride, before it is completely
decomposed. The cloudy appearance of the electrolyte
due to mercurous chloride disappears again after some
interval of time. The procedure is very similar when
mercurous chloride is placed upon the bottom of a beaker,
covered with water containing hydrochloric acid or sodium
chloride, and electrolysed. Other mercury compounds
which may be decomposed in this way are mercuric
sulphide and cinnabar. This method has in fact been in
use for a considerable time at the mines in Almada, in
Spain, for determining the percentage of mercury in pieces of
pure cinnabar. It is only possible, however, to use very
feeble currents to effect the separation, and on this account
the electrolysis requires twelve to eighteen hours ; this is
too long a period for practical purposes.

In order to effect the removal of the deposited mercury
from the basin or from the conical electrode, nitric acid is
used ; and the solution is hastened by heating. It fre-
quently occurs, however, that round the top edge of the

144 THE ELECTROLYTIC PROCEDURE

deposit a dark-coloured band, insoluble in nitric acid, re-
mains upon the platinum. In these cases one may either
attempt to remove it by raising the basin to a red heat,
or by using the basin as anode with an electrolyte
containing nitric acid, and a stout copper wire as kathode.
This deposit can be removed by either of these methods, in
most cases very quickly.

It is noteworthy that the electrodes lose slightly in
weight with every successive mercury determination ; on
this account it is necessary to reweigh them after each
electrolysis of a mercury solution.

Since nearly all the methods described for the electro-
lytic determination of mercury yield equally good deposits
on the kathode, it follows that the selection of the best
method rests upon the time required for the deposition.
The nature of the solution, or the form in which the
mercury is obtained in the ordinary course of the analysis,
must, however, be permitted to exert some influence upon
the selection of the method. If the mercury is obtained in
an acid solution, it is of course most convenient to use
this directly for the deposition ; if mercuric sulphide is
obtained, the method with sodium sulphide is used. For
solutions of the neutral salts in the case of which any one
of the described methods is directly applicable, the
potassium cyanide method is to be preferred.

The electrolytic method for mercury determinations can
be advantageously used in the case of pure salts or their
solutions. It is not equally well suited for determining the
amount of the metal in the ores of mercury, because, as will
be pointed out later, the separation from several of the
other metals is difficult, or for technical purposes incon-
venient.

ANTIMONY

This metal exhibits electrolytic characteristics which
differ somewhat from those of any of the metals that have
hitherto been considered. Its position in the list of metals

DEPOSITION FROM PUEE SALT SOLUTIONS 145

given earlier would lead one to suppose that antimony can
be deposited from acid solutions ; this supposition is found
to be correct. Classen and V. Reiss (I.e.) have shown that
this metal may be deposited from solutions containing
hydrochloric acid ; while Gore and Sanderson have noted
that solutions containing ammonium or sodium chloride
are equally serviceable. 1 The deposits obtained in this
way are, however, not sufficiently adherent to the electrode
to be regarded as satisfactory ; and that obtained from the
hydrochloric acid solution is further unfitted for analytical
purposes on account of its explosive properties. Gore, and
Classen and V. Reiss, have recommended solutions contain-
ing oxalates of the alkali metals, but these yield metallic
deposits which are still less adherent than those obtained
from the first- named solutions.

The addition of ammonium pyrophosphate produces no
better results.

The electrolysis of a solution of antimonyl potassium
tartrate (tartar emetic), or of an antimony solution con-
taining tartaric acid, yields a deposit which is entirely
satisfactory ; the electrical resistances of these solutions
are however so great (as with all other solutions contain-
ing chiefly tartrates) that the separation takes place too
slowly for practical use.

The sulpho-salt of antimony, as proposed by Parrodi and
Mascazzini, 2 Luckow, 3 Classen and Y. Reiss, 4 Classen, 5 and
Classen and Ludwig, 6 is the best to use for obtaining
satisfactory antimony deposits. It is necessary to prepare
a saturated solution of sodium sulphide from the pure
crystallised salt, for use in this method. This solution
requires filtering before employment. A solution of 1 grm.
tartar emetic, in water, is treated with this sodium sul-
phide solution until the precipitate which first forms is

1 Berichte, Eef. 1891, 340.

' Oazz. chim. Ital. 8, 1879 ; Zeitschr. /. anal Chem. 18, 588.

3 Zeitschr. f. anal Chem. 1880, 19, 1.

4 Berichte, 1881, 14.

Ibid. 1884, 17, 2476. Ibid. 1885, 18, 1104.

146 THE ELECTROLYTIC PROCEDURE

redissolved. The mixture, after dilution to 150 c.cms., is
electrolysed at the normal temperature with a current
density of from '50 to 1 -0 ampere.

The E.M.F. required will be between 1-3 and 1-8 volts ;
the separation will demand from six to seven hours. It is
more advantageous to heat the electrolyte to 70 or 80 C.,
and to use a current density of between 1 '0 and 1 '5 amperes.
The E.M.F. required under these conditions will lie between
2 4 5 and 3'2 volts ; the time will be only one and a half hours.
In order to test whether any antimony still remains in the
electrolyte, the small test portion of the latter is heated
with a few drops of dilute sulphuric acid. If antimony be
present, the sulphur which separates owing to decomposi-
tion of some of the sodium sulphide will be reddish in
colour, by reason of the admixture of antimony sulphide.

One may also use the method of testing for the end of
the electrolysis, described fully under ' Copper,' dependent
upon raising the level of the electrolyte in the basin or
beaker by addition of water. This method is, however,
not very trustworthy, if the amount of remaining metal be
small. When the electrolysis is completed, the electrolyte
is washed out of the basin ^before breaking the circuit, and
the washing and drying of the deposit are carried out as
usual. The deposit of antimony obtained from these
sulphide solutions is bright, metallic, and silver grey in
colour ; and if a well-worn electrode has been used as the
kathode, it is extremely adherent to the platinum surface.

It is therefore advisable, if such an electrode be not at
hand, to artificially roughen one by means of a sand blast,
before carrying out electrolytic determinations of this metal.
While a deposit of metallic antimony of the character just
described is obtained at the kathode, the anode becomes
coated with a yellowish white deposit of sulphur, which may
easily be removed by rubbing. Equally good results are
obtained, when any other salt of antimony than that known
as tartar emetic is converted into the sulpho-salt by means
of sodium sulphide.

DEPOSITION FROM PURE SALT SOLUTIONS 147

The special advantage of this method apart from the
easy separation of antimony from other metals which it
affords lies in the fact that the form in which the metal
is submitted to electrolysis is that in which it is obtained,
in the customary course of analysis. It must, however, be
noted that the substitution of potassium sulphide for
sodium sulphide is not feasible ; from such a solution com-
plete separation of the antimony does not occur.

If the sodium sulphide solution used in preparing the
electrolyte should contain pblysulphides, the presence of
which is indicated by its dark yellow colour, the separation
of the antimony will not be quantitative, and may even be
checked very early in the electrolysis. It is therefore best
to use a solution containing only the monosulphide ; and
one containing free alkali is to be preferred to one holding
too much sulphur. Polysulphides are, however, produced
in the electrolyte during the course of the electrolysis, and
the current be allowed to continue for too lengthy a
period of time, the edges of the deposit may be dissolved ;
and this redissolved antimony cannot then be separated
from the electrolyte. The deposition of antimony from a
sodium sulphide solution can be carried out at the normal
temperature daring the night by means of a current density
of '30 to -40 ampere ; the E.M.F. required will be between
1 -7 and 1 -8 volts, and the time for complete separation about
twelve or fourteen hours.

It is however preferable to carry out the electrolysis
with hot solutions and strong currents in a short time,
since the above-named difficulty caused by re-solution of
the deposit is often found to occur when the current is
left in operation over night. If the solution obtained for
electrolysis should contain polysulphides, these must be
decomposed by means of hydrogen peroxide, and a fresh
amount of sodium monosulphide must then be added to
the solution.

The removal of the deposit of antimony from the
platinum electrode is effected by means of hot nitric acid.

148 THE ELECTROLYTIC PROCEDURE

A white oxide of antimony is formed, which passes into
solution, if tartaric acid be added to the nitric acid ; or
this oxide may be removed by simply rinsing out with water
or by rubbing the electrode with a cloth. If a greyish
discoloration should remain at its edges, this may be
removed by treatment with dilute hydrochloric acid.

The determination of antimony by the electrolysis of its
sulpho-salt is one of the most convenient and useful of all
electrolytic methods, and is one that is frequently used in
actual practice. It is especially convenient, because the
form of solution in which the antimony is obtained in the
usual course of separation from the other metals can be
directly electrolysed. Further, this method affords a simple
means for separating antimony from tin and arsenic a
separation that by the usual methods of analysis is difficult
to effect. This will, however, receive fuller notice later.

ARSENIC

This metal cannot be quantitatively separated by means
of electrolytic methods of analysis. From solutions con-
taining hydrochloric acid no deposition occurs, since the
arsenic combines with the hydrogen at the surface of the
kathode and forms arseniuretted hydrogen, which escapes
as a gas. Solutions to which oxalates and similar salts
have been added give incomplete deposits on electrolysis ;
while even the sulpho-salt solutions fail to give satisfactory
results according to Luckow, 1 Classen and V. Reiss, 2 and
Moore. 3 Vortmann has described attempts to separate
the arsenic as an amalgam with mercury ; 4 the results
were unsuccessful, owing to incomplete separation of the
arsenic.

TIN

Tin in its electrolytic characteristics closely resembles
antimony. Solutions of stannous and stannic chloride may

1 Zeitschr. f. anal. Chem. 19, 1. 2 Berichte, 14, 1 622.

Clvnn. News, 53, 209. ' Berichte, 24, 2750.

DEPOSITION FROM PURE SALT SOLUTIONS 149

be decomposed by the current, and yield a deposit of
metallic tin. Other solutions of tin containing free hydro-
chloric acid give similar results. The deposit inclines to a
coarse crystalline structure ; and both on this account, and
on account of the disagreeable features of the electrolysis
of chloride solutions, except when absolutely necessary
this method is little used.

Tin, like zinc, forms with excess of sodium hydrate a
complex salt sodium stannate.

If a tin solution containing such an excess of sodium
hydrate be submitted to electrolysis, either heated or at
the normal temperature, with a current of from '50 to 1*0
ampere, a separation of spongy tin will result ; and only a
very small amount will be obtained as a metallic deposit of
a silver white colour. The E.M.F. required will be from
4*0 to 4 '5 volts. The greyish spongy deposit of tin is not
composed of loose flaky particles, as in the case of the
other metals, but consists of a network of numberless tiny
shining needles. Solutions containing, in addition to the
caustic hydrate, potassium cyanide, yield deposits of a
similar character under the above conditions of current,
E.M.F., and temperature.

Classen and V. Reiss (/.c.) have proposed the use of the
double oxalate of tin and ammonium. This solution yields
a metallic deposit ; but when neutral, white particles
of stannic oxide separate during the electrolysis ; and an
addition of oxalic acid is necessary to bring these again into
solution. In these cases, therefore, in which this method
is to be used it is more advantageous to employ the acid
oxalate of ammonium, or to add to the neutral double
oxalate solution, oxalic or acetic acid. In order to prepare
such a solution for electrolysis, '50 grm. stannous chloride
is dissolved with the aid of a little hydrochloric acid, the
excess of acid is neutralised with ammonium hydrate, and
a solution of 4 grms. ammonium oxalate is added. The
precipitate which at first forms redissolves. The mix-
ture is then acidified with oxalic acid ; or an equal weight

150 THE ELECTROLYTIC PROCEDURE

of the acid oxalate of ammonium might have been used, in
place of the 4 grms. of the neutral salt. With a current
of from '30 to '40 ampere in density at the normal tempera-
ture the separation requires an E.M.F. of from 2 '8 to 3*5
volts, and lasts from six to seven hours. The neutral
double salt solution may also receive an addition of 10 c.cms.
acetic acid, and be electrolysed with a current of '50 ampere
at the normal temperature. In this case the E.M.F. re-
quired is between 3-3 and 3-8 volts, and the separation
demands from five to six hours. Towards the end of the
deposition an increase of the current strength is necessary,
in order to effect the separation of the last traces of the
metal from the electrolyte. The deposits obtained from
these solutions are adherent, silvery, white, and metallic ;
they are washed and dried in the customary manner.

Since tin is frequently obtained in the usual course of
analysis as stannic or stannous sulphide, it is a great
advantage that tin can be quantitatively separated from
solutions of the sulpho-salt. A complete separation is,
however, only possible when using the ammonium salt
solution ; the deposition is incomplete when the potassium
or sodium salt is used. In order to prepare the solution
of ammonium sulphide, sulphuretted hydrogen is passed
through ammonium hydrate until no more of this gas is
absorbed ; and in order to produce polysulphides (in the
case of antimony these had to be avoided) some powdered
sulphur is added. A solution of '50 grm. stannous chloride
in water containing a little hydrochloric acid is now pre-
pared, the tin is precipitated by sulphuretted hydrogen, and
the well- washed precipitate is dissolved in from 10 to 15
c.cms. of the above ammonium sulphide solution.

After dilution to 150 c.cms. the solution is electrolysed
either at the normal temperature, with a current of between
50 and '70 ampere in density, or better, at a temperature of
50 to 60 C., with a current of from 1 -0 to 2-0 amperes. In
the latter case an E.M.F. of between 3'3 and 4'0 volts will be
required, and the time occupied in the separation will be

DEPOSITION FROM PURE SALT SOLUTIONS 151

only one hour, as compared with the five or six hours de-
manded in the former case. The deposits obtained in this
way are bright steel grey. The end of the electrolysis is
checked by treating the small test portion of the electrolyte
with a few drops of dilute sulphuric acid and by gently heat-
ing the mixture. If the turbidity which is produced owing to
the decomposition of the ammonium sulphide remains white
no tin is present ; if it changes to a greyish-brown tint, the
whole of the metal has not been deposited. When the
deposition of the tin is completed the remainder of the
electrolyte must be washed from the basin before the cir-
cuit is broken. In order to effect the removal of the de-
posit from the electrode, it may be warmed with
concentrated hydrochloric acid.

Engels has recently shown that a clear solution of a
tin salt to which a little hydroxylamine and sulphuric acid
have been added, can be prepared for electrolysis in such a
way that no separation of stannic oxide will occur during
the deposition of the tin, and that the character of the
deposit obtained at the kathode, even under the most
divergent conditions of current and temperature, will be
all that can be desired. If on solution of the salt in
water a slight turbidity is produced, this is removed by
the addition of a little oxalic acid. This solution which
should contain an amount of the salt = '30 grm. tin then
receives an addition of '30 to '50 grm. hydroxylammonium
sulphate or chloride, 2 grms. tartaric acid, and 2 grms.
ammonium acetate, and after dilution to 150 c.cms. it is
electrolysed at a temperature of 60 to 70 C., with a current
density of from -70 to I'O ampere. The E.M.F. required
will lie between 4*2 and 5*6 volts, and the time demanded
for the complete separation of the tin as a silver- white
metallic deposit will be three to three and a half hours.

In consequence of the difficulties that arise when
commercial stannous chloride is dissolved in water, and in
order to avoid the unpleasantness of manipulating stannic
chloride, it is advantageous to prepare the double salts of

152 THE ELECTEOLYTIC PROCEDURE

tin with ammonium or the alkali metal chlorides, for
electrolytic experiments.

The stannous and stannic salts of this type are all
soluble in water.

Of the methods described above, only one the sulpho-
salt method is actually used in technical laboratories.
As a general rule, tin is obtained in the ordinary course of
analysis either in the form of oxide when it is more
simple to ignite and to weigh this, than to bring into
solution for electrolysis or as stannous or stannic sul-
phide. In the latter case, the electrolytic method with
ammonium sulphide is useful. This latter method is also of
technical importance in the separation of antimony, arsenic,
and tin.

GOLD

Gold, being one of the noble metals, may be deposited
from acid solutions.

Solutions containing free hydrochloric acid may be used,
or even neutral solutions of gold chloride. Luckow l and
Brugnatelli 2 have shown that the double salts with sodium
or ammonium chlorides may also be used. The deposits
of gold obtained from these solutions are unfortunately
powdery in nature, and brown in colour, even when
currents of moderate intensity are used.

The solution of gold with excess of potassium cyanide,
recommended by Elkington, Ruolz, de la Rive, Luckow,
and Smith and Moore, 3 is that chiefly used for obtaining
deposits of this metal ; and in this respect gold shows
its kinship to silver. In order to carry out such an electro-
lysis a solution of gold chloride, containing about "10 grm.
gold, receives an addition of 1*5 grm. potassium cyanide ;
and after dilution to the usual volume the mixture is
electrolysed with a current density of '10 ampere. The
deposition will be complete in from ten to twelve hours.

1 Zeitschr. f. anal Chem. 19, 14. 2 Phil. Magazine, 21, 187.
3 Berichte, 1891, 2175.

DEPOSITION FROM PURE SALT SOLUTIONS 153

The deposit of gold adheres firmly to the platinum of
the electrode, and, if one would avoid attacking the latter
when dissolving off the coating of gold by means of aqua
regia, it is necessary, as in the case of zinc, to protect the
electrode with a coating of silver or of copper.

If for any reason it may be desired to dispense with
this coating, the gold deposit may be removed from the
electrode by covering it with a dilute potassium cyanide
solution, and by electrolysing this with a stout copper wire
as kathode, and the platinum electrode with its coating of
gold as anode.

Gold resembles antimony and tin in its property of
forming soluble double salts with the alkali-metal sul-
phides, the solutions of which on electrolysis yield useful
deposits. The sodium double salt is exceptionally suited
for this use ; while Smith and Wallace l have shown that
the double ammonium sulphide does not yield quantitative
results. In order to prepare such a solution, sodium
sulphide solution (saturated) is added to a solution of gold
chloride until the precipitate which first forms is re-
dissolved. This solution is diluted to about 125 c.cms., and
on electrolysis by means of currents of from "10 to *20
ampere at the normal temperature, an adherent deposit of
a brilliant yellow colour is obtained.

Smith has shown that useful deposits may likewise be
obtained from gold solutions containing 5 c.cms. free
phosphoric acid, 2 by means of current densities of *08 to "10
ampere ; and Bersoz proved so long ago as 1847 that
similar results could be obtained by use of gold solutions
containing sodium pyrophosphate. 3

Only the methods with potassium cyanide, and that
depending upon the formation of the sulpho-salts, are used
in practical work. For gold determinations by the electro-
lytic method it is advisable always to choose the smaller and

1 Proc. Chem. Soc. Frankl. 3, 20.

2 Amer. Chem. Jour. 1891, 13, 206.

s Anal. Chem. Pharm. 1847, 65, 164.

154 THE ELECTEOLYTIC PROCEDURE

lighter electrode as kathode, since as a rule the amount of
gold obtained is very small.

On account of the difficulty of separating gold from the
other metals by electrolysis, no displacement in technical
laboratories of the usual dry assay methods by the intro-
duction of the electrolytic methods described above is likely
to occur.

PLATINUM

This metal, which likewise belongs to the group of the
noble metals, may be deposited from solutions containing
free hydrochloric or sulphuric acid. If the current used be
not extremely feeble (under '10 ampere) the metal will be
obtained as a black loosely adherent powder at the kathode,
instead of as a bright and dense metallic deposit.

In this case, as in that of gold, it is preferable to pro-
tect the electrode surface with a coating of silver or copper.
A solution of platinum chloride, or of the double chloride
of potassium and platinum, to which a few drops of con-
centrated sulphuric acid have beea added, is recommended
by Classen and Halberstadt, l Riidorff, 2 and V. Miller and
Kiliani 3 for obtaining deposits of this metal. An E.M.F.
of 2 volts (that given by a single accumulator cell) suffices
to carry out this electrolysis. The solution is heated and
is electrolysed with current densities of between '01 and *03
ampere.

An E.M.F. of from 1-1 to 1*7 volts will be required ;
and the time necessary to deposit -20 to '30 grm. platinum
will be from four to five hours. The only test which can
be applied in order to determine the end of the electro-
lysis is that depending upon the addition of water
to the solution and consequent raising of level of the
electrolyte.

Smith has proposed the use of solutions containing

1 Berichte, 17, S 2477. 2 Zeitschr. f. angew. Cliem. 1892, 696.
8 Lehrbuch der Analyse.

DEPOSITION FROM PURE SALT SOLUTIONS 155

sodium phosphate and free phosphoric acid for obtaining
deposits of platinum. 1 A current of '07 ampere must be
used ; and ten hours are requisite to effect the separation
of '10 grm. metal.

Classen and Halberstadt (I.e.) have experimented with
platinum solutions containing potassium or ammonium
oxalate ; while Wahl has made attempts to obtain useful
deposits of the metal from solutions containing only oxalic
acid in addition to the platinum salt. 2

The double salt of platinum with potassium cyanide
has also been used with success, for obtaining deposits of
this metal.

The electrolytic determination of platinum is of no
practical importance.

PALLADIUM

Schucht has shown that palladium is separated con-
currently as metal and as oxide from solutions containing
free nitric acid, and also from those containing excess of
sodium hydrate. 3 When, however, the solutions corre-
sponding to those given above for platinum are employed,
the metal alone is obtained. If the currents be feeble,
dense metallic deposits are obtained ; if strong currents be
used, the metal separates at the kathode as a black powdery
mass. Joly and Leidie" electrolysed a solution of the double
chloride of potassium and palladium when engaged upon
an atomic weight determination. 4

Smith and Keller have shown that palladium may also
be deposited from ammoniacal solutions. 5

In order to obtain a palladium deposit from such a
solution, the chloride is dissolved in water, and is treated
with ammonium hydrate and hydrochloric acid, and finally

1 Amer. Chem. Jour. 1891, 13, 206.

2 Jour. Frankl. Inst. 132, 62.

3 Berg. u. HUttenzeitg. 1880, 39, 121.

4 Compt. Rend. 116, 146.

3 Amer. Cliem. Jour. 1890, 12, 252.

156 THE ELECTROLYTIC PROCEDURE

20 c.cms. ammonium hydrate are added in excess. After
dilution to the usual volume, this mixture is electrolysed
with a current of from '07 to -10 ampere. If the electro-
lysis be allowed to continue through the night, -20 to *30 grm.
palladium can be separated in this way.

The remark made concerning the practical importance of
the electrolytic methods for determining platinum applies
in this case also.

IRIDIUM

There is but little known concerning the deposition of
this metal from its solutions by means of the electric
current. Schucht states that it may be separated in adherent
and bright metallic form from solutions acidified with dilute
sulphuric acid. 1

From solutions containing sodium phosphate and free
phosphoric acid, on the other hand, no deposition is obtained,
and Smith has suggested the use of this solution in order to
effect the separation of iridium from platinum and pal-
ladium. 2

RHODIUM

There is little more known concerning the electrolytic
separation of this metal than in the case of iridium. Joly
and Leidie' have shown that the metal is separable from
solutions slightly acidified with sulphuric acid. 3 The
solution of the sesquichloride to which alkali metal
chlorides and hydrochloric acid have been added may also
be decomposed by the current with separation of the metal.
Smith states that a complete deposition of the metal is
obtained when a solution of the double chloride of rhodium
and sodium to which sodium phosphate and phosphoric acid
have been added is electrolysed. 4

1 Berg. u. Huttenzeitg. 1880, 39, 122.

2 Amer. Chem. Jour. 1892, 14, 435.

3 Compt. Rend. 1891, 112, 793.

4 Amer. Chem. Jour. 1892, 14, 435.

DEPOSITION FROM PURE SALT SOLUTIONS 157

THALLIUM

This metal in many of its electrolytic characteristics
resembles lead. It possesses the property of separating
from certain solutions both as metal and as oxide. It is
quite possible so to conduct the electrolysis with other
solutions that only the metal separates at the kathode ; but
this metallic deposit is so easily oxidised that it is not
fitted for quantitative determinations. Schucht has shown
that under certain conditions it is possible to obtain a
deposition of pure thallium peroxide (similar to that of
lead) from solutions containing free nitric acid. 1 This
method, however, cannot be used for quantitative purposes,
as the separation occurs on both electrodes. The same
result is found to occur if an alkaline or ammoniacal
solution be used ; while from neutral solutions and
from solutions containing sulphuric acid the deposition is
incomplete. The solutions prepared with an excess of
potassium cyanide and ammonium oxalate i.e. solutions
of the double cyanides and the double oxalates yield with
currents of *10 ampere the whole of the thallium as metal
at the kathode, but these deposits cannot be washed or
dried without the occurrence of oxidation. Neumann has
shown that in such cases the determination of the weight
of metal can be effected by placing the electrode with
its metallic deposit in a suitable apparatus and adding
hydrochloric acid ; the volume of hydrogen liberated
is quantitatively proportional to the weight of metal dis-
solved. 2 The electrolytic determination of thallium is
therefore always somewhat inconvenient and lengthy.

BISMUTH

Bismuth likewise belongs to that group of metals which
separate on electrolysis of their solutions, concurrently as
metal and oxide. Schucht has shown that the electrolysis

1 Berg, u. Huttenzeitg. 1880, 39, 121. 2 Berichte, 21, 356.

158 THE ELECTEOLYTIC PROCEDURE

of the neutral salt solutions yields metal at the kathode and
yellow bismuthic acid at the anode. 1 It is not possible so to
carry out this electrolysis that no metal is deposited ; but
it may be conducted so that no peroxide is formed. It is,
however, only in exceptional instances that the metal is
obtained, even in small amounts, as a silvery white and'
metallic deposit ; as a rule the deposited metal is not only
dark in colour, but is a powdery deposit of spongy formation,
and is only loosely adherent to the electrode.

It is true that results have been obtained in very
carefully conducted analyses, which, in spite of the unsuitable
character of the deposit, have closely agreed with those
obtained by other methods of analysis. The results
obtained, however, with these deposits of a powdery
character must always be regarded with distrust.

According to Smith and Knerr, 2 and Thomas and
Smith, 3 a solution of bismuth sulphate containing -15 grm.
bismuth, to which 3 c.cms. dilute sulphuric acid have been
added, if diluted to 150 c.cms. and electrolysed with a
current of *30 ampere, gives in about three hours a fairly
adherent deposit, which may be washed with care without
loss.

Wieland also states that an adherent deposit may be
obtained from solutions containing 5 c.cms. free nitric acid
if the current density be kept below '05 ampere. 4 Accord-
ing to Smith and Saltar, on the other hand, the nitric acid
used ought not to be greater in amount than is necessary
to effect the solution of the basic salt. 5 If a greater amount
than this be used, oxygen products of bismuth will separate
at the anode.

The end of the electrolysis must be determined by
treating the test-sample of the electrolyte with sulphuretted
hydrogen.

1 Berg. u. Hiittenzeitg. 1880, 39, 121.

a Amer. Chem. Jour. 1886, 8, 206. 3 Ibid. 1883, 5, 114.

4 Berichte, 17, 1612.

5 Zeitschr. /. anorg. Chem. 1893, 3, 416.

DEPOSITION FROM PURE SALT SOLUTIONS 159

Classen and V. Reiss, 1 Classen and Eliasberg, 2 and Vort-
mann 3 have also made experiments bearing upon the use
of the double oxalate salts of bismuth with ammonium
or potassium, for electrolytic separations.

This method suffers under two disadvantages in the
first place, bismuth oxalate is only slightly soluble in
ammonium oxalate; and, secondly, oxygen compounds
separate at the anode as before. The method proposed by
Brand (I.e.), in which sodium pyrophosphate and ammonium
hydrate are used, also yields unsatisfactory results ; and
thai proposed by Riidorff, in which the bismuth solution is
treated with quite a selection of chemicals potassium
oxalate, potassium sulphate, and sodium pyrophosphate is
similarly useless.

Other proposals are the addition of citric acid to either
acid or alkaline bismuth solutions, by Thomas and Smith,
Schmucker, 4 and Smith and Frankel, 5 and the addition of
tartaric acid to alkaline or ammoniacal solutions.

None of these methods give really satisfactory results.
On this account, when it is desired to effect the electrolytic
determination of bismuth it is necessary to make use of
the fact that bismuth combines with mercury to form an
amalgam, and to deposit these two metals together. This
method will receive further notice under the heading
' Amalgams.'

URANIUM, MOLYBDENUM, MAGNESIUM, ALU-
MINIUM, CHROMIUM, CALCIUM, BARIUM,
STRONTIUM, POTASSIUM, SODIUM

Uranium and molybdenum can only be electrolytically
separated from their solutions in the form of oxides, and
the separation of these is frequently incomplete.

Magnesium, aluminium, and chromium can never be

1 Berichte, 14, 1620. 2 Ibid. 19, 326.

3 Ibid. 24, 2750. 4 Zeitschr. f. anorg. Chem. 5, 199.

5 Amer. Chem. Jour. 12, 428.

160 THE ELECTROLYTIC PROCEDURE

separated as metals from solutions of their salts in water
under the customary conditions as regards concentration
and current of electrolytic analyses. In some cases the
separation of the hydroxide occurs.

The alkali metals, under similar conditions, are also
incapable of depositions.

The alkaline earth metals, when present in hydrochloric
or nitric acid solutions, are not deposited ; if, however,
organic acids be present in the solution, the decomposition
of these leads to the formation of carbonates or hydroxides,
which then separate as flocculent precipitates in the electro-
lyte.

AMALGAMS

Attention was directed in the theoretical portion of this
work to the fact that, under certain conditions, mixtures
of metals or alloys could be deposited from electrolytes
which contained two or more salts in solution. The electro-
lytic method of covering metals with brass is an example of
the technical application of this method of procedure. It
is also employed for analytical purposes ; as when solutions
of mercuric chloride are mixed with other rnetal-salt
solutions, in order to obtain the separation of an amalgam.
Luckow was certainly the first to carry out experiments in
this direction, 1 but it was more especially Vortmann who
investigated the electrolytic deposition of amalgams, and
who studied the behaviour of many metals towards mercury,
when they underwent simultaneous deposition with the
latter metal. 2 The aim of these investigations was to dis^
cover whether the amalgam method could not be used to
obtain trustworthy results in the case of those metals
which in the ordinary way gave unsatisfactory deposits.

Experiments in which the metals are to be separated as
amalgams are best carried out as are depositions of the

o . *

metal mercury alone in a platinum basin.

Zinc amalgam, Luckow recommends a solution of zinc,

1 Zeitschr.f. anal. Chem. 19, 1.

2 Berichte, 24, 1891, 2752.

DEPOSITION FKOM PUBE SALT SOLUTIONS 161

made slightly acid with sulphuric acid, to which has been
added a solution of an equal weight of mercuric chloride,

If too great an excess of acid be present, the deposition
is completely stopped. Yortmann used ammonium oxalate
solutions, and also ammoniacal ammonium tartrate solu-
tion. When using the former, the zinc and mercury salt
solutions were mixed in the proportions of 1 : 2 or 1 : 3
(zinc to mercury) ; and the electrolysis of this mixture
yielded a silvery-white amalgam. In the case of the latter
solution the proportion must be at least 1:3; otherwise
the deposit will be spongy in character. In carrying out
such depositions, the weighed amounts of zinc sulphate or
chloride and of mercuric chloride are brought into solution,
the necessary chemical reagents are added, and the electro-
lysis is completed as in the case of the single metals.
This method of electrolytic determination cannot be recom-
mended for zinc, on account of the injury it causes to the
platinum basin. The loss in weight of this can rise to as
much as '05 grm. with each single determination.

Cadmium amalgam. Vortmann has proposed for this
the same solutions as those recommended by him for the zinc
amalgam. The method with the oxalates can, however, be
used only for very small amounts of cadmium, owing to the
comparative insolubility of the cadmium ammonium oxalate.
When more than '30 grm. of the metal is to be dealt with,
the solution containing tartrates is to be preferred. In
order to prepare such a solution, cadmium nitrate, or sul-
phate, and mercuric chloride are dissolved, 3 grms. tartaric
acid and excess of ammonium hydrate are added, and after
dilution to the usual volume the mixture is electrolysed.
If the proportions of cadmium to mercury present in the
solution is represented by the ratio 1 : 4 or 1 : 5, the
amalgam obtained is hard ; if the proportion falls to 1 : 8
the amalgam will be partly fluid.

Lead amalgam. In order to obtain this amalgam, a
solution containing the lead salt and mercuric chloride is
mixed with 3 to 5 grms. sodium acetate, potassium nitrite,

162 THE ELECTKOLYTIC PKOCEDUBE

and a few cubic centimetres acetic acid. Solutions made
slightly acid with nitric acid may also be used ; but the
addition of potassium nitrite must still be made, in order
to prevent the formation of lead peroxide.

Deposits of amalgams without any formation of per-
oxide may also be obtained from alkaline solutions contain-
ing the two salts, tartaric acid, potassium iodide, and excess
of sodium hydrate.

The last method is to some extent an unpleasant one to
use, since iodine separates at the anode owing to the decom-
position of the potassium iodide ; and this liberated iodine
forms gas-holding clots which swim upon the surface of the
electrolyte. The lead amalgam when kept dry is stable in
the air, but when moist it oxidises very easily. It is there-
fore necessary to wash the deposit quickly with water and
alcohol, to dry it by an air-blast, and to keep the electrode
when dry in a desiccator.

The results are satisfactory. Since the method of elec-
trolytic determination by means of peroxide deposition at
the anode (see p. 128) is so simple and so entirely satisfac-
tory, the above methods are without practical import-
ance.

Bismuth amalgam. Bismuth is the metal for the elec-
trolytic determination of which the amalgam method is
advantageous ; for if this separation as an amalgam were
not feasible in the case of bismuth, only very small
amounts of the metal, and even those rarely in the
metallic form, could be electrolytically deposited from its
solutions.

By use of the amalgam method, amounts up to '60 or
70 grm. bismuth may be separated. The mixed solution
of the bismuth and mercury salts may be subjected to
electrolysis in various forms.

The ammonium oxalate double salt solution has been
found unsuitable for this deposition.

The form of solution in which free nitric acid is present
may be used ; the acid must be present in sufficient amount

DEPOSITION FKOM PUEE SALT SOLUTIONS 168

to keep the basic bismuth salt in solution. Too great an
excess of acid, however, causes a deposition of bismuthic
acid at the anode. In order to carry out an electrolysis
with this solution, '50 grin, bismuth oxide and 2 grms. mer-
curic oxide are dissolved in the required amount of nitric
acid ; and the solution of mixed nitrates is electrolysed
with currents up to 1 ampere in strength. &t the usual
temperature this electrolysis requires an E.M.F. of 3^
volts. The relative amounts of the two salts in the elec-
trolyte must be at least 4 of mercury to 1 of bismuth ; a
silvery-white amalgam will then be obtained. The end of
the electrolysis is determined by use of ammonium sulphide.
The remaining electrolyte must be displaced by water
before the circuit is broken. The washing and drying of
the deposit are effected in the usual manner. The bis-
muth amalgam is not subject to oxidation on exposure
to air, and it is little affected by heat. If too little mer-
cury has been used, a black deposit of bismuth will be
found covering the amalgam. In order to prevent in
all cases the separation of oxygen compounds of bismuth
at the anode, a little tartaric acid is added to the elec-
trolyte. These oxides, when they have once separated at
the anode, are in most cases not to be brought into solution
again.

If a dark band should remain on the platinum electrode
after solution of the amalgam, this may be removed by igni-
tion, or by use of the electrode as anode in a dilute nitric
acid solution, with a stout copper wire or strip of sheet
copper as kathode.

Vortmann has also used solutions containing hydro-
chloric acid. In order to prepare such a solution, he added
potassium iodide and hydrochloric acid to the solution of
the bismuth and mercury salts, until a clear liquid was pro-
duced. On electrolysis this solution yields a gas-holding
scum of iodine upon the surface of the electrolyte. In order
to avoid too great an excess of hydrochloric acid when using
this method, Vortmann added 50 c.cms. 96 per cent, alcohol

M 2

164 THE ELECTROLYTIC PROCEDUEE

to the solution of the bismuth and mercury chlorides with
hydrochloric acid. This addition assists the solution of the
chlorides.

The relative proportions of the metals and of the other
reagents which should be employed are therefore as
follows : '20 to '80 grm. bismuth oxide ; 1 to 2 grms. mer-
curic chloride ; hydrochloric acid in sufficient amount to
dissolve the bismuth oxide ; 50 c.cms. 96 per cent, alcohol.
The method yields good results.

In actual work, it is more customary to deposit the
bismuth amalgam from the nitric acid solution than from
that prepared with hydrochloric acid. The reason for this
preference of the nitric acid method apart altogether from
the general objection to the electrolysis of solutions con-
taining chlorides is to be found in the fact that in the
ordinary course of analysis the nitric acid solution is often
obtained, or can easily be prepared. It must certainly be
held to be a great convenience that in the case of bismuth
the amalgam method is sufficiently trustworthy to be used
in place of the unsatisfactory methods of obtaining metal
separations.

Antimony amalgam. Formerly the electrolytic deter-
mination of antimony was beset with difficulties, and this
metal was therefore included in the number of those which
it was attempted to separate and to determine as amal-
gams. The experiments in the case of antimony were
made with solutions containing the mixed salts and sodium
sulphide. Now that a simple and easy method is known
by which it is possible to deposit antimony as metal in
sufficiently large amounts and in short periods of time, the
amalgam method has become superfluous.

The experiments made upon arsenic salts showed that
the deposition of this metal as an amalgam was in-
complete.

It is seen from the above that at present the amalgam
method of determining the amount of metal in a solution
is only of importance in the case of bismuth.

DEPOSITION FROM PURE SALT SOLUTIONS 165

SEPARATION OF THE METALS

In Chapter VII. of Part I. it was shown that when
the current is passed through a liquid containing two
or more metal salts in solution, the metals are, according
to circumstances, deposited either together as alloys or
amalgams, or the deposition is only partial, and certain of
the metals in the mixture are deposited, while others
remain in the solution. The investigation of this
phenomenon, in so far as it relates to its application to
analytical purposes, is chiefly confined to the discovery of
the conditions under which certain metals are deposited
separately from mixed solutions. The various separations
possible by this mode of procedure may be divided into the
four following groups :

Group I. Separations of the metals from mixtures
containing two or more different metals, by deposition of
the one as metal at the kathode, of the other as peroxide
at the anode. The separation of copper from lead in a
nitric acid solution is an example of an application of the
method, which is much used in technical laboratories.

Group II. Separations of different metals by the
maintenance of the electric current used for the electro-
lysis, at a definite maximum, as regards E.M.F."(ref. p. 35).
Since the decomposition values of different salts vary, it is
possible to effect separations by means of an E.M.F. vari-
able at will, if the two values lie sufficiently far apart.

Kiliani and Freudenberg have shown that if the E.M.F.
of the current be kept below that required to effect the
electrolysis of the salt with the higher decomposition value,
the one metal will be deposited, while the other will remain
in solution.

Group III. The separations in this group are effected
by artificially increasing the decomposition value of the one
metal salt. This may be achieved either by raising the salt
to a higher level of oxidation, or by converting it into a
complex salt through the addition of other salts to the

166 THE ELECTROLYTIC PROCEDURE

solution. In either case the metal passes into the anion
group on electrolysis ; and its separation from this only
occurs in a secondary manner by use of an increased
E.M.F., or in some cases does not occur at all.

An example of this class of separation is to be seen
in the method adopted to part antimony and arsenic
in a sodium sulphide solution. The arsenic is previously
raised to the arsenic acid stage of oxidation. As an ex-
ample of the complex salt method of separation, the
parting of iron and cobalt from zinc after addition of
potassium hydrate may be cited.

Group IV. The separation of certain metals from others
may be effected by the addition of strong mineral acids to
their salt solutions.

In this way the deposition of iron, cobalt, nickel, cad-
mium, and zinc is prevented. Since those metals are in all
cases first separated for which the least E.M.F. is required,
it will be the noble metals, gold, silver, copper, and mercury,
that are first deposited ; while if a considerable excess of
acid be present the remainder of the series of metals given
on page 35 will not be deposited, a liberation of the hydro-
gen of the acid being produced instead.

These methods of separation are not all applicable in the
case of every metal ; neither are the four groups given
above sharply divided the one from the other in all cases.
In many instances a combination of the methods given
under two or more of the above groups is used.

For example, one may have a solution containing copper,
zinc, lead, and iron for analysis that is, a solution of a
brass containing lead and iron as impurities.

In this case the separation of the copper and lead is
effected in a nitric acid solution by the method of Group I. ;
but at the same time the copper is being separated from
the iron and zinc by the method of Group IV., since these
cannot be deposited from a strongly acid solution.

They remain in the electrolyte, and after the copper has
been removed they are determined by some special method.

SEPARATION OF METALS 167

The different separations that are possible will be found
described under the headings of the individual metals. The
current strengths given in these descriptions of the methods
are again based upon a kathode or anode surface area of
100 sq. c.m., even when this is not expressly stated to be
the case. In those cases in which no details are given con-
cerning the weights of the salts of the individual metals
which are to be used in carrying out these methods, those
given in Part III., B, may be taken. Reference to the pre-
vious division of the work may also be made in those cases
in which nothing is stated concerning the character or the
treatment of the metallic deposits.

COPPER

This metal one of the group known as the noble
metals can very easily be deposited in useful form from
solutions containing free mineral acids (ref. p. 93). This
characteristic makes the elaboration of a method of sepa-
ration from the metals zinc, iron, nickel, and cadmium
possible, while it also indicates that a separation from
those metals which form peroxide deposits at the anode can
be carried out.

Copper from Zinc. The separation of these two metals
can be effected in various acid solutions, provided that a
sufficient amount of acid be present. The mineral acids
are found to yield the best results, and of these, nitric and
sulphuric acids are most generally used.

The solution of '50 grm. of each of the salts, zinc
sulphate and copper sulphate, is treated with 1 to 2 c.cms.
cone, sulphuric acid, diluted to about 150 c.cms. and
electrolysed with a current density of between '50 and I'O
ampere. The E.M.F. required will be from 2 -5 to 2 -8
volts, and the electrolysis may be carried out either at the
normal temperature or at 50 C. The deposition of the
copper will be complete in from one and a half to two
hours ; but the last traces of the copper are difficult to

168 THE ELECTROLYTIC PROCEDURE

remove from the solution, and on this account the time
required is often greater than that named.

The remarks made on p. 95 concerning the character
and the treatment of the deposit apply in this case also.

The remaining acid zinc solution is neutralised after
the complete deposition of the copper, and bhe zinc is de-
posited by one of the trustworthy methods of electrolytic
determination given under " Zinc." If the copper deter-
mination has been carried out in the basin electrode, and
the liquid displaced before breaking the circuit, it is neces-
sary to evaporate the excess of water, in order that after the
addition of the necessary reagents the zinc solution may be
electrolysed in the same basin electrode, This is now pro-
vided with the required coating of copper, and the weight
is already known. If the electrolysis has been conducted
in a beaker with the jacket form of electrode, the evapora-
tion of the diluted solution is unnecessary, and the electro-
lytic deposition of the zinc may be directly proceeded with. 1
Nitric acid may be used in place of sulphuric acid in
effecting this separation. In this case the deposition of
the copper occurs rather more slowly.

The solution of the two sulphates is treated with about
5 c.cms. cone, nitric acid, diluted to the usual volume, and
after heating to about 50 C. it is electrolysed with a
current density of from *50 to I'O ampere. The E.M.F. re-
quired will be between 2 -5 and 3-0 volts, and the deposition
will demand about three hours.

Since, during the electrolysis of the copper salt, part of
the nitric acid is converted into ammonia by the action of
the current, a too lengthy duration of the electrolytic
separation, or an insufficiency of acid in the solution, may
cause the electrolyte to lose its acid reaction through the
formation of ammonium nitrate. If this should occur, the
zinc may be deposited with the copper.

The liquid containing the zinc salt, after the complete

1 When a jacket kathode is used, so little wash water is produced
that evaporation is unnecessary.

SEPARATION OF METALS 169

separation of the copper, should not be used directly after
neutralisation of any remaining free acid for deposition of
the zinc. The presence of nitrates in solution is not
favourable to the attainment of good metallic deposits of
this metal.

It is most advantageous to evaporate this solution with
sulphuric acid, and then to convert the resulting sulphate
of zinc into one of the forms given under ' Zinc ' as most
suitable for the electrolytic deposition of the metal. The
further conduct of the electrolytic separation is then exactly
as described under 'Zinc.'

Smith has recommended the use of a solution of the
sulphate salts of copper and zinc, to which 30 c.cms. of a
saturated solution of sodium phosphate, and 3 c.cms. phos-
phoric acid, have been added.

Very weak currents must be employed with this solution.
The copper is deposited first quite free from admixture with
zinc. The deposition takes place very slowly, and the
method is not so simple as the two methods already de-
scribed, in which sulphuric or nitric acid solutions are used.

Classen has effected the separation of copper and zinc
by means of his oxalate method. 1

The salt solutions of the two metals receive an addition
of ammonium oxalate, and are thereby converted into the
double oxalate form. The electrolysis is conducted with a
neutral or feebly acid solution, and the E.M.F. required is
about 2 volts. The method suffers from the disadvantages
already discussed under ' Copper ' (see p. 100).

The electrolytic method of separation of copper and zinc
is made use of in technical laboratories for the analysis of
brass. In Part III., D, further reference will be made to
this use of the method.

The first-named methods of separation i.e. those effected
in solutions acidified with the mineral acids are the only
two which are of technical importance.

Copper from Iron. The separation of these two metals
1 Berichte, 17, 2467.

170 THE ELECTROLYTIC PROCEDURE

can, in a similar manner to that of the two former ones, be
effected in an acid solution, one containing free sulphuric
acid having been found to be the most suitable. In order
to prepare such a solution of the mixed salts, about 1 grm.
each of cupric and ferrous sulphates are dissolved in water,
the solution is treated with 3 c.cms. cone, sulphuric acid,
and after dilution to 150 c.cms. it is electrolysed at the
normal temperature, with a current density of 1 ampere.
The E.M.F. required will be from 2-75 to 3-0 volts ; the
time necessary for the complete deposition of the copper
will be from two to two and a half hours. The remaining
electrolyte must be washed out of the basin before breaking
the circuit ; after evaporating to a suitable volume, it is
neutralised with ammonium hydrate, and between 4 and 6
grms. ammonium oxalate are added to the solution.

If a jacket electrode is to be used, this evaporation is un-
necessary. The electrolysis is carried out at a temperature
of 30 to 40 C., with a current density of between I'O and
1-5 ampere, and an E.M.F. of from 3'4 to 3'8 volts. The
iron will require between three and four hours for complete
deposition. The results obtained by this method are good.
The ammonium sulphate which results from the neutralisa-
tion of the excess acid by ammonium hydrate is without
prejudicial influence upon the separation of the iron from
the double oxalate salt solution.

If 5 c.cms. cone, nitric acid be added to the solution of
the two salts, and, after the usual dilution, the electrolysis
be carried out at the normal temperature with a current
density of 1 ampere, and an E.M.F. of from 2'9 to 3-3 volts,
the complete deposition of the copper will require from four
to five hours. The remaining electrolyte must be displaced
before the current connections are broken.

If one proceeds, as before, to electrolyse the iron solu-
tion after neutralisation of the free acid with ammonium
hydrate, and addition of from 4 to 6 grms. ammonium
oxalate, a separation of ferric hydrate will occur in the
electrolyte during the deposition of the iron. This precipi-

SEPARATION OF METALS 171

tate of iron can be dissolved by the use of oxalic acid, but
such an addition always has a prejudicial influence upon
the results, and in very many cases the deposition of the
iron is incomplete. It is, on this account, necessary, when
a nitric acid solution has been used to effect the separation
of the copper, to remove the nitric acid by evaporation
with sulphuric acid, before proceeding to this electrolytic
separation of the iron.

Classen has used his oxalate method to effect separations
of these two metals. The solution containing the two
metals as sulphates, together with between 6 and 8 grms.
ammonium oxalate in 150 c.cms. water, is treated with oxalic,
tartaric, or acetic acid, and small quantities of the acid
are added from time to time during the course of the
electrolysis in order to maintain the acid reaction of the
electrolyte.

The electrolysis is conducted at a temperature of
between 50 and 60 C. with a current density of 1 ampere.
The E.M.F. required under these conditions will be from
2 '9 to 3-4 volts ; and the complete deposition of the copper
(which separates quite free from iron) will demand about
three hours, if 1 grm. copper sulphate has been used.

If sufficient care be not given during the electrolysis to
the maintenance of the acidity of the electrolyte, oxalic acid
being especially easily decomposed by the current, iron will
be deposited with the copper at the kathode. When the
deposition of the copper has been completed satisfactorily,
the remaining electrolyte may be simply neutralised with
ammonium hydrate, and the iron deposited forthwith from
this solution at the normal temperature by means of a
current of from 1*0 to 1*5 ampere density.

The E.M.F. required will be from 3'0 to 3-3 volts, and
the time about three hours. When the iron is to be
estimated in this way, only oxalic acid, of those named,
may be employed for acidifying the electrolyte during the
copper deposition; the electrolysis demands constant super-
vision, and does not always give satisfactory results.

172 THE ELECTROLYTIC PROCEDURE

Vortmann has recommended the use of ammonium
sulphate and ammonium hydrate with the sulphate salts
of the two metals, when the amount of iron is considerable.
A precipitate of flocculent ferrous or ferric hydrate of
course occurs in such a solution, but this is said not to be
detrimental to the deposit of copper. A current density of
between '10 and *60 ampere is employed.

Apart from the objection that exists to the use of solu-
tions containing precipitates in suspension for electrolysis,
it is by no means certain that by the use of this method
small amounts of iron will not be deposited with the copper.

Smith has also recommended the use of solutions con-
taining sodium phosphate and free phosphoric acid for
effecting the electrolytic separation of copper and iron.
The remarks made upon this method as applied to the
separation of copper and zinc are also applicable in this case.

A consideration of the separate methods described above
shows that the method with free sulphuric acid is clearly
superior both in simplicity and in reliability to any of the
others.

It is necessary to note, in conclusion, that the separation
of the two metals copper and iron by the electrolytic
method is attended with difficulties when the latter metal
is present in considerable amount. This is especially the
case when a nitric acid solution is employed. Not only
does the already deposited copper partly redissolve, but,
according to Schweder, the deposition remains incomplete. 1

Copper from Cobalt or Nickel. This separation can be
effected in a manner precisely similar to that described for
the separation of copper from zinc or from iron namely, by
means of solutions of the salts containing an excess of free
mineral acids, from which copper alone will be deposited.
In this case also sulphuric acid is found to be eminently
fitted for use as the acidifying agent. In order to prepare
a solution of the mixed salts for electrolysis, 1 grm.
each of copper and nickel sulphates (or cobalt sulphate)
1 Berg- u. Hiittenzeitg. 30, 5, 11, 31.

SEPARATION OF METALS 173

are dissolved in the necessary amount of water, and 3 c.cms.
cone, sulphuric acid are added. After dilution to the usual
volume the solution is electrolysed with a current density
of about 1 ampere at the normal temperature. From two
and a half to three hours will be requisite to effect the
complete removal of the copper from the electrolyte. The
further treatment of the deposit and of the electrolyte is as
described under the separations Copper- Zinc and Copper-
Iron.

In place of the addition of sulphuric acid, 5 c.cm. of
nitric acid may be used, in which case the deposition of the
copper occurs under approximately the same conditions as
those given above.

In either case the deposit of copper is perfectly free
from nickel (or cobalt). Of the two methods, that with
sulphuric acid is to be preferred, since, after the electro-
lyte has been removed from the basin electrode and
the washings have been added, it is only necessary to add
ammonium hydrate in excess to the solution, now contain-
ing only nickel (or cobalt), and to decompose the heated
solution by means of a strong current, as described under
' Nickel.'

The nitric acid solution requires a previous evaporation
with sulphuric acid in order to convert the nitrate salts into
sulphates, if later disturbing influences upon the electrolytic
process are to be avoided ; whereas the sulphuric acid solu-
tion is, after addition of ammonium hydrate, at once ready
for the deposition of the nickel or cobalt.

Classen has recommended the use of the ammonium
oxalate double salt, with the addition of oxalic or tartaric
acid, for effecting the separation of these metals. A
quantitative separation is, however, only possible if the
E.M.F. be kept at or below 1-3 volts, since a higher E.M.F.
produces an alloy of the two metals at the kathode. This
low E.M.F. will only produce a very small current, and
consequently the deposition of the copper occupies much
time.

174 THE ELECTROLYTIC PROCEDURE

Heydenreich has stated that about four hours are
requisite to deposit -25 grm. copper. 1

The deposition of copper from a solution of a copper salt
to which ammonium oxalate has been added in excess only
commences when an E.M.F. of I'l volts has been attained,
and the margin between this and that named above is ex-
tremely small. The conditions are precisely similar in the
case of either metal. The oxalate solution when freed from
its copper contents may be used directly for the electrolytic
separation of the nickel (or cobalt) ; neutralisation of the
free acid by means of ammonium hydrate being alone
necessary to prepare it for this further electrolysis. On
account of the time required to carry out the deposition of
the copper, this method cannot be regarded as a convenient
or useful one.

Smith has also recommended the use of his sodium
phosphate method for effecting the separation of these
metals. The remarks already made concerning this method
under Copper-Zinc apply in this case also, and it is unneces-
sary to repeat them here.

The method of separation of copper from nickel or
cobalt by use of a sulphuric acid solution of the metals is
the only one of technical importance. It is noteworthy
that it was from this solution that Gibbs, in the year 1864,
separated the two metals nickel and copper in an examina-
tion of ' mint nickel/ and thus gave proof of the applicability
of electrolysis to practical analytical purposes.-

Copper from Cadmium. One would suppose that cad-
mium, which as regards both its chemical and its electrolytic
characteristics and properties stands closely allied to zinc,
would show a similar relationship to zinc in the manner of
its separation from copper, and that the addition of the
required amount of free mineral acid to the solution of the
two salts would suffice to effect an easy separation of copper
from cadmium. This supposition is, however, correct only

1 Zeitschr.f. Elcktrochcm. 1894, 1,290.
- Zeitschr. f. anal. Chem. 3, 334.

SEPARATION OF METALS 175

to a very limited degree. The deposition of the copper
occurs satisfactorily when nitric acid solutions are made
use of. In order to carry out such a separation '30
to -50 grin, cadmium sulphate is dissolved with 1 grm.
copper sulphate in water ; the mixed solution receives
an addition of 5 c.cms. nitric acid, and after dilution to
150 c.cms. it is electrolysed at the normal temperature with
a current density of from -80 to 1-0 ampere. The E.M.F.
required will be from 2-8 to 2-9 volts ; the deposition of
the copper will demand about four and a half hours.
Equally good results can be obtained if 10 c.cms. nitric acid
be used, and if the electrolysis be carried out during the
night with a small current density of from -20 to '30
ampere, by means of an E.M.F. of from 1-9 to 2'2 volts.
When this method of procedure is adopted, it is necessary
to increase the current to a density of at least 1 ampere the
next morning, in order to effect the separation of the last
traces of copper from the solution. The displacement of
the remaining electrolyte must occur before the current
connections are broken.

If it be desired to effect the determination of the cad-
mium in this solution, it is made alkaline with sodium
hydrate, and is then treated with a freshly prepared solution
of pure potassium cyanide until the first-formed precipitate
is redissolved.

The electrolytic deposition of the cadmium is then
carried out as described under ' Cadmium.'

If the ammonium oxalate double salt solution be pre-
ferred for use in obtaining the cadmium deposit, it is
necessary to evaporate the solution from the first part of
the analysis with sulphuric acid, in order to convert the
nitrates into sulphates, since the presence of the former leads
to incomplete and unsatisfactory deposit when the double
oxalate method is employed. The solution of sulphates
obtained in this way is neutralised with ammonium hydrate,
8*0 grms. ammonium oxalate are added, and the solution
made up to the proper volume is then electrolysed with a

176 THE ELECTKOLYTIC PROCEDURE

current of from -50 to '80 ampere density at a temperature
of 60 C., with the precautions relative to the presence of
oxalic acid described at length under ' Cadmium. 7

The two metals may also be separated in a solution con-
taining sulphuric acid in place of nitric acid, but this separa-
tion is not possible in such solutions under all circumstances.

If, for example, a solution be prepared containing *30
grm. cadmium sulphate, 1-0 grm. copper sulphate, and 3
c.cms. cone, sulphuric acid in 150 c.cms. water, and this
be electrolysed, as in the case of the nitric acid solution, at
the normal temperature with a current of 1*0 ampere
density, an alloy of the two metals, copper and cadmium,
will be obtained at the kathode.

Under the above circumstances the E.M.F. will be
about 2 '8 volts. If, however, the E.M.F. be not allowed
to exceed 2 volts, and if the electrolysis be accordingly
undertaken with a weaker current, it is quite possible to
obtain a deposition of copper quite free from cadmium. In
this case the electrolysis should be carried out at a tempera-
ture of 60 C. ; the separation under these conditions lasts
about eight hours.

The solution containing sulphuric acid that remains
when the deposition of the copper is completed, after
neutralisation, may be directly used for the deposition of the
cadmium, either by the potassium cyanide or the ammonium
oxalate method. One may, however, in this case make use
of a third method, and electrolyse the cadmium sulphate
solution, after neutralisation with ammonium hydrate, and
without the addition of any other reagents, with a current
of 1 ampere density at a temperature of between 50 and
60 C. The E.M.F. required will be from 3'0 to 3-5 volts,
and the deposition will demand between three and a half
and four hours. The deposit of cadmium obtained will be
bright and metallic.

If between 5 and 6 grms. pure potassium cyanide be
added to a solution of the sulphates of copper and cad-
mium, of which each is present in an amount equivalent to

SEPARATION OF METALS 177

50 grm. of the metal, and if this solution after dilution to
between 130 and 150 c.cms. be electrolysed with a current
produced by an E.M.F. not exceeding 2'6 volts, Smith and
Freudenberg state that cadmium alone will be deposited,
while copper will remain in solution. 1 It is therefore pos-
sible to effect a separation in this manner.

It is remarkable that in this case the ' decomposing
values ' of the complex cyanides of copper and cadmium do
not follow the order of those of the neutral and acid salts
(see p. 46).

From solutions which contain a mixture of the double
oxalates of these two metals, a separation cannot be effected
by the use of moderately strong currents. It is, however,
not impossible that by the use of an E.M.F. not exceeding
2 volts a separation might be obtained in such solution.
The separation of copper from cadmium in solutions con-
taining sodium phosphate and free phosphoric acid is
possible when extremely weak currents are employed. The
deposition of the copper demands about twelve hours, and
this fact alone places this method far behind the others as
regards convenience or usefulness.

The most simple and convenient method to use for effect-
ing separation of copper and cadmium is that depending
upon the use of a nitric acid solution of the mixed salts.

Copper from Aluminium, Magnesium, Chromium, Cal-
cium, Barium, Strontium, Potassium, and Sodium. The
separation of copper from these metals is effected without
difficulty, if the electrolysis be carried out with solutions
containing a sufficiency of sulphuric or nitric acid, and under
the current conditions mentioned by Copper-Zinc and
Copper-Iron.

Copper from Lead. The separation of copper from lead
is easily effected in a nitric acid solution, for out of such
solutions copper is deposited in bright metallic form at the
kathode, whereas lead is separated as peroxide at the anode.

If a solution containing these two metals has received
1 Jour. anal. u. appl. Chem. 3, 385.

178 THE ELECTROLYTIC PROCEDURE

an addition of the requisite amount of nitric acid, the
electrolysis will result in the deposition of each metal at
the opposite electrode, and numerous experiments have
proved that the separation is absolutely complete. In order
to carry out such an electrolysis, 1 grm. each of copper and
lead nitrate is dissolved in water, 15 c.cms. of cone, nitric
acid are added to the solution, and the clear mixture is then
diluted to 150 c.cms. If copper sulphate should be used in
place of the nitrate, a white precipitate of lead sulphate will
be formed on adding the lead nitrate, and this compound
can only be brought into solution by gentle heating with
an excess of ammonium hydrate.

If this method has been used it will be necessary to
neutralise with nitric acid, before adding the measured
quantity of acid named above. The solution, which must
be perfectly clear, is heated to 60 F., and the current con-
nections are then made in such a manner that the larger
electrode surface, i.e. the platinum basin or cone, functions
as anode for the reception of the deposit of lead peroxide.
The remarks made under ' Lead J relative to the advantage
of using a well-worn electrode apply of course with equal
force in this separation. The current density required will
be from 1-0 to 1-5 amperes; the E.M.F. will be only 1-4 volts.

One hour will suffice to complete the separation of all
the lead as peroxide at the anode, but a longer period will
be requisite to complete the deposition of the copper, since
it has always been observed that when the electrolysis is
first commenced only the peroxide is separated, and that
the deposition of the copper commences later and takes
place more slowly. After the whole of the lead is deposited
the current connection is broken, there being no necessity
at this point to pay any attention to the fact that the
greater portion of the copper is still in solution. If the
jacket electrode has been used, it is simply necessary to
lift this with its coating of peroxide out of the solution, and
to wash and dry it as described fully under * Lead.' A new
jacket electrode is then fixed in position in the solution,

SEPARATION OF METALS 179

which is now free from lead, but the current connections
are reversed, and the cone now functions in the usual
manner as kathode.

The copper which in the previous electrolysis had sepa-
rated upon the electrode that is now acting as anode is
redissolved in the electrolyte, and is separated with that
which originally remained in solution upon the new kathode.
If a basin electrode has been used, the electrolyte must
be displaced before the current connections are broken
when the whole of the lead has been separated as peroxide ;
and the deposit of the latter must be treated as already
described. The displaced liquid together with the wash
water is evaporated down to a volume of about 130 c.cms.,
and after neutralisation with ammonium hydrate 10 c.cms.
nitric acid are again added in order to raise the electrolyte
to the degree of acidity required.

The electrolysis of the copper salt is then carried out at
the normal temperature with a current density of 1*0
ampere, and an E.M.F. of from 2 -2 to 2-5 volts. In this
case a fresh platinum basin is used as kathode ; while the
disc electrode with its deposit of copper obtained during tho.
deposition of the lead peroxide is now used as anode, and
speedily loses its coating of copper. The time required
to effect the deposition of the whole of the copper will be
from four to five hours. The simplest test to apply in order
to ascertain whether all the lead has been deposited as per-
oxide in the first portion of this electrolysis, is to add a
little to the volume of the electrolyte, and to watch the
freshly covered anode surface for traces of a deposit of the
dark-coloured lead peroxide. The deposit of copper obtained
in the second part of the separation is washed and dried in
the usual manner.

This method for effecting the separation of copper and
lead is one which has attained a very wide field of useful-
ness in technical laboratories.

Copper from Manganese. The latter metal is deposited,
as already noticed, from nearly all its solutions in the form

180 THE ELECTROLYTIC PROCEDURE

of peroxide, but this deposition occurs with especial ease
and certainty in the case of solutions containing a little free
sulphuric acid. If copper be present in such a solution, it
would reasonably be assumed that on electrolysis this metal
would be deposited at the kathode, while the manganese
would be found at the anode, i.e. a separation might be
expected to occur. This supposition is found to be correct,
for such a separation does occur, and is complete. In order
to carry out this electrolysis, '50 grm. each of copper and
manganese sulphates (or nitrates) are dissolved in water, and
after dilution of the solution to between 130 and 150 c.cms.
ten drops of cone, sulphuric acid are added. The mixture
is then heated to 50 to 60 C., and is electrolysed with a
current varying between '50 and 1 '0 ampere in density. The
electrolysis will require between two and three hours. The
remarks made concerning the use of the larger electrode for
the reception of the lead peroxide deposit under < Copper-
Lead ' apply here, as do those also made under ' Lead,' con-
cerning the use of a well-worn electrode surface. It is not
advisable to use a larger amount of the manganese salt
than that named above, since there is some danger of the
manganese deposit scaling.

The deposit of manganese peroxide after washing and
drying is ignited and weighed, the results being calculated
upon the formula Mn 3 O 4 . Equally reliable results may be
obtained by drying the manganese peroxide deposit at 60 C.,
and by using the formula MnO 2 -f H 2 in the after calcula-
tion. In this case the factor '523 is used to convert the weight
of the deposit into its equivalent of Mn. The deposit of
copper obtained simultaneously at the kathode is treated in
the usual manner.

Since manganese may be deposited as peroxide from a
solution containing free nitric acid, a form of solution from
which copper also may be obtained in very satisfactory
deposits, it is possible to separate these two metals in solu-
tions containing a small amount of free nitric acid. A few
c.cms. nitric acid are added to the diluted solution of the

SEPAEATION OF METALS 181

two salts, and the mixture after heating to 50 C. is
electrolysed with a current density of about '50 ampere.

The deposition will be complete in three hours. The
use of high-current densities is not recommended, on account
of the detrimental effect of these upon the coherence of the
deposit of manganese peroxide. The other conditions of
the separation by this method are similar to those described
above.

If the amount of free nitric acid present be allowed to
exceed 3 to 4 per cent., no deposition of manganese peroxide
will take place at the anode, a formation of red permanganic
acid will occur in its stead.

Smith has used solutions containing 30 c.cms. saturated
sodium phosphate solution, and 10 c.cms. phosphoric acid,
in order to effect separations of copper and manganese. If
very feeble currents be employed with such solutions, the
copper alone is deposited, while the manganese remains
completely in solution. The two methods first named,
however, excel Smith's method not only in simplicity but
also in speed.

Classen has proposed his double oxalate method for
separating copper and manganese.

The extremely slight adherent properties shown by the
manganese deposit obtained from these solutions, together
with the unsatisfactory character of the copper deposit,
place this method also, in respect to simplicity and relia-
bility, far behind the two methods first described.

Copper from Silver. The principal factors in the
development of the fairly simple methods for the separa-
tion of copper from the other metals that have hitherto
been described have been the properties displayed by
certain metals which, on electrolysis of their salt solutions,
yield peroxides at the anodes, and of others which yield no
deposit at all when the electrolysis is carried out in acid
solutions. In effecting the separation of copper from the
metals silver, mercury, bismuth, &c., the 'decomposition
values ' of whose salts lie very close to those of the

182 THE ELECTEOLYTIC PKOCEDUEE

salts of copper, it is necessary to attempt to bring about a
separation by the employment of an E.M.F. lying midway
between those represented by the ' decomposition values '
of the two salts concerned ; or, if this be impossible, an at-
tempt must be made to bring about a greater divergence
in these values by use of other forms of solution.

The separation of copper and silver can be undertaken
/ in different solutions. Freudenberg l and Kiliani 2 have
shown that these metals may be separated in nitric solu-
tions when the E.M.F. is not allowed to exceed 1'3 or 1-4

\ volts.

In order to effect such an electrolysis, the metals may
be dissolved in nitric acid, or -5 grm. of each of the
neutral nitrate salts is dissolved in water, and after addi-
tion of 2 to 3 c.cms. nitric acid, and dilution to the usual
volume, the electrolysis is carried out by means of the
E.M.F. named above. The current-density obtained with
this E.M.F. will only be '10 ampere ; and the deposition of
silver will require six to seven hours at the normal
temperature, or three to four hours if the electrolyte be
heated. The remaining electrolyte is removed from the
basin electrode by a syphon, and after addition of the
washings the volume is reduced within the required limits
by evaporation. The copper deposition is then effected in
this solution after the addition of a few cubic centimetres
of nitric acid, with a current density of from '50 to 1-0
ampere. The time required will be between one and two
hours.

If a cone electrode be used, the evaporation of the
solution may be avoided, and in this case one merely fixes
a fresh electrode in the beaker containing the electrolyte
and the washings, after addition of the necessary amount
of nitric acid, arid proceeds with the electrolysis with a
stronger current. If the E.M.F. be allowed to exceed that
mentioned above, an alloy of the two metals will be

1 Zeitschr.f.phys. Chem. 1893, 12, 197.

2 Berg- u. Hiittenzeitg. 1883, 375.

SEPARATION OF METALS 183

obtained at the kathode. This method of separation is
simple, and yields very satisfactory results.

Smith and Frankel 1 and Smith and Spencer 2 have used
solutions of the double cyanides for this purpose. In order
to prepare such a solution, between 1 and 2 grms. of pure
potassium cyanide is added to the neutral solution of the
two salts, which must be present in amount equal to '40 to
50 grm. metal, and after dilution to the usual volume
(150 c.cms.) the electrolysis is carried out with currents up
to "10 ampere in density.

If the deposition be carried out at the normal tempera-
ture from eight to twelve hours will be requisite to effect
the complete separation of the silver ; if the solution be
heated to 60 C., the whole of the silver can be deposited
in four hours. Freudenberg has found that the E.M.F.
must not be permitted to exceed 2-3 to 2*4 volts, otherwise
the copper will be deposited with the silver ; a fact also
discovered by Smith and Frankel in experiments carried on
with very small current strengths. The addition of potas-
sium cyanide may certainly be increased in amount. Under
the conditions given, the deposit of silver is obtained per-
fectly free from copper. The latter metal is deposited from
the remaining electrolyte simply by increasing the current
density. The details of the procedure will be found under
1 Copper.' -,

Classen has recommended his oxalate method for effect- VO
ing the separation of copper and silver. The addition of ip
ammonium oxalate solution to a solution of the salts of
these two metals produces precipitates of their oxalates, one /
only of which copper oxalate is soluble in excess of the /
reagent. The separation is therefore not an electrolytic /
but a chemical one, and there is the disadvantage that the /
deposition of copper from its double oxalate solution is/
never satisfactory. In order to effect the electrolytic depo-/

1 Amcr, Chem. Jour. 12, 104.

* Zeitschr. f. Elektrochcm. 1894, 542 ; Elektroclwm. Zeitschr,
1894, 180.

184 THE ELECTKOLYTIC PROCEDURE

sition of the silver, the silver oxalate precipitate must be
dissolved in potassium cyanide, and the solution thus ob-
tained then electrolysed in the usual manner.

A further disadvantage of this method is that the pre-
cipitate of silver oxalate carries with it some copper, which
cannot be redissolved by the ammonium oxalate. It
follows therefore that this method stands far behind the
two methods first described as regards simplicity and ac-
curacy.

The electrolytic method of separating copper and silver
could receive a practical application in the analysis of
silver coining- metal, and this use of it will receive further
notice in Part III., D.

Copper from Quicksilver. According to Smith, the
separation of these two metals can be effected in solutions
of the double salts with potassium cyanide. 1 In order to
carry out such an electrolysis from '50 to I'O grm. copper
sulphate, and at the most '50 grm. mercuric chloride, are
dissolved in water, and to this solution from 2-0 to 4'0
grms. pure potassium cyanide are added. The clear solution
after dilution to the usual volume is electrolysed with a
current density of from -06 to '08 ampere.

At the normal temperature about sixteen hours will be
found necessary to effect the separation of the mercury ; if
the electrolyte be heated to 60 C., the deposition of this
metal can be effected in from three to four hours. The
mercury separates as a dead silver-white deposit, and is
perfectly free from copper if, as Freudenberg has pointed
out, the E.M.F. be kept below 2-5 volts. If the E.M.F.
should exceed this limit, copper will separate with the
mercury at the kathode.

The copper may be directly deposited from the cyanide
solution by use of a stronger current, or the cyanides may
be destroyed by heating with sulphuric acid (under a
draught-hood), and the copper then separated electrolyti-
cally from the resulting sulphuric a*id solution.
1 Electrolyse.

SEPAEATION OF METALS 185

Copper from Bismuth. The separation of these two
metals in a solution containing nitric acid is not possible,
since the ' decomposing values ' of their salts lie too closely
the one to the other. Smith effects their separation by
using a solution of the bismuth salt to which 3 or 4 grms.
citric acid have been added, followed by an excess of
sodium hydrate solution.

The separately prepared solution of the double cyanide
of copper and potassium is then added to this mixture.
Bismuth does not form a double cyanide. During the
electrolysis of this solution of the mixed salts, the E.M.F.
must not be allowed to exceed 2 '7 volts, otherwise the
bismuth deposit will contain copper.

On account of the very small current density about
05 ampere which it is possible to employ, not more than
20 grm. bismuth can be deposited in twenty-four hours.

Copper from Arsenic. The earlier experimenters who
attempted to separate these two metals in acid solutions
always obtained deposits of copper contaminated with
arsenic. Freudenberg was the first to show that a copper
deposit perfectly free from arsenic could be obtained from
a solution containing between 10 and 20 c.cms. dilute
sulphuric acid, if an E.M.F. of 1*9 volts be not exceeded
for production of the current passed through the electrolyte.
Though similar results could be obtained with nitric acid
solution, those containing sulphuric acid are to be given
the preference. According to Drossbach, 1 McKay, 2 and
Oettel, 3 copper can also be obtained as a bright metallic
deposit, completely free from arsenic, by use of very feeble
currents, and ammoniacal solutions of copper (see under
Copper,' p. 98) in which the arsenic may be present either
in the arsenious or arsenic form of combination. The
explanation of this lies in the fact that arsenic is an
element which possesses both basic and acid properties in
its combinations, and that in alkaline solutions the arsenic

1 Client. Zeitn. 1892, 819. - Ibid. 1890, 509.

; < Ibid. 1894, 879.

186 THE ELECTROLYTIC PROCEDURE

forms part of the complex anion, and does not exist
separately as an ion.

Arsenic acid and the arsenates especially are only re-
duced with difficulty, and on this account it is advisable to
convert any arsenic that may be present into this form of
combination by means of nitric acid, and afterwards to
make the solution alkaline. The oxidised solution of the
two metals is treated with 30 c.cms. ammonium hydrate, and
this alkaline solution is then decomposed by a current, the
E.M.F. of which should not exceed 1'9 volts. The deposi-
tion of the copper will require from six to eight hours.

The deposit obtained in this way will be perfectly bright,
whereas a deposit of copper containing arsenic has always a
more or less dirty or black appearance. The arsenic that
remains in the electrolyte after complete separation of the
copper must be determined by gravimetric methods of
analysis, since no electrolytic method is applicable for this
metal.

Smith has stated that a precisely similar method of
separation to that just described for ammoniacal solutions
is applicable to potassium cyanide solutions. The mixed
solution of the copper salt and the alkali metal arsenate is
treated with an excess of potassium cyanide, and after
dilution is electrolysed with a very feeble current.

A deposit of copper quite free from arsenic is also
obtained in this case.

Copper from Antimony and Tin. If one attempts to
separate these metals by means of a current of moderate
density in an acid solution, it will be found that both metals
are deposited simultaneously at the kathode. Copper is,
however, deposited alone when the E.M.F. does not exceed
1*8 volts ; and it is therefore possible to obtain a deposit of
copper free from antimony, especially when the amount of
the latter metal present is small, if the electrolysis be not
allowed to continue for too long a period that is, if it be
stopped as soon as the whole of the copper is removed from
the solution. Such a method of separation cannot, however,

SEPARATION OF METALS 187

be recommended. Antimony is a metal that resembles
arsenic in its behaviour in alkaline solutions. One can
therefore separate copper and antimony in an ammoniacal
solution, especially if the precaution be taken to raise the
antimony salts present to the higher stage of oxidation by
means of nitric acid. If this precaution be omitted, anti-
mony may be deposited at the kathode with the copper, as
in the case of an antimony trichloride solution, which has
simply been treated with excess of ammonium hydrate.
Schmucker has effected the separation by this method, by
using an oxidised solution of the two metals to which 8
grms. tartaric acid and 30 c.cms. ammonium hydrate have
been added. l The electrolysis is carried out with a current
of '10 ampere density, and five hours is requisite to deposit
10 grm. copper. The latter metal will be obtained quite
free from antimony.

This method may also be used to effect the separation
of copper and tin. The separation of copper from antimony
and tin by electrolytic methods is not, however, of any
technical importance, since this separation is so easily
effected by the chemical method with nitric acid.

LEAD

Lead from Copper. (See p. 177.)

Lead from Silver. Lead is deposited as peroxide at the
anode from solutions" containing free nitric acid ; and it is
therefore possible to effect separation of lead from those
metals which are deposited at the kathode from such solu-
tions. The method of separation of copper from lead is
based upon this principle, but when one employs a similar
method to effect the separation of silver and lead the silver
exhibits a disturbing characteristic, in that under certain
conditions it separates partly at the anode as peroxide.
This, of course, prevents any quantitative separation of the
two metals, for the lead peroxide deposit is contaminated
with silver.

1 Jour. Amcr. Chem. 15, 195.

188 THE ELECTEOLYTIC PROCEDURE

Since it is possible, however, under certain conditions
(see ' Silver 3 ) to obtain deposits of this metal by use of
nitric acid solutions without any separation of silver per-
oxide, investigations have been made to discover the condi-
tions which regulate the deposition of the silver in presence
of lead. Luckow has stated that these are the presence of
at least 18 per cent, free nitric acid, and the addition of a
small amount of oxalic acid. l With such an electrolyte the
lead peroxide obtained at the anode is free from silver.
Smith and Moyer state that if 15 c.cms. nitric acid be present
to each 180 c.cms. of the solution, and if a feeble current
be used, equally good results may be obtained. In spite of
these results this method of separation for silver and lead
cannot be regarded as absolutely trustworthy.

Lead from Bismuth. In the attempts that have been
made to effect the electrolytic separation of these two
metals, the same phenomena are found to occur as in the
case of silver and lead. The bismuth is deposited always,
partly as metal at the kathode, and partly as peroxide at
the anode. No separation is therefore possible in a nitric
acid solution ; the lead peroxide deposit, according to Classen
and Ludwig, 2 and Smith and Moyer, 3 always contains
bismuth.

Lead from Mercury. Although in nitric acid solu-
tions lead is always deposited as peroxide, while mercury
is always obtained as metal, it is not always possible to
effect a separation of these two metals in such a solution.
If less than a 15 per cent, excess of free nitric acid
be present, and if moderately strong currents be used,
Smith and Moyer state that part of the lead will be
deposited as an amalgam at the kathode. 4 According to
Heydenreich. the conditions necessary in order to obtain a
complete separation of lead and mercury are the presence
of between 20 and 30 c.cms. free nitric acid in the 150 c.cms.

1 Zeitschr. f. angew. Chemie, 1890, 345.

2 Berichte, 19, 326.

. 3 Jour.f. anal. u. appl. Chem. 1893, 7, 252.
1 Ibid. 1803, 7, 252 ; Zeitschr. f. anorg. Chem. 4, 267.

SEPAEATION OF

volume of the electrolyte, and the use of a current of about
20 ampere in density. l

Lead from Arsenic. If a solution containing lead
nitrate, a soluble salt of arsenic acid, and free nitric acid be
electrolysed, the separation of the lead as peroxide at the
anode will be found to be nearly always incomplete.

The deposit obtained during the same time at the
kathode will be a mixture of arsenic and lead, while another
portion of the arsenic will be evolved at the kathode surface
as arseniuretted hydrogen.

The greater the amount of arsenic in the solution, the
less will be the amount of lead separable at the anode
as peroxide ; the excess of nitric acid present also affects
this result. If the electrolysis be continued, after all the
lead has been separated from the solution either as metal
at the kathode or as peroxide at the anode, part of the
former redissolves and migrates to the anode, where it is
deposited as peroxide ; but the author has made experi-
ments which prove that the separation of the lead at the
anode is never complete. 2

Lead from Manganese. From acid solutions manganese
separates as peroxide at the anode, and, since lead yields a
similar deposit in nitric acid solutions, one would surmise
that the electrolysis of a solution of the mixed salts of
these metals containing free nitric acid would yield a
mixed deposit of peroxides at the anode. This is found by
experiment to be the case. If the excess of nitric acid
added to the electrolyte be, however, over 4 per cent.,
no deposition of manganese peroxide occurs at the anode ;
in place of this there is a formation of permanganic acid,
recognisable by the pink coloration which it produces
round the anode. If a solution of the two salts containing
about 20 per cent, nitric acid be electrolysed at the normal
temperature by means of a weak current, the deposition of
lead peroxide will take place slowly, and the solution will

1 Zeitschr. f. Elektrochem. 1896, 3, 151.

2 Chem. Zeitg. 1896, 20, No. 39.

190 THE ELECTROLYTIC PROCEDURE

remain colourless. If, however, the electrolyte be heated
to 60 or 70 0., and the electrolysis be carried out with a
current of from 1-5 to 2*0 amperes in density (E.M.F.
2-5 to 2-7 volts), the whole of the lead will be deposited as
peroxide in a short time, and the liquid will assume a rose
colour owing to the formation of permanganic acid and its
salts. The method yields approximately accurate results
when carried out as described above, and when the amount
of manganese present does not exceed '03 grm. for 150 c.cms.
of the electrolyte. If the manganese present exceeds this
amount, or if the electrolysis be permitted to continue for
too long a time, the author has found that a flocculent pre-
cipitate of a hydrated manganese peroxide is formed, and
that the lead peroxide deposit is no longer free from the
other metal. 1

Lead from Zinc, Iron, Nickel, Cobalt, and Cadmium.
Lead can be separated in a very simple manner from all
those metals placed above hydrogen in the list given on
p. 35 which cannot be deposited in acid solutions. The
solution of the mixed salts simply requires to be acidified
with 15 to 20 per cent. cone, nitric acid, and to be electro-
lysed with the current conditions given under ' Lead ' (see
p. 128).

Lead peroxide will be deposited at the anode, while the
other metal remains in solution. The separation is com-
plete. After deposition of the whole of the lead, the
remaining liquid, which will still contain much free nitric
acid, is treated with the chemical reagents necessary to
produce the salt of the metal present, that is recommended
for use under the single metal separations. In few cases
only is neutralisation sufficient ; and a conversion of the
nitrates into sulphates will be found to be necessary in the
greater number of instances.

1 Chem. Zeitg. 1896, 20, No. 39.

SEPARATION OF METALS 191

SILVER

Silver from Copper. (See p. 181.)

Silver from Lead. (See p. 187.)

Silver from Bismuth. Freudenberg has stated that if
a solution of the nitrates of silver and bismuth (about *30
grm. each metal) be treated with 2 to 3 c.cms. nitric acid,
and, after addition of 2 to 4 grms. ammonium nitrate and
dilution to 150 c.cms., the mixture be electrolysed with a
current the E.M.F. of which does not exceed 1'3 volts, an
electrolytic separation of these two metals will be obtained.
If the electrolysis be permitted to continue through the
night, '30 to '40 grm. silver may be easily deposited. The
remaining electrolyte which contains the bismuth is used
for the deposition of the latter by the amalgam method.

Silver from Mercury and Gold. This separation
cannot be effected either by the use of nitric acid solutions
or by the use of cyanide solutions, since the ' decomposition
values ' of these salts of the concerned metals lie too close
one to the other.

The electrolytic determination of silver and mercury
may, however, be carried out as follows. The two metals
are deposited together from a solution at the normal
temperature by means of a current of *50 ampere density.
The E.M.F. required will lie between 1*7 and 2'2 volts;
and for '30 grm. silver about four and a half hours will be
necessary to effect complete deposition. After drying, the
weight of the combined metals on the electrode is deter-
mined ; the mercury is then driven off by ignition, and the
weight of the remaining silver obtained. The deposit of
the two metals is grey in colour and spongy in character ;
but in spite of this the method yields correct results.

Silver from Antimony and Arsenic. The separation of
silver from these metals is possible by electrolytic methods
if solutions containing free nitric and tartaric acids be used,
and if the antimony and arsenic present be previously
raised by chemical methods to the higher stage of oxidation.

192 THE ELECTROLYTIC PROCEDURE

Under these conditions it is not safe, however, to exceed an
E.M.F. of 1-5 volts.

The deposit of silver obtained is not very well suited
for correct weighing. In the case of arsenic the E.M.F.
used may be slightly greater than in the case of antimony ;
but 1*7 volts must not be exceeded even in this case.

Silver may also be separated from arsenic and antimony
in a solution which contains free ammonium hydrate and
ammonium sulphate, since from such a solution silver can
be deposited by means of an E.M.F. of 1'20 or 1-30 volts.
This low E.M.F. causes, however, the deposit of silver to
be but loosely adherent to the platinum basin.

The separation of these metals may also be carried out
with solutions containing 1*0 grm. pure potassium cyanide
for each "10 grm. metal present. Freudenburg has stated
that from such a solution the silver can be obtained as a
firmly adherent deposit. The E.M.F. used may be some-
what higher than in the case of the nitric acid solution, but
it must not exceed 2-4 volts. As before, it is best to raise
the arsenic and antimony to the higher stage of oxidation
before commencing the electrolysis. Smith states that the
separation of silver from these metals by this method suc-
ceeds perfectly when tartaric acid is used in excess in the
solution. l

Silver from Platinum and Palladium. In order to
effect the separation of silver from the first of these metals,
the solutions of their mixed salts is neutralised, and after
the addition of 2 or 3 grms. potassium cyanide it is electro-
lysed with a current, the E.M.F. of which does not exceed
2-50 volts.

The separation of silver from palladium is not possible
in this way.

Silver from Cadmium, Zinc, Cobalt, Nickel, and

Iron. The separation of silver from the metals which

cannot be deposited in an acid solution, of which those

named above are examples, is conveniently carried out by

1 Amer. Chem. Jour. 12, 428.

SEPAKATION OF METALS 193

electrolysis of the solution of the mixed salts, after acidify-
ing with nitric acid.

The E.M.F. used should lie between 2'0 and 2 "2
volts.

Solutions of the double cyanide salts containing an
excess of potassium cyanide (2 to 3 grms. pure KCN) may
also be used to effect the separation of silver from the
metals named above.

This method is to be preferred to that first described
since the deposit of silver obtained by it is more satisfactory.
The separation of silver from zinc can be effected in such
a solution if the E.M.F. used does not exceed 2*5 volts.
The current density possible with this E.M.F. is between
05 and '08 ampere ; the temperature should be 60 C.
The same conditions apply in the separation of silver from
nickel, but in this case if the electrolysis be allowed to con-
tinue for too long a period nickel may be deposited with
the silver. The presence of a cobalt salt in the electrolyte
renders it more difficult to effect the separation of the silver ;
and in this case the E.M.F. used may rise to a maximum of
2*7 volts. In the case of cadmium the ' decomposition values '
of the two double cyanide salts lie very near together, and in
order to obtain the silver free from cadmium it is necessary
to use an E.M.F. of only 1'9 volt. The current obtained
by use of this E.M.F. will be only -04 ampere in density. .

In all these cases it is best to use the solutions at a
temperature of between 50 and 60 C. In order to deter-
mine by electrolytic methods the amount of the second
metal in the solution, it is necessary after complete deposi-
tion of the silver to treat the remaining electrolyte with
sulphuric acid under the draught-hood, and then to apply
the method which is most strongly recommended for the
concerned metal in Part III., B. .

MERCURY

This metal is Closely .related to silver in its electrolytic
characteristics ; and, its separation from the other metals

194 THE ELECTEOLYTIC PROCEDURE

is effected by methods very similar to those used for silver.
In nitric acid solutions an E.M.F. of only 1'3 volts suffices
to produce a deposit of mercury.

Mercury from Copper. See p. 184.

Mercury from Lead. See p. 188.

Mercury from Silver. See p. 191.

Mercury from Bismuth. In spite of all assertions to
the contrary, the separation of these two metals can be
effected in solutions of their nitrates containing an excess
of nitric acid, if, as Freudenberg has pointed out, the
E.M.F. of the current used does not exceed 1'30 volt.

Although the current density obtained with this
E.M.F. is extremely small, and as a consequence the time
demanded for the electrolysis is rather long, the method is
a practicable one. The deposit of'mercury obtained is not
composed of minute globules, but is a smooth metallic
coating.

If stronger currents be used the two metals will be
simultaneously deposited as an amalgam, a fact which is made
use of in the electrolytic method for determining bismuth.

Mercury from Arsenic and Antimony. The separation
of mercury from these two metals can be effected, if the
electrolysis be carried out with solutions containing free
nitric acid by means of currents, the E.M.F. of which does
not exceed 1*8 volts. A solution of the mixed salts con-
taining tartaric acid and an excess of ammonium hydrate
may also be used, if the arsenic and antimony are present
in the form of their higher oxides.

The current conditions in this case are as above. In
order to carry out such a separation, the chlorides are
dissolved with the addition of 1 grm. tartaric acid,
the solution is diluted, and after neutralisation with
ammonium hydrate a further 20 c.cms. of this reagent is
added. The mixture is then electrolysed by a current the
E.M.F. of which is kept between 1'60 and 1'70 volts.

In order to determine the antimony in the electrolyte
remaining when the first method is used, the excess of

SEPAEATION OF METALS 195

nitric acid must be carefully evaporated, and sulphuretted
hydrogen then passed through the diluted solution. The
precipitate of antimony pentasulphide is then dissolved in
sodium sulphide, and the electrolysis of the resulting solu-
tion is conducted as described under ' Antimony.'

Mercury from Tin. It was stated under ' Mercury '
that it was possible to completely deposit that metal from
the alkaline solutions of its sulphide in sodium sulphide,
while under ' Tin ' it was noted that the latter metal could
not be deposited from such solutions. A method of separa-
tion may therefore be based upon this difference.

If both metals be present in solution in presence of free
alkali and excess of sodium sulphide, it is merely necessary
to employ the current conditions given under ' Mercury '
(see p. 142) in order to obtain a complete separation.
The remaining electrolyte containing the tin must be
boiled with 30 grms. ammonium sulphate, in order to con-
vert the sodium sulphide into ammonium sulphide before the
deposition of the tin can be proceeded with. This method
of depositing tin will be found more fully described under
* Antimony and Tin 5 (see p. 201).

A separation of mercury from tin can also be effected
by the method with tartaric acid and ammonium hydrate
described under * Mercury and Antimony.' In this case, a
few grams tartaric acid and 30 c.cms. ammonium hydrate
are added to the mixed salts solution, and the electrolysis
is carried out with a feeble current and an E.M.F. not
exceeding 1*70 volts.

Mercury from Gold. This separation can only be
effected in solutions containing an excess of potassium
cyanide by means of currents the E.M.F. of which does
not exceed 1*90 volts. It is also necessary that the
electrolysis should be stopped when all the mercury is
deposited, otherwise the mercury deposit will be found to
contain some gold. Smith states that the deposition of the
mercury under these conditions is extremely slow. !

1 Amcr. Chcm. Jour. 11, 264, 352 ; 12, 428 ; 13, 417.

o 2

196 THE ELECTROLYTIC PROCEDURE

Mercury from Palladium, Platinum, and Osmium. If
the solution of the salts of any one of these metals and
mercury ('20 grm. of each metal) be treated with an excess
of potassium cyanide, and then be electrolysed with a
current of '20 ampere in density, a separation of the metals
will be found to occur.

The mercury will be obtained as a deposit at the
kathode, whereas the other metal will remain in solution.
According to Smith, 1 and to Smith and Frankel, 2 the sepa-
ration requires from fourteen to sixteen hours.

Mercury from Manganese. From a solution containing
free sulphuric acid, mercury can be separated as a metal
and manganese as peroxide. This form of solution can
therefore be used to effect the separation of these two metals.
The electrolysis is carried out under the conditions described
under ' Manganese.' It is, however, necessary to note here
that only very small amounts of either metal must be present
in the solution, on account of the tendency of large amounts
of manganese to separate from such solutions in a non-adherent
form at the anode. The use of the larger electrode surface
as anode does not remove this difficulty. With regard to
mercury, the deposition of larger amounts is attended by a
running together of the minute globules and formation of small
balls, which are easily detached from the kathode surface.

Mercury from Iron, Cadmium, Nickel, Cobalt, and
Zinc. The separation of mercury from these metals can be
effected by the method used to separate copper and silver
from the same group of metals namely, by the electrolysis
of a nitric acid solution. The E.M.F. used to effect the
deposition of the mercury in such a solution may rise to a
maximum of 2'4 volts. The deposition occurs easily, and
the separation is complete.

The remaining solution after deposition of the whole of

the mercury should be treated with sulphuric acid in order

to convert the nitrates into sulphates, since only in few

cases can a nitrate solution be used without harmful results

1 Amer. Chem. Jour. 11, 264, 352 ; 12, 428 ; 13, 417. - Ibid. 12, 428.

SEPAKATION OF METALS 197

for the electrolytic determination of the above -named metals.
For the details of this treatment with sulphuric acid,
see p. 169.

A solution containing tartaric acid and ammonium
hydrate may also be used to effect the separation of these
metals, in place of the nitric acid solution. The E.M.F.
required is practically the same as that named above.

The fact that mercury can be deposited in very satis-
factory form from solutions containing an excess of
potassium cyanide by means of almost any E.M.F. or
current density has already been noted ; and the solution
of the double cyanide salt may also be used to effect the
separation of mercury from the metals named above.

The ' decomposition value ' of the double cyanide salt of
mercury and potassium is equivalent to about 1'60 volts ;
and the E.M.F. of the current used to effect the separation
may be allowed to rise to 2 '50 volts without any of the other
metals named being deposited with the mercury at the
kathode.

If from 2 to 3 grms. pure potassium cyanide be added
to the neutral solution of the mixed salts, a current of '08
ampere density can be obtained from the above E.M.F.
The solution is heated to 50 or 60 C. before electrolysis,
and the time required to deposit '50 grm. mercury is
between five and six hours.

When cobalt is present the time required to deposit
the mercury is increased. When using the double cyanide
solution for the separation of mercury and cadmium it is
necessary to keep the E.M.F. used for the electrolysis at
from 1-80 to 1'90 volts, and in this case the deposition
of the mercury is conveniently carried out at night. The
separation from cadmium in an acid solution demands,
however, less time and attention.

Mercury may also be separated from aluminium,
magnesium, and the alkali metals in acid or cyanide
solutions by use of any E.M.F., or current density, that
may be thought suitable by the experimenter.

198 THE ELECTEOLYTIC PROCEDURE

GOLD

Gold from Silver. See p. 191.

Gold from Mercury. See p. 195.

Gold may be deposited from a hydrochloric acid solution
by means of a current with an E.M.F. of only one volt, but
the deposit so obtained is not very adherent, and it is much
the better plan always to make use of the double cyanide
solution. This on electrolysis yields an even and bright
coating of the metal upon the kathode surface.

The ' decomposition value ' of the double cyanide of gold
and potassium is somewhat higher than those of the corre-
sponding salts of silver and mercury. In such a solution
the separation of gold from zinc, copper, nickel, cobalt, and
iron may be effected by means of a current the E.M.F. of
which does not exceed 2-5 volts.

If the amount of each metal present be about -10 grm.,
and if from I'O to 2'0 grms. pure potassium cyanide have
been used in preparing the solution, a current density of
between -05 and '10 ampere may be used. According to
Smith and Wallace, 1 from three to three and a half hours
will be requisite to complete the deposition. If a lower
current density be used, it is perfectly feasible and safe to
allow the electrolysis to continue through the night.

Smith and Muhr 2 state that gold may also be separated
from palladium, platinum, and osmium by this method.
In this case, from 2 to 2| grms. potassium cyanide are used
with a current density of about -05 ampere, and the time
required is between twelve and fourteen hours.

Gold can also be deposited from a sodium sulphide
solution, and this fact renders it possible to effect a separa-
tion by electrolysis of gold and arsenic.

A similar method cannot be applied to effect the separa-
tion of gold from antimony and tin. It was hoped that
as gold cannot be deposited from an ammonium sulphide

1 Jour. Amer. Chem. Soc. 1895, 17, 612.

2 Berichte, 1891, 2171.

SEPARATION OF METALS 199

solution, a separation from tin would be effected by electro-
lysis of such solutions ; the experiments, however, did not
yield successful results. Arsenic may not only be sepa-
rated from gold in the sulpho-salt solution described above,
but such a separation is also possible in the double cyanide
solution.

The separation of gold and antimony may be effected by
adding from -50 to 1*0 grm. tartaric acid, and then excess
of potassium cyanide to the solution of the mixed metal
salts. The antimony remains in solution when this mixture
is electrolysed, and the gold deposit is obtained perfectly
free from antimony.

PLATINUM

Platinum from Silver. See p. 192.

Platinum from Mercury. See p. 196.

Platinum from Gold.- See p. 198.

The electrolysis of chloroplatinic acid can be effected
with an E.M.F. of only 1*1 volts, but this compound
requires for its complete decomposition at least 1'5 volts.
Since the solutions of mercury, gold, copper, and antimony
containing hydrochloric acid require an E.M.F. of only 1'6
volts to produce electrolytic decomposition, this method is
not adapted for separating platinum from these metals. In
the case of arsenic and tin the separation is possible, but
even for these metals it is not completely satisfactory.
The separation of platinum from arsenic can be effected by
means of the same E.M.F. in a sulphuric acid solution.

Platinum may be separated from the metals nickel,
cobalt, iron, cadmium, and zinc, in any acid solution, by
means of a current the E.M.F. of which lies between
1-8 and 2'0 volts, and the current density between -07 and
08 ampere.

In solutions containing excess of potassium cyanide, the
platinum is so strongly held in combination that the separa-
tion from silver, gold, and mercury is easily effected. The
platinum remains in solution, while the other metals are

200 THE ELECTROLYTIC PROCEDURE

deposited. These separations are described under the
headings of the various metals. (See above.)

BISMUTH

Bismuth from Copper. See p. 185.

Bismuth from Lead. See p. 188.

Bismuth from Silver. See p. 191.

Bismuth from Mercury. Seep. 194.

Bismuth can be deposited from either nitric or sulphuric
acid solutions, and it follows from this that it may be
electrolytically separated from iron, nickel, cobalt, zinc,
and cadmium. These separations, however, suffer from the
disadvantages noted under ' Bismuth.' The deficiencies of
the direct method of deposition of this metal may, however,
be overcome by adding a weighed amount of a mercury
salt to the solution of the salts of the two metals whose
separation is required. The bismuth is then obtained as an
amalgam at the kathode ; and a complete separation from
the other metals is possible if the conditions noted as
requisite for the separation of mercury from nickel, cobalt,
iron, zinc, and cadmium are maintained during the electro-
lysis (see p. 196).

Bismuth may be separated from arsenic, but not from
antimony, by use of an E.M.F. of 1 '9 volts with a sulphuric
acid solution.

ANTIMONY

Antimony from Copper. See p. 186.

Antimony from Silver. See p. 191.

Antimony from Mercury. See p. 194.

Antimony from Gold. See p. 199.

Antimony from Arsenic. These two metals behave
alike in alkaline or hydrochloric acid solutions, when
present in the lower state of oxidation. If, however, both
are present in forms equivalent to the pentoxide, the
antimony alone can be electrolytically deposited. This
method for separating these two metals is, however, not

SEPAKATION OF METALS 201

practicable with hydrochloric acid solutions, as the antimony
deposited possesses many unsatisfactory characteristics.
The best solution to use for effecting this separation is that
of the sulpho- salts prepared by use of excess of sodium sul-
phide. In order to carry out such an electrolytic separa-
tion of arsenic and antimony, 1 grm. each of tartar-emetic
.and of sodium arsenate are dissolved in water, and to the
solution 1 to 2 grms. sodium hydrate and 50 c.cms. of a
saturated solution of sodium sulphide are added. The
mixture is diluted to 150 c.cms. heated to between 50 and
70 C., and electrolysed with a current of from I'O to I'D
amperes in density. The E.M.F. required will lie between
1*7 and 2 - volts ; the time will be from one and a half to
two hours. If it be desired to use currents of from -30 to
40 ampere in density, and to allow the electrolysis to run
during the night, it is necessary to direct attention to the
irregularities that may arise from the formation of polysul-
phides. When using this method antimony is obtained as
a silvery grey deposit, while arsenic remains in solution.
In order to determine the latter, the solution remaining
after deposition of all the antimony is decomposed with
sulphuric acid, the precipitate of arsenic pentasulphide and
sulphur is treated with hydrochloric acid and potassium
chlorate, and the arsenic determined by the gravimetric
method as the ammonium and magnesium salt. The above
method for effecting separations of antimony from arsenic is
frequently used in technical laboratories, in place of the incon-
venient and troublesome gravimetric method of analysis.

Antimony from Tin. While antimony is easily depo-
sited from a concentrated sodium monosulphide solution,
tin cannot be separated from such a solution by electrolysis
until it has been strongly diluted, and upon this difference
a method of separation has been based. In order to
carry out such an electrolysis, 1 grm. tartar-emetic and
50 grm. stannous chloride, or in its place 1 grm. of the
double chloride of tin and ammonium, are dissolved in
water, and to this solution 1 to 2 grms. sodium hydrate and

202 THE ELECTKOLYTIC PKOCEDUKE

50 c.cms. of a cold saturated solution of sodium monosulphide
are added. The mixture after dilution to the usual volume
is heated to the boiling point or at least to 60 to 70 C.,
and is electrolysed at this temperature with a strong current
of from 1-0 to 1-5 amperes density. The E.M.F. required
will lie between '90 and 1'7 volts, and to separate the
antimony contained in 1 grin, tartar-emetic from one and a
half to two hours will be demanded. The deposit will be
bright and steel grey in colour, and it will be found best
to employ a roughened electrode as kathode. Using cur-
rents of 1*0 ampere it is possible to deposit '16 to "20 grm.
antimony per hour from such solutions, and this fact is im-
portant, because by its aid one can approximately calculate
the duration of the electrolysis.

This must not be allowed to continue longer than is
required for the complete deposition of the antimony, since
long- continued electrolysis of these solutions produces poly-
sulphides, and from solutions containing these tin may be
deposited with the antimony. 1 For this reason it is not
advisable to attempt this separation at the normal tempera-
ture by means of feeble currents during the night ; under
such conditions one is almost certain to obtain a deposit of
antimony containing tin. The method first described, in
which a strong current arid a high temperature are employed,
has, on the other hand, been frequently used in technical
laboratories, and has been found to give absolutely correct
results. If these are not attained by others, the failure
can only be ascribed to their non-observance of some of the
requisite conditions. The remarks made under 'Antimony '
concerning the preparation of the sodium sulphide, the treat-
ment of the deposit, &c., apply in this case also (see p. 145)
If one has made use of a platinum basin electrode for

1 When tin is present in very small amounts only, the separation
of antimony and tin by means of strong currents is sufficiently
accurate for practical purposes. When the tin is present in larger
amount it deposits with the antimony if currents over -30 ampere in
density be used, since the E.M.F. of such currents will exceed that
required to decompose the tin sulphide namely, T20 to 1-30 volts.

SEPARATION OF METALS 203

receiving the deposit of antimony, it is necessary to displace
the remaining electrolyte by water before breaking the
circuit. This electrolyte, freed from antimony, cannot be
used directly, however, for the determination of the tin. It
is first requisite to reduce it by evaporation to a volume of
about 150 c.cms., and to convert the sodium sulphide into
ammonium sulphide by the aid of 25 to 30 grms. ammonium
sulphate and fifteen to thirty minutes' boiling. The end
of this reaction is indicated by the brown colour that
the liquid assumes, and by its smell. When it is com-
pleted, the solution is made up to the usual volume, and
the deposition of the tin effected at a temperature of 70
C., with a current of from 1*0 to 2'0 amperes in density.
The E.M.F. required will be from 3-3 to 4-0 volts, and the
time about one hour.

From this solution between -30 and '40 grm. tin can be
deposited per hour by means of a current of one ampere.

The deposition of the tin may also be undertaken at the
normal temperature instead of at 70 C. The deposit is
bright and of a greyish colour ; its further treatment has
already been described under ' Tin ' (see p. 151). In carry
ing out this separation, if the instructions given above
relative to the amount and the degree of concentration of
the sodium sulphide solution have been exactly followed,
the electrolysis of the solution will yield a yellow deposit
at the anode composed of sulphur alone ; if, however, too
little sodium sulphide has been employed, both antimony
and tin sulphides will be deposited with the sulphur at the
anode, and these will, in most cases, not pass into solution
again during the electrolysis. If, instead of the salts
named above, a mixture of the two sulphides of antimony
and tin similar to that obtained in the ordinary course of
analysis be used, the preparation of the solution and its
electrolysis are carried out exactly as described above. An
alteration in the method is only called for should the pre-
cipitate of the mixed sulphides be suspectedioiiQniam much
free sulphur. .^I^LSE L -'Sft4/J^V

OF THB '^

DIVERSITY }

204 THE ELECTROLYTIC PROCEDURE

If this were the case, a sodium polysulphide solution
would be formed, and the separation of the antimony would
be incomplete. A similar solution is formed when substances
containing antimony and tin are opened up by fusion with
dehydrated sodium hyposulphite, or with soda and sulphur.

In these cases the solution containing the poly sulphides
is treated with excess of anammoniacal solution of hydrogen
peroxide, and is warmed. The sulphur is oxidised, and the
solution becomes colourless.

The resulting liquid is evaporated to a small volume, 50
c.cms. sodium monosulphide is again added, and the sepa-
ration of the antimony and tin is carried out as already
described.

The electrolytic method for effecting the separation of
antimony and tin excels the ordinary analytical methods of
separation for these two metals in both simplicity and
speed. On this account the method has been very widely
made use of in technical and analytical laboratories. 1

CADMIUM

Cadmium belongs to that group of metals the 'de-
composing values ' of whose salts lie above those of the
corresponding salts of hydrogen or acids.

It occupies, however, a distinct place in this group,
since, unlike iron, cobalt, nickel, and zinc, it may be
deposited from a solution containing a small amount of free
sulphuric acid. It is also much more easily deposited from
the double cyanide solution than the other metals of the
group. The behaviour of this metal on electrolysis finds
an analogy in its behaviour towards sulphuretted hydrogen.

The separation of cadmium from those metals which can
be deposited in acid solutions is most satisfactorily under-
taken with nitric acid solutions. The separation of cadmium
from the metals iron, cobalt, nickel, zinc, is more difficult.

1 [Waller states in Zeits. /. Electrochem. 4, 247, that if an E.M.F.
of '70 volt be exceeded, Tin will be deposited with the Antimony.
Translator's note.']

SEPARATION OF METALS 205

Cadmium from Copper. See p. 174.

Cadmium from Lead. See p. 190.

Cadmium from Silver. See p. 192.

Cadmium from Mercury. See p. 196.

Cadmium from Zinc. The separation of cadmium from
zinc, a meta] to which it is closely allied, is possible in the
solution of the double cyanides.

In order to carry out such a separation, the neutral
solution of the sulphates of the two metals is treated
with between 4 and 5 grms. pure potassium cyanide,
diluted to about 150 c.cms., and electrolysed with a
current the E.M.F. of which, according to Freudenberg ]
and Smith and Frankel, 2 must not exceed 2'6 volts.
The deposition of the cadmium occurs exceedingly slowly,
and eighteen to twenty hours are requisite for '30 grm. of
the metal. The deposit exhibits the silvery white colour
of the metal.

The zinc remains in solution under the current conditions
named, but by use of a stronger current it may be deposited
directly from the same solution upon a new kathode. One
may also convert the double cyanide of zinc salt into some
other form suited for the deposition of this metal, but the
first method is the simpler.

Since cadmium can be deposited from a solution which
is slightly acidified with sulphuric acid, while the deposi-
tion of zinc from such a solution is impossible, this form of
solution may also be used to effect the separation of these
two metals.

The solution of the two salts is treated with 3 to 4 c.cms.
of a concentrated ammonium sulphate solution and 2 to 3
c.cms. dilute sulphuric acid. The mixed solutions are then
diluted to 150 c.cms., and the electrolysis is conducted with
a current of '('8 ampere density under an E.M.F. of
between 2-8 and 2 '9 volts. The separation is complete ;
the cyanide method is, however, to be preferred, since the

1 Zeitsclir.f.phys. Client. 12, 116.
'-' Amer. Chem. Jour. 3, 385.

206 THE ELECTKOLYTIC PROCEDURE

deposit of cadmium obtained from a sulphuric acid solution
is not always metallic in character.

A solution containing a small quantity of free acetic
acid may be used in place of the sulphuric acid solution. In
order to prepare such a solution, the acetates or sulphates
of the two metals are dissolved in water, 3 grms. sodium
acetate is added, and the liquid is acidified by means of a
few drops of acetic acid. After dilution of this solution to
150 c.cms. it is heated to 70 C., and electrolysed with a
currentthe E.M.F. of which is 2-2 volts. According to Yver, 1
from three to four hours are required to deposit -20 grm.
cadmium. The deposit obtained is crystalline in structure.
Eliasberg states that a separation of these two metals
may be obtained by use of the double oxalate solution. 2
In order to prepare this, the neutral solution of the two
salts is treated with 8 to 10 grms. potassium oxalate and
2 grms. ammonium oxalate, and is then diluted to the
usual volume. The electrolysis is carried out with this
solution after warming, by means of a current of '01
ampere density ; from six to seven hours will be required
to deposit *20 grm. cadmium.

The zinc may then be deposited from the same solution
by means of a stronger current.

The solutions prepared for electrolysis with sodium
phosphate and free phosphoric acid have also been
recommended by Smith for use in effecting the separation
of cadmium and zinc. 3

Smith and Knerr have also shown that the solution
prepared by adding 3 to 4 grms. sodium tartrate and some
free tartaric acid to the neutral solution of the two salts
yields on electrolysis, by means of a current of '30 ampere
density, a deposit of cadmium free from zinc. 4

Cadmium from Nickel and Cobalt. The separation of

1 Bull, de la Soc. Chem. 34, 18 ; Zeitschr. f. anal. Chem. 20, 1881,
417.

* Zeitschr. f. anal. Chem. 24, 548.

3 Aincr. Chem. Jour. 12, 329 ; 13, 206. 4 Ibid. 8, 200.

SEPARATION OF METALS 207

cadmium from these two metals can be effected in a
solution slightly acidified with sulphuric acid.

The solution of the mixed salts is treated, as in the
case of the separation of cadmium from zinc, with am-
monium sulphate and a little free sulphuric acid, and is
electrolysed with a current the E.M.F. of which does not
exceed 2 -8 or 2 -9 volts. The deposit of cadmium obtained
from this solution is completely free from nickel or cobalt.
The remarks made under the separation * Cadmium-Zinc '
concerning the character of the deposit obtained from the
sulphuric acid solution apply in this case also.

The double cyanide method described under Cadmium-
Zinc may also be used to separate cadmium from cobalt.
The neutral solution of the mixed salts is treated with 4 to 5
grms. pure potassium cyanide, and is electrolysed with a
weak current, the E.M.F. of which, according to Smith
and Frankel 1 and Freudenberg, 2 must not exceed 2*6 volts.
These authorities state that this method is not applicable
to the separation of nickel and cadmium. Smith and
Wallace, however, have found that if 2 grms. sodium
hydrate be added to the cyanide solution, and if the
electrolysis be allowed to proceed during the night, using a
current of "20 ampere density, a deposit of cadmium com-
pletely free from nickel can be obtained. 3

Solutions containing phosphates and a small amount of
free phosphoric acid have also been proposed for effecting
the separation of cadmium from nickel and cobalt.

Cadmium from Iron. These metals may be completely
separated by means of electrolysis in a solution contain-
ing ammonium sulphate and free sulphuric acid, similar
to that recommended for effecting the separation of cad-
mium from zinc, nickel, and cobalt. The E.M.F. of the
current employed must not exceed 2 '8 volts. The deposit
of cadmium obtained as in the previous separation is not

1 Amer. Chem. Jour. 12, 104.

- Zeitschr. f. phys. Cliem. 12, 116.

3 Jour. anal. u. appl. Cliem. 6, 87.

208 THE ELECTROLYTIC PROCEDURE

always satisfactory in character. The separation of these
two metals may also be effected in a phosphate solution.

Cadmium from Manganese. The latter metal is de-
posited from a sulphuric acid solution as peroxide at the
anode, whereas cadmium, as already noted, may be deposited
from such a solution if the amount of free acid present is
not too great. A separation of these two metals is there-
fore possible. The amount of the metals present in the
electrolyte must be small.

The larger electrode is used as anode, and it is pre-
ferable to use one with a roughened surface for this
purpose. The solution is prepared as already described
under Cadmium -Zinc.

Cadmium from Aluminium, Chromium, Magnesium,
Calcium, Barium, Strontium, Potassium, and Sodium.
The separation of cadmium from these metals is easily
effected either in solutions slightly acidified with sulphuric
acid, or in those containing an excess of potassium cyanide.
No deposition of the other metals occurs.

Cadmium from Arsenic, Antimony, and Tin, In a
solution containing cadmium and arsenic, the separation of
the two metals may be effected by raising the arsenic to
the arsenic acid stage of oxidation, adding 2 to 3 grms.
potassium cyanide to the neutral solution, and electrolysing
with a weak current. According to Freudenberg, and
Smith and Frankel, 1 the separation is complete if the
E.M.F. be not allowed to exceed 2-6 volts ; but the time
required is great namely, ten hours. Cadmium may also
be separated from arsenic, antimony, and tin by use of an
ammoniacal solution containing tartaric acid, if these
metals be present in the higher state of oxidation. The
cadmium obtained, however, from this solution is not in a
form adapted for weighing.

Cadmium from Wolfram, Molybdenum, Osmium. Smith
states that cadmium may be separated from these metals
in solutions containing an excess of potassium cyanide.
1 Amer. Chein. Jour. 12, 428.

SEPARATION OF METALS 209

IRON

The methods by which this metal can be separated from
those which are deposited in acid solutions have already
received notice under the concerned metals. As a general
rule, solutions strongly acidified with the mineral acids are
used.

The separation of iron from the group of similar
metals cobalt, nickel, and zinc offers on the contrary
considerable difficulty, since in order to obtain deposits
free from carbon and exact results, practically only one
form of solution is available namely, that containing
ammonium oxalate. Solutions containing tartaric or citric
acid cannot be used.

Iron from Copper. See p. 169.

Iron from Lead. See p. 190.

Iron from Silver. See p. 192.

Iron from Mercury. See p. 196.

Iron from Cadmium. See p. 207.

Iron from Gold. See p. 198.

Iron from Bismuth. See p. 200.

Iron from Cobalt and Nickel. The attempts made to
separate these metals in solutions of their double oxalate
salts failed on account of the fact that the ' decomposition
values' of these salts lie very closely one to the other.
Classen has, however, published a method by which this
difficulty is overcome. 1 To the solution of the mixed salts
of nickel arid iron, 8 grins, ammonium oxalate are added,
and after dilution the solution is electrolysed a,t a tempera-
ture of 60 or 70 C., with a current of between 1 and 2
amperes density. The E.M.F. required will be from
3 to 4 volts, and the time between two and three hours,
for -30 grm. of the metals. If a weaker current be used, it is
necessary to increase it to at least 1 ampere in density
towards the end of the electrolysis, in order to remove the
last traces of the metals from the electrolyte. The deposit
1 Electrolyse.

210 THE ELECTROLYTIC PROCEDURE

obtained at the kathode is a bright and steel-grey alloy of
iron and nickel. It is washed and dried as described under
these metals. After weighing, the deposited alloy is brought
into solution by warming with sulphuric or hydrochloric
acid. The alloy is not easily soluble, and the acids must
not be used too dilute. In any case considerable time will
be required. The iron is present in this solution in the
ferrous state, and its estimation is effected by titration with
potassium permanganate. Owing to the green tint of the
acid solution of the alloy of iron and nickel, the end of the
reaction with the permanganate is obscured, and an addition
of a solution of cobalt sulphate is made previous to the
titration in order to overcome this difficulty.

This addition in some cases produces the desired result,
and a colourless solution is obtained for the permanganate
titration ; but it is not always effectual, and the results
obtained in these cases are inexact. The separation of iron
from cobalt may be carried out in a similar manner. An
addition of nickel sulphate is used here to neutralise the
pink colour of the solution. The results obtained are
similar to those with nickel and iron.

Vortmann has proposed another method for separating
iron from nickel and cobalt. The iron in the solution of
the mixed salts is oxidised by means of bromine, and 6 to
8 grms. ammonium sulphate arid a slight excess of am-
monium hydrate are then added. A flocculent precipitate
of ferric hydrate is produced ; this remains suspended in
the solution. The nickel or cobalt is then deposited by
means of a current of from *40 to '80 ampere in density.
This deposit always contains a small amount of iron, which
is removed by dissolving the deposit, and redepositing it
from the comparatively pure solution thus obtained. This
double deposition makes the method a troublesome one, and,
apart from this, no method can be recommended in which
one of the substances in the electrolyte is present as a
flocculent precipitate during the electrolysis.

No simple and rapid electrolytic method for the separa-

SEPARATION OF METALS 211

tion of iron from cobalt and nickel exists. There is
further little need for such, since the quantitative sepa-
ration of these metals is easily effected by chemical
methods.

Iron from Zinc. This separation cannot be effected in
solutions of the double oxalate salts for the same reason
as that given under ' Iron- Nickel.' Classen has therefore
recommended the deposition of these two metals as an
alloy, under similar conditions to those given above, and
after weighing the alloy, the determination of the iron by
titration with permanganate. 1

In this case there is no difficulty arising from the colour
of the iron-zinc solution ; but the simultaneous deposition
of the two metals is not without some objectionable
features. If a double oxalate solution, in which the two
metals are present in about equal amounts, be electrolysed,
more of the zinc than of the iron will be deposited at first.
As the electrolysis proceeds, a portion of the zinc depo-
sited will pass into solution again, and an evolution of gas
will occur. The deposition of the alloy only takes place
satisfactorily when the amount of zinc present is less than
one-third that of the iron.

Vortmann has recommended the use of a solution con-
taining a sufficient excess of potassium cyanide to hold the
cyanides of the metals in solution, and sodium hydrate. 2
This latter forms sodium ferrocyanide with the iron, and
this salt is not decomposed by the current in presence of
free alkalies. Too great an excess of potassium cyanide
delays the deposition of the zinc.

A current density of from -30 to '60 ampere is
employed. As regards the practical utility of this method,
the remarks under 'Iron Nickel' apply here.

Iron from Manganese. A great number of experi-
ments have been carried out with all forms of salts, in
order to discover a reliable method for obtaining a complete
separation of these two metals, but without success. In
1 Electrolyse. 2 Monats. f. Chem. 14, 536.

212 THE ELECTROLYTIC PROCEDURE

most of these experiments the aim has been to obtain the
manganese as peroxide at the anode, or to keep it in
solution while the iron is deposited at the kathode. The
results obtained showed that the deposition of the iron was
incomplete (at least for the first deposition), and that when
the manganese was separated as peroxide this latter con-
tained iron.

This difficulty arises in connection with the method
proposed by Classen. 1 The solution of the two metals is
prepared by treating it with 6 or 8 grms. ammonium
oxalate, and after heating to 50 or 60 C., the electrolysis
is conducted with a current of 1 ampere in density, and
of 3-1 to 3-8 volts as regards E.M.F. Only a small
portion of the manganese is obtained at the anode as per-
oxide under these conditions. If less ammonium oxalate
be used, permanganic acid and its salts will be formed at
first at the anode, and later a peroxide deposit will be ob-
tained containing iron. As a rule the liquid is rendered
completely turbid by a brown flocculent precipitate, which
partly settles in adherent form upon the kathode. The
method gives inexact results in spite of all assertions to the
contrary.

The method proposed by Brand, 2 in which a solution
containing sodium pyrophosphate and ammonium oxalate is
used, also yields inaccurate results.

If one attempt to effect the separation of iron from
manganese in a solution containing 20 to 30 grms. ammonium
acetate, an incomplete deposition of the manganese as per-
oxide occurs, owing to the formation of a ferrous salt which
dissolves the peroxide again at the anode. Engels has proposed
to add oxidising agents in order to overcome this difficulty. 3
If chromic acid be used to oxidise the ferrous salt, a com-
plete deposition of the manganese as peroxide can be
obtained, but the deposit will be found to contain up to '02
grm. iron, probably in the form of oxide.

1 Electrolyse. 2 Zeitschr. f. anal. CJwm. 28, 581.

3 Zeitschr. f. Elektrochem. 2, 414.

SEPARATION OF METALS 213

Iron from Aluminium. If a solution of an iron salt
containing alum be treated with 8 grms. ammonium oxalate
and be then electrolysed, a deposit of iron alone will be ob-
tained at the kathode at the commencement of the electro-
lysis. As aluminium can in no case be deposited from an
aqueous solution, the separation is complete. In course of
the electrolysis of solutions containing ammonium oxalate,
carbonic acid is formed at the anode.

This leads to the formation of ammonium carbonate in
the electrolyte, and to the precipitation by the latter of
aluminium as a flocculent hydroxide.

The separation of aluminium hydrate does not produce
any impurity in the deposit of iron. The electrolysis is
conducted at the normal temperature, with a current that
does not exceed 1 ampere in density. Stronger currents
than 1 ampere heat the electrolyte and accelerate the for-
mation of ammonium carbonate.

The E.M.F. required will lie between 3 and 3-8 volts, and
about four hours will be demanded for the deposition of -10
grm. iron.

As a rule a smooth deposit of iron is obtained, but
towards the end of the electrolysis the aluminium hydroxide
has a tendency to adhere to the deposit on the kathode.

When this has occurred, it may be removed without
injury to the coating of iron by wiping with a cloth. The
aluminium must be estimated by gravimetric methods.

If it be thought necessary to avoid the separation of the
aluminium as hydroxide in the electrolyte, the solution
containing the iron salt and the alum is treated with 1 grm.
potassium tartrate, and after heating to 50 or 00 0. is
electrolysed with a current of about 1 ampere in density.
The E.M.F. required will be from 4 to 5 volts, and about
five and a half hours will be requisite for -10 grm. iron.

This solution will remain clear to the end of the electro-
lysis. A bright deposit of iron will be obtained, but it will
be found to contain some carbon. The amount of this
latter impurity does not exceed 1 mg. for the above-

214 THE ELECTKOLYTIC PKOCEDUEE

named weight of potassium tartrate, so that the results
obtained are only slightly erroneous.

Iron from Chromium. A solution of either a ferric or
ferrous salt containing any soluble salt of chromium sesqui-
oxide may be prepared for electrolysis by adding 8 grms.
ammonium oxalate.

The solution is then heated to 60 C., and is electrolysed
by means of a current of from 1 to 2 amperes in density.
The E.M.F. required will be from 3-3 to 3'7 volts ; in order
to deposit '10 grm. iron from three to four hours will be
necessary. The deposit obtained is bright and metallic.
The chromium salt is raised to the chromic acid state of
oxidation during the electrolysis, and the chromium is
determined in this solution by gravimetric methods.

COBALT AND NICKEL

Since cobalt and nickel belong to the group of metals
which as a general rule cannot be deposited in acid solutions
by means of the electric current, the separation of these
two metals from many of the others is easily accomplished.
The methods in use have already been described under the
headings of the different metals, and a few methods of
separation from metals of the same group have also already
received mention.

Cobalt and Nickel from Copper. See p. 172.

Cobalt and Nickel from Silver. See p. 192.

Cobalt and Nickel from Mercury. See p. 196.

Cobalt and Nickel from Bismuth. See p. 200.

Cobalt and Nickel from Lead. See p. 190.

Cobalt and Nickel from Cadmium. See p. 206.

Cobalt and Nickel from Iron. See p. 209.

Cobalt from Nickel. Yortmann has proposed two
methods } for effecting the separation of these two metals, so
closely allied in their general properties and characteristics.
The first consists in the use of a solution of the neutral
sulphates of the two metals to which sulphates of the

1 D. R. P. Kl. 40, 78236. Monatsheftc f. Chemie. 14, 548.

SEPAKATION OF METALS 215

alkali or alkaline earth metals have been added, together with
a soluble chloride salt. The solution is then electrolysed with
a current, the direction of which is continually changed,
oxidation and reduction alternately occur at the electrode,
and the cobalt is said to separate as hydrate while the
nickel remains in solution.

The method is unsuitable for the purpose of electrolytic
analysis, since quantitative results cannot be expected with
it. The other method depends upon the use of solutions
containing tartrates of the alkali metals, and a little potas-
sium iodide. The results obtained with this method are also
unsatisfactory, and a reliable electrolytic procedure for the
separation of cobalt and nickel does not therefore exist.

Cobalt and Nickel from Zinc. There are two ways in
which the separation of these metals can be effected.
Either the nickel or the zinc may be deposited, while the
second metal remains in solution. According to Vortmann,
in order to deposit the zinc from such a mixture of salts,
the solution, which should contain about '20 grm. each of
the concerned metals, should be treated with 5 to 6 grms.
sodium potassium tartrate and with an excess of sodium
hydrate, and should then be diluted to a volume of 150
c.cms. * The electrolysis is conducted at the normal tempera-
ture with a current of from '30 to '60 ampere in density.
From two and a half to three and a half hours will be required
in order to deposit the whole of the zinc.

Nickel monoxide often separates at the anode during
this electrolysis. Towards the end of the deposition of zinc
it frequently happens that a flocculent precipitate of nickel
hydrate separates in the electrolyte, and ultimately this
hydrate may settle upon the zinc at the kathode in fine
brown streaks. This, however, can only occur when the
electrolysis has been permitted to continue for too lengthy
a period of time. The most simple manner of determining
whether the whole of the zinc has been deposited is to hang
a narrow strip of brass over the edge of the basin electrode
1 Mwatsch. /. Clicinie, 14, 536.

216 THE ELECTROLYTIC PROCEDURE

and to note whether any deposition of zinc occurs upon it.
The solution remaining after the whole of the zinc has been
removed is acidified with sulphuric acid, and, after addition
of excess of ammonium hydrate, is made use of for the
deposition of the nickel by the ammonium sulphate method
described under ' Nickel.' The solution may also be treated
with 25 c.cms. ammonium hydrate, and 15 to 20 grms.
ammonium carbonate, and electrolysed at a temperature of
50 to 60 C., with a current of from -80 to 1-0 ampere in
density. From one to two hours will be required in order
to deposit '20 grm. nickel.

A method of separation depending upon the deposition
of the nickel has been proposed by von Foregger. 1

The electrolyte is prepared by treating the solutions of
the two sulphates, which should contain about '20 grm. of
each metal, with 10 grms. ammonium sulphate, 10 grms. am-
monium carbonate, and 10 c.cms. strong ammonium hydrate.
The mixed salt solution is then diluted to 150 c.cms., and is
electrolysed at a temperature of 50 or 60 C., with a
current which at first does not exceed "30 to '50 ampere in
density, but which is later increased to a density of 1 '0 to
1*5 amperes.

The nickel separates as an adherent deposit at the
kathode, whereas the zinc remains in solution even at the
higher current density.

It is striking that the deposit of nickel is sometimes of
a brownish colour, due not to admixed zinc but to enclosed
nickel sesquioxide.

This, when it occurs, renders the results too high. The
electrolyte remaining after the separation of the nickel can
be prepared for the electrolytic determination of the zinc
by treating with an excess of sodium hydrate. The depo-
sition of the zinc from this solution is then carried out at
60 or 70 C., with a current of from -80 to 1*0 ampere in
density. About three and a half hours will be required to
deposit the zinc.

1 Dissertation, Bern, 18UO.

SEPARATION OF METALS 217

The methods of zinc deposition depending upon the use
of the cyanide or oxalate double salts may also be used, if
the necessary steps be taken to convert the zinc present in
the solution that remains after deposition of the nickel, into
these forms.

The two methods given above may also be used together ;
that is to say, the zinc is deposited according to the first,
and the nickel in the remaining electrolyte is then deposited
in accordance with the directions of the second.

Cobalt and Nickel from Manganese. Classen has prot
posed to use a solution of the sulphate salts of these metals,
to which about 8 grms. ammonium oxalate have been
added, for effecting their separation. 1 The deposition of
the nickel or cobalt is then effected similarly to that of
iron from a corresponding solution, at a temperature of
Between 50 and 60 C., by means of a current of about
1-0 ampere in density. The E.M.F. required will be
from 3*1 up to 3'6 volts. The cobalt or nickel separate
at the kathode, whilst the deposition of the manganese
is prevented by the ammonium oxalate present in the
solution.

A formation of a dark-brown flocculent precipitate of
manganese compounds occurs, however, and these settle
upon and adhere to the metallic coating on the kathode.
It is impossible wholly to avoid this precipitation, either by
altering the temperature at which the electrolysis is carried
out, or by varying the amount of ammonium oxalate used.
The method is inexact.

Brand has proposed to separate cobalt fro*m manganese in
solutions containing sodium pyrophosphate, 2 but the method
does not lead to successful results. Nickel may, however,
be separated from manganese in such a solution if the
amount of the two metals present is very small, and if the
electrolyte contains in addition to the sodium salt 15 per cent,
ammonium hydrate.

Neither of the two methods described for the separation

1 Electrolyse. * Zeitttchr. f. anal. Client. 28, 581.

218 THE ELECTKOLYTIC PKOCEDURE

of cobalt and nickel from manganese can be recommended
as trustworthy.

Nickel and Cobalt from Aluminium and Chromium.
The separation of these metals is effected in the same way
as that of iron from aluminium and chromium.

ZINC

Since zinc also belongs to that group of metals which
are separated from their salts with greater difficulty than
hydrogen is separated from its salts (the acids), it follows
that the separation of zinc from many of the metals is easily
accomplished. The methods used to effect such separations,
and also other separations from the metals of the same
group, have already received mention as follows :

Zinc from Copper. See p. 167.

Zinc from Lead. See p. 190.

Zinc from Silver. See p. 192.

Zinc from Mercury. See p. 196.

Zinc from Gold. See p. 198.

Zinc from Bismuth. See p. 200.

Zinc from Cadmium. See p. 205.

Zinc from Iron. See p. 211.

Zinc from Cobalt and Nickel. See p. 215.

Zinc from Manganese, Aluminium, and Chromium.
The methods described under iron and cobalt for the
separation of these metals from manganese, aluminium,
and chromium may also be used to effect the separation of
zinc from the latter metals. The remarks concerning the
trustworthiness of the methods also apply in the case of
zinc.

MANGANESE

Manganese, which is nearly always deposited as
peroxide, can be separated in acid solutions from a con-
siderable number of the metals. These separations have
already received full description, under the concerned

PRACTICAL EXAMPLES 219

metals. The separation of manganese from those metals
which cannot be deposited in acid solutions is attended by
difficulties, and the results obtained in most cases are un-
satisfactory.

These separations have likewise received mention under
the individual metals.

SEPARATION OF SEVERAL METALS

If many metals be present in one solution, the methods
of electrolytic separation employed are varied according to
the electrolytic character of the metals present.

Magnesium, aluminium, chromium, calcium, barium,
strontium, potassium, and sodium always remain in
solution, as they cannot be deposited at the kathode under
the ordinary current conditions.

The remaining metals can be easily separated into two
large groups by electrolysing solutions containing a definite
excess of certain acids.

In this way it may occur that many of the metals are
deposited together ; a separation of the metals in such a
composite deposit is only possible after redissolving.

For example, if a solution containing silver, copper,
cadmium, and zinc be obtained for analysis, one would
first deposit the silver and copper together, and then
dissolve this mixed kathode deposit in order to effect the
separation of the silver from the copper. If lead be
present in such a mixed acid solution, the method of
separation is also again very simple.

After electrolysis, those metals which can be deposited
in the presence of free acid will be found at the kathode,
the lead as peroxide at the anode, and the metals of the
group Zinc-Iron will be found still in solution. In some
cases, dependent upon the metals present, similar group
separations are possible in solutions of the cyanides or other
salts.

It is more advantageous, however, in practical analytic
work, when dealing with solutions which contain several

220 THE ELECTROLYTIC PROCEDURE

metals, to separate these by purely chemical methods to
such an extent, that either the electrolytic work is confined
to depositions of single metals, or to separations for which
definite data are available.

Examples of these combined chemical and electrolytic
methods of analysis are given in Part III. D.

D. PRACTICAL EXAMPLES

Alloys of Copper and Zinc, containing Lead and Iron
as Impurities (Brass, Tombac). About -50 grm. of the
sample obtained by boring or filing the alloy is dissolved,
with the aid of gentle heat, in dilute nitric or sulphuric
acid The amount of acid requisite for the later electro-
lysis is 5 to 10 c.cms. strong nitric acid, or 3 to 5 c.cms. cone,
sulphuric acid. The warm solution of the alloy is diluted to a
volume of 150 c.cms., and is electrolysed either in a beaker
with a cone electrode, or in the platinum basin, with a current
of about 1 ampere in density, and under the conditions given
in detail under Copper-Zinc on p. 167.

If the alloy under analysis be brass containing lead as
an impurity, a nitric acid solution should be used with
an anode that has been previously weighed. The lead
separates upon the latter as peroxide ; and as lead is only
present in very small amounts in brass, the smaller
electrode will in this case serve to receive it.

When the whole of the copper has been deposited
(three to four hours will be requisite for this) the electrodes
are removed from the electrolyte in the beaker, or the
basin electrode is washed out before breaking the current
circuit.

The remaining solution of zinc in nitric acid is treated
with a small amount of sulphuric acid, and is evaporated
in order to drive off the free and combined nitric acid.
The sulphates of zinc and iron thus obtained are dissolved in a
small quantity of water, and the latter is precipitated as

PRACTICAL EXAMPLES 221

hydroxide, most simply by addition of a slight excess of
ammonium hydrate to the aqueous solution of the
sulphates. 1

This iron is then determined by the gravimetric method,
or the hydroxide may be dissolved, and the iron deter-
mined electrolytically in an ammonium oxalate solution.

The sulphuric acid solution of zinc and iron that
remains when the copper has been deposited from a
sulphuric acid electrolyte is treated in the same manner
in order to separate the iron. The slightly alkaline am-
moniacal zinc solution obtained in either case is treated with
a few grams of pure potassium cyanide, or with ammonium
oxalate or lactate, according to one of the methods de-
scribed under zinc on pp. 113-122, and the zinc is deposited
as metal upon an electrode which has been previously coated
with copper or silver. The preparation of the deposits of
copper and zinc for weighing is, of course, carried out as
already described in detail under these metals.

It is more convenient to precipitate the iron by means
of ammonia, and to estimate it separately, than to electro-
lytically deposit zinc and iron together, and then to use
the unsatisfactory electrolytic method for separation of
these two metals.

Brass is composed as a rule of 65 per cent, copper and
35 per cent. zinc. Lead and iron are generally only present
as impurities in very small amounts.

Alloys of Copper and Silver (Mint-Silver). In order
to carry out this analysis '20 to '60 grm. of the borings or
filings of the alloy are dissolved in a small amount of
nitric acid. This solution is then either directly used for
the electrolytic separation of the silver and copper under
the conditions described on p. 182, or it is neutralised
with sodium hydrate, treated with excess of pure potas-
sium cyanide, and electrolysed as described under this
method on p. 183.

1 [If much zinc be present, a larger excess of ammonium hydrate
will be required to keep this metal in solution. Translator's note.'}

222 THE ELECTROLYTIC PROCEDURE

The German and United States mint-silver contains
90 per cent, of the metal ; that used in France varies from
83'5 per cent, up to 90 per cent. ; while there is 92-5 per
cent, silver in the coinage-silver used at the mint in
England.

Alloys of Copper and Nickel (Mint-Nickel). The
solution of this alloy for electrolysis is obtained by
dissolving -30 to *50 grm. of the prepared sample in dilute
nitric or sulphuric acid, and by adding in the former case
still another 5 .c.cms. cone, nitric acid. The deposition of
the copper is then carried out under the current con-
ditions detailed under Copper-Nickel (see p. 173). In
order to determine the nickel in the remaining electrolyte,
the nitric acid is removed by evaporation with an excess of
sulphuric acid, and, after treatment with an excess of
ammonium hydrate, the nickel is deposited directly from
the resulting ammoniacal solution of sulphate salts. When
sulphuric acid has been used to dissolve the alloy, more
time is required to effect this, but the later evaporation
with this acid is unnecessary ; and after deposition of the
copper in the acid solution, one can simply add excess of
ammonium hydrate, and proceed at once to deposit the
nickel.

The details of the procedure will be found on p. 106.

If small amounts of iron be present as an impurity in
the alloy, this will cause a precipitate of ferric- hydrate to
form when the ammoniacal solution of nickel is being pre-
pared for electrolysis.

This is removed from the solution by nitration, and the
iron in it is determined either by the gravimetric method,
or by redissolving and deposition from an ammonium
oxalate solution.

The nickel coins used as currency in Germany contain
75 per cent, copper and 25 per cent, nickel.

Alloys of Copper, Zinc, and Nickel (German-Silver).
Three different methods may be employed to effect the
electrolytic separation of the metals that occur in this

PRACTICAL EXAMPLES 228

alloy. First, one may use a nitric acid solution, to de-
posit the copper alone, and then separate the nickel and
zinc in the remaining electrolyte. Or, one can make use
of an alkaline sodium potassium tartrate solution, and
deposit the zinc and copper together as an alloy, while
the nickel remains in solution to be later deposited alone.

The third method depends upon the use of an am-
moniacal solution containing ammonium carbonate, from
which on electrolysis copper and nickel are deposited as an
alloy, zinc remaining in solution.

In order to carry out the first method, between -20 and
40 grm. of the alloy, preferably in the form of thin
shavings, is dissolved in dilute nitric acid in a beaker.
When the solution is complete, a further 20 to 30 c.cms.
cone, nitric acid are added, the solution is diluted to 150
c.cms., and after cooling to the normal temperature it is
electrolysed either in the beaker with a cone electrode, or
in the platinum basin.

The density of current used should be from -50 to I'O
ampere. The E.M.F. required will be from 2-5 to 2-8 volts,
and the time from two to three hours. The remaining
solution is then evaporated with sulphuric acid in order to
remove the nitric acid and to convert the nitrates into
sulphates, and after neutralising it is treated by either of
the methods detailed on pp. 215, 216. In the one case zinc
is first deposited ; in the other, the nickel is determined
first, and the zinc in the remaining electrolyte.

The second method is carried out as follows : To the
solution of '20 to -40 grm. of the alloy in nitric acid, after
evaporation with sulphuric acid to convert the salts into
sulphates, 6 grms. sodium potassium tartrate and 4 to
5 grms. sodium hydrate are added, and the mixture is
then diluted to 150 c.cms. and heated to 40 or 50 C. The
solution is then electrolysed with a current of -60 to -70
ampere in density. The whole of the copper and zinc will
be deposited as an alloy in three to four hours. As the
copper is deposited more rapidly than the zinc, the red

224 THE ELECTROLYTIC PROCEDURE

colour of the coating on the kathode will gradually pass
into a grey. The mixed deposit after washing is dissolved in
a few cubic centimetres of dilute nitric or sulphuric acid, and
the separation of the copper and zinc in this solution is then
undertaken as described under Copper-Zinc (see p. 168).

The current used should be about 1 ampere in density ;
the time required will be from two to three hours.

The solution containing the nickel is treated with
15 grms. ammonium carbonate, and after heating to 30 to
50 C. is electrolysed with a current of between -80 and
1*0 ampere density. In from two to four hours the whole
of the nickel will have been deposited as a bright metallic
coating at the kathode.

In order to effect the separation of these three metals
by the third method, between '20 to 40 grm. of the alloy
is again dissolved in nitric acid and evaporated with
sulphuric acid in order to convert the nitrates into sul-
phates. The solution is then treated with 10 grms. am-
monium carbonate, 15 grms. ammonium sulphate, and
10 c.cms. ammonium hydrate. The solution diluted to the
usual volume is heated to 50 C., and electrolysed with a
current of '50 ampere density.

The deposition of the copper-nickel alloy demands from
four to five hours, but if the electrolysis is allowed to con-
tinue for too lengthy a period of time, the deposit may
become brown, owing to the formation of nickel oxides.
This deposit is redissolved in sulphuric or nitric acid, and
the nickel arid copper are separated by the method de-
scribed on p. 173. In order to effect the deposition of the
zinc from the remaining electrolyte, one may add either
excess of ammonium oxalate or of potassium cyanide to
the solution, and electrolyse under the current conditions
given under the descriptions of these methods on pp. 115
and 118. One may also prepare the solution for deposi-
tion of the zinc by evaporating off the greater part of the
ammonia, and by adding 2 to 3 grms. sodium hydrate.

If the alloy should contain small amounts of iron as an

PEACTICAL EXAMPLES 225

impurity, this will be precipitated on the addition of the
ammonium hydrate or ammonium carbonate to the electro-
lyte. It is separated by filtration, and estimated either by
the gravimetric method or by the electrolytic method in
a double oxalate solution.

The composition of German silver varies between the
following limits : copper, 50 to 66 per cent. ; zinc, 19 to
31 per cent. ; nickel, 10 to 18 per cent.

Alloys of Copper, Zinc, Nickel, and Silver (Old Swiss
Nickel Coinage). The earlier Swiss nickel coinage metal
was a true alloy of copper, nickel, and zinc, with a little
silver, whereas ' China silver ' is merely ' German silver '
coated with silver. In order to analyse this alloy by
electrolytic methods one may proceed in two different ways.
By the first method -20 to '40 grm. of the alloy, preferably
in the form of filings, is dissolved in a small quantity of
nitric acid, and after dilution the silver is precipitated as
chloride by addition of a few drops of hydrochloric acid.
The solution is then warmed, and, after filtering off the
silver, is treated for the separation of the copper, nickel,
and zinc by one of the methods just described under
'German Silver.' The silver in the separated silver
chloride may either be determined in the usual gravi-
metric way, or the chloride may be dissolved in potassium
cyanide solution, and the silver be deposited from this
solution according to the conditions given under silver
(seep. 136).

In the latter case the smaller electrode is used as
kathode, on account of the small amount of silver present
in the alloy.

The second method depends upon the deposition of the
silver and copper as an alloy directly from the nitric acid
solution in a manner similar to that described under
' German Silver.' The alloy is then redissolved and the
two metals separated according fco the method given on
p. 182. It is better, however, to use at first a low E.M.F.
and a feeble current, in order to obtain a deposit of the

226 THE ELECTROLYTIC PROCEDURE

silver alone ; afterwards the copper may be deposited by
means of a stronger current. The remaining metals
nickel and zinc may then be separated by the method
given on p. 215.

Alloys of Copper and Tin (Bronze). In order to pre-
pare a solution of this alloy for electrolysis, '20 to - 40 grm.
of the extremely finely divided metal is dissolved in 6 c.cms.
nitric acid of 1*5 sp. gr., and 3 c.cms. water are then added.
When the first action has subsided, the solution is heated to
boiling, 15 c.cms. boiling water are added, and the tin oxide
is, after settling, filtered off, and washed. The filtrate
contains all the copper, which may be deposited under the
conditions given on p. 93, after the addition of a further
5 to 10 c.cms. nitric acid to the filtrate from the tin oxide. If
the above instructions regarding the concentration of the
nitric acid solution have been carefully carried out, the tin
oxide which separates will be found free from copper ; this
may not be the case if other proportions of acid and water
have been used in bringing the alloy into solution.

The tin oxide collected on the filter may either be dried,
ignited, and weighed ; or while still moist it may be dis-
solved in a solution of ammonium sulphide, and the result-
ing stannic sulphide solution electrolysed under the con-
ditions given on p. 150.

The solution of the alloy may also be effected by means
of aqua regia. In this case the solution is evaporated to
dryness, and the residue is treated with a solution of
sodium sulphide. The tin passes into solution, and the
copper remains as insoluble copper sulphide. The latter
is filtered off, and, after washing with water containing
sulphuretted hydrogen, is dissolved in the necessary amount
of nitric acid, and the copper determined electrolytically
by the method given on p. 93. The solution of tin is boiled
with ammonium sulphate in order to convert the sodium
sulphide into the ammonium salt, and the tin is then
deposited from this solution. Ammonium sulphide can-
not be directly used in the treatment of the residue

or THB

JNIVERSITY

PEACTICAL EXAMPLES

from the aqua regia solution, since copper sulphide is
slightly soluble in solutions of the polysulphides of am-
monium.

The following are alloys of copper and tin : Bronzes
of the ancients ; cannon metal (9 to 10 per cent, tin) ; bell
metal (20 to 25 per cent, tin) ; and speculum metal (30 to
35 per cent. tin).

Alloys of Copper, Tin, and Zinc (German Mint Copper,
Modern Bronze). In order to prepare a solution of this
alloy for electrolysis -20 to '50 grm. of the alloy in a finely
divided state is dissolved in nitric acid under the conditions
as regards concentration of the solution mentioned under
* Bronze.' The tin oxide is filtered off, washed, and the tin
determined either gravimetrically or by electrolytic deposi-
tion from an ammonium sulphide solution (see p. 1 50). The
filtrate containing copper and zinc is then treated exactly
as described on p. 168 for the separation of these two metals.
If lead be present as an impurity in the bronze, it will be
separated as peroxide at the anode during the deposition of
the copper from the nitric acid solution, and it is estimated
as usual from the weight of this peroxide deposit.

German mint copper contains 95 per cent, copper, 4
per cent, tin, and 1 per cent. zinc.

Alloys of Copper, Tin, Zinc, and Phosphorus (Phosphor
Bronze). If a finely divided sample of this alloy be treated
with nitric acid as described under ' Bronze,' there will
remain a residue of stannic phosphate which may be
separated and weighed. Another sample of the alloy is
treated in the same way, but the insoluble stannic phos-
phate is digested with sodium sulphide, and the solution
containing the tin is then treated with ammonium sulphate
and electrolysed as described on p. 150.

The difference between the weight of tin thus found
and that of the stannic phosphate yields by calculation the
weight of phosphorus contained in the alloy. The residue
of copper and zinc sulphides remaining from the treatment
with sodium sulphide solution is dissolved in nitric acid,

228 THE ELECTROLYTIC PROCEDURE

and these two metals are then separated by the method
given on p. 168.

Alloys of Zinc and Tin (Counterfeit Silver-leaf). A
small portion of the alloy is dissolved in nitric acid under
the conditions of solution described for 'Bronze,' and the
tin is determined as there recommended. The nitric acid
solution is freed from this acid by evaporation with sulphuric
acid, and, after neutralising, the zinc sulphate solution is
electrolysed as described under ' Zinc ' (see p. 115).

Alloys of Copper and Aluminium (Aluminium
Bronze). The finely divided sample of the alloy is prepared
for electrolysis by dissolving in nitric acid, and by addition
of a further amount of nitric acid. This solution is then
used for deposition of the copper as described on p. 93.
The aluminium is determined by the gravimetric method in
the remaining electrolyte. The alloy containing 3 per cent,
copper is white, that containing 5 to 10 per cent, is golden
yellow.

Alloys of Copper and Gold (Mint Gold). A solution of
this alloy is prepared by dissolving a small amount in aqua
regia, and by evaporating the solution to dryness. The
residue is treated with a small quantity of hydrochloric
acid, and afterwards with sodium hydrate solution and with
2 grms. pure potassium cyanide. The gold is deposited
from this solution first, under the current conditions given
under ' Gold ' (see p. 152), while the copper remains in
solution, and is only deposited when a stronger current is
employed.

German mint gold contains 90 per cent, gold, whereas that
coined at the English mint contains 91-66 per cent. gold.

Alloys of Lead and Tin (Solder). In order to prepare a
solution of this alloy for electrolysis *30 to -50 grm. of the
sample in small pieces is treated with a mixture of 6 c.cms.
cone, nitric acid and 3 c.cms. water, and, when the first
reaction is over, the whole is heated to a boiling temperature.
The solution containing the tin as insoluble oxide is then
diluted with 15 c.cms. water and filtered. The tin oxide is

PEACTICAL EXAMPLES 229

washed, and the tin is determined either by direct
weighing or by the electrolytic method described on p. 150.
In the latter case the moist stannic oxide is dissolved in
ammonium sulphide solution. The nitrate containing the
lead as lead nitrate receives a further addition of nitric
acid, and the lead is then determined electrolytically by
deposition as peroxide in the manner described under
'Lead '(see p. 128).

Alloys of Lead, Tin, and Bismuth (Rose's Metal). This
alloy is prepared for electrolysis by treating the finely
divided sample with nitric acid under the conditions
described for ' Solder.' The tin oxide requires, however, in
this case washing with water that contains nitric acid, in
order to remove the basic bismuth nitrate that would
otherwise remain with the tin on the filter. Since lead
and bismuth cannot be separated by electrolytic methods
in a nitric acid solution, it is necessary to evaporate the
solution to a syrup consistency many times upon the water-
bath, using water each time to bring the metal salts into
solution.

When all the nitric acid has been driven off, a dilute
solution of ammonium nitrate is added, and the insoluble
basic nitrate salt of bismuth is filtered off. The filtrate
after addition of nitric acid is used for deposition of the
lead as peroxide. The tin is determined in the tin oxide
as described under ' Bronze ' (see p. 226). The bismuth
nitrate precipitate may either be dried, ignited, and
weighed as bismuth oxide, or dissolved, and the bismuth
deposited as an amalgam in the manner described on p. 162.

Alloys of Tin, Lead, Bismuth, and Cadmium (Wood's
Metal). In order to analyse this alloy by electrolytic
methods, the solution of the sample and determination of
the tin and bismuth are carried out exactly as in the case
of Rose's metal. The filtrate from the basic bismuth nitrate
precipitate contains lead and cadmium, and, after addition
of a sufficiency of nitric acid to keep the latter metal in
solution, it is electrolysed.

230 THE ELECTROLYTIC PEOCEDURE

The lead is obtained as peroxide at the anode.

The remaining electrolyte is treated according to one of
the methods given under ' Cadmium ' on p. 122, in order to
determine this latter metal.

Alloys of Tin and Mercury (Tin Amalgam). The alloy
is prepared for electrolysis by dissolving a few decigrams in
nitric acid, under the conditions of solution described for
' Solder.' The tin is determined as there noted ; while
the mercury may be directly deposited from the nitric acid
solution by means of the current and E.M.F. mentioned on
p. 140. The solution of the alloy may also be effected by
the use of aqua regia at a gentle heat. The free chlorine is
then driven off by further heating, and, after neutralising
with ammonium hydrate, the solution is treated with am-
monium chloride and ammonium sulphide solutions.

A precipitate of mercury sulphide is obtained which
may be separated and redissolved, and the solution, after re-
moval of the excess of free acid, used for deposition of the
metal according to one of the methods given on p. 140.
The solution of the tin sulpho-salt is used directly for the
electrolytic determination of the tin.

Alloys of Lead and Antimony (Hard Lead, Type
Metal), These alloys contain as a rule, in addition to the
lead, 18 to 25 per cent, antimony and small quantities of
copper and iron. The simplest method of analysis is as
follows : 2*5grms. of the alloy are placed in a ] -litre flask with
10 grms. tartaric acid, 15 c.cnis. water, and 4 c.cms. cone,
nitric acid, and gently warmed. The clear solution is
treated with 4 c.cms. cone, sulphuric acid, diluted, and after-
cooling the flask is filled up to the mark. 50 c.cms. of the
nitrate, corresponding to '50 grm. of the alloy, are then
made strongly alkaline with sodium hydrate solution, and
are boiled with 50 c.cms. of a saturated sodium sulphide
solution. The resulting liquid is at once filtered, and the
filtrate while still hot electrolysed with a strong current,
under the conditions given on p. 145, for the deposition of
antimony. In order to determine the copper that may be

PRACTICAL EXAMPLES 231

present, the residue that remains from the treatment with
sodium sulphide solution is dissolved in nitric acid, the
solution is diluted, filtered, and the copper deposited as
described on p. 93. If it be thought necessary to deter-
mine the lead separately, *50 grm. may be used instead of
2*5 grms. of the alloy, and the lead sulphate precipitated by
the addition of sulphuric acid, collected and weighed. It is
better, however, to treat the solution of the metals directly
with sodium hydrate and sodium sulphide.

The insoluble residue that remains after this treatment
is made up of the sulphides of copper and lead. It is
dissolved in nitric acid, and the separation of these two
metals effected by the method described on p. 178.

The method first described excels the gravimetric method
greatly in simplicity, and is very frequently used in tech-
nological laboratories for the analysis of this alloy.

Alloys of Antimony, Tin, and Arsenic (Britannia
Metal). The solution of this alloy is prepared for electro-
lysis by dissolving the sample in aqua regia, evaporating
off the excess of acid, treating the residue with a small
quantity of sodium hydrate solution, and then with 50 c.c'ms.
of a saturated solution of sodium sulphide. All three
metals pass into solution as sulpho- salts, but arsenic cannot
be separated electrolytically from the solution, since it is
present in the higher state of oxidation. The separation of
the antimony is effected by the method given 011 p. 201
under ' Antimony-Tin.' If the electrolyte remaining after
the deposition of the antimony be treated with hydrochloric
or sulphuric acid, a precipitate of the sulphides of arsenic
and tin mixed with sulphur is obtained. This is filtered
off, and the arsenic sulphide separated by digestion with a
solution of ammonium carbonate. The residue is then
washed and dissolved in ammonium sulphide, from the
solution in which the tin may be separated directly as
described on p. 150.

The arsenic must be determined by gravimetric methods.
It may either be precipitated, or distilled off from the

232 THE ELECTROLYTIC PROCEDURE

solution before the deposition of the antimony, or its
determination may be left to the last. The former method
is to be preferred. In either case the antimony is deposited
from a sodium sulphide solution. This is then converted
into an ammonium sulphide solution and electrolysed to
obtain a deposition of the tin, according to the method
described fully under * Antimony-Tin ' on p. 201.

The separation and determination of these three metals
in the presence of each other by gravimetric methods is
difficult and troublesome, and electrolysis proves itself in this
case a very useful aid to the ordinary methods of analysis.

Refined Soft Lead. The refined lead found in com-
merce always contains traces of other metals - silver,
copper, bismuth, iron, nickel, zinc, tin, antimony, arsenic-
which together make up some hundredths of 1 per cent.

It is customary in the analysis of this lead to estimate
only the impurities. The analysis is conducted as follows :
200 grms. of the sample of lead cut into small pieces are
put into a 2-litre flask, with 325 c.cms. nitric acid and
1275 c.cms. water. The solution of the lead is effected by
the aid of gentle heat upon the sand-bath. To the clear
solution 62 c.cms. cone, sulphuric acid are added, and, after
cooling, the flask is filled up with water to the mark, and
1750 c.cms. of the liquid containing lead sulphate in sus-
pension are filtered and evaporated to dryness in a porce-
lain basin. The residue is digested with water, and then
brought upon a filter.

The insoluble residue (A) upon the filter is digested with
25 c.cms. of a saturated solution of sodium sulphide.

The filtrate (B) is acidified with hydrochloric acid, and
sulphuretted hydrogen gas is then passed through it ; this
separates it into a precipitate (c) and a filtrate (D). Pre-
cipitate (c) is digested with 25 c.cms. sodium sulphide
solution. The digestion leads to the solution of the
arsenic, antimony, and tin which may be contained in (c) ;
the solution of the sulpho-salts obtained is added to that
obtained earlier in the analysis. If a qualitative examina-

PRACTICAL EXAMPLES 233

tion of the lead has shown that no arsenic is present, the
antimony is deposited from this solution, and, after treat-
ment with ammonium sulphate, the tin is deposited ac-
cording to the method described on p. 150. If arsenic be
present, the solution containing the sulpho-salts is decom-
posed with sulphuric acid, and the precipitate of the sul-
phides mixed with free sulphur is digested with a solution
of ammonium carbonate for the removal of the arsenic.
The antimony and tin sulphides that remain are then re-
dissolved in sodium sulphide and separated as already
described. The residue (c) is boiled with aqua regia, the
silver chloride which separates is filtered off, and the silver
either determined by the gravimetric method or by electro-
lysis of a potassium cyanide solution of this chloride. The
former is the better plan.

The nitrate from the silver chloride is evaporated to
dryness with a small quantity of sulphuric acid, in order to
effect the separation of the remainder of the lead as sul-
phate ; the residue is taken up with water, and the
filtrate from the lead sulphate is neutralised with ammonia,
and the bismuth precipitated by means of ammonium
carbonate.

The bismuth is then electrolytically deposited as an
amalgam according to the method given on p. 163. The
ammoiiiacal filtrate from the insoluble basic bismuth com-
pound contains the copper and cadmium. These metals
are either separated by the method given on p. 175, or the
solution is treated with 1 grm. pure potassium cyanide and
with a small amount of sodium sulphide, whereby cadmium
sulphide is precipitated. The cadmium sulphide is filtered
off, dissolved in nitric acid, and, after evaporation of the
solution with sulphuric acid, the cadmium is determined
electrolytically by one of the methods given on pp. 122-126.

The copper contained in the cyanide solution may
either be deposited directly, or after treatment with
sulphuric acid, under the current conditions given on
pp. 96-99.

234 THE ELECTROLYTIC PROCEDURE

The filtrate (D) from the precipitate with sulphuretted
hydrogen is boiled to drive off the excess of gas, and, after
oxidation by means of bromine water, is treated with an
excess of sodium hydrate solution. The zinc passes into
solution and is deposited either directly from this, or from
some other salt solution obtained by chemical means from
the sodium zincate solution. The precipitate of hydroxides
produced by the addition of sodium hydrate to the nitrate
(D) is dissolved in dilute sulphuric acid, the iron is pre-
cipitated with ammonium hydrate, and is determined either
by the volumetric or the electrolytic method. The gravi-
metric method cannot be used, as the aluminium always
present in sodium hydrate would be weighed with the iron
oxide, and would cause an error in the results. The
remaining solution containing nickel and cobalt is treated
with ammonia hydrate, and these metals electrolytically
determined by the method given on p. 106. The results
obtained are calculated upon 179 '12 grms. lead, since the
volume of the lead sulphate produced in the measuring
flask has to be allowed for.

Raw Lead, Argentiferous Lead. This grade of lead
contains from 1 to 4 per cent, of impurities. According to
the degree of purity from 10 to 50 grms. of the sample are
dissolved in nitric acid and water.

For every 10 grms. of lead 60 c.cms. water and 16 c.cms.
nitric acid should be employed. The antimony present is
kept in solution by aid of 5 to 10 grms. tartaric acid. In
order to precipitate the lead as sulphate, 3 c.cms. cone,
sulphuric acid are used for each 10 grms. lead dissolved,
and 2'15 c.cms. are deducted as the volume of the lead sul-
phate produced. On account of the presence of tartaric
acid the nitrate from the lead sulphate is not evaporated
quite to dry ness, and the treatment with sodium hydrate
and sodium sulphide follows at once.

The remainder of the analytical procedure is conducted
as already described under ' Soft Lead. 5

Commercial Zinc. The zinc of commerce always con-

PEACTICAL EXAMPLES 235

tains, in addition to lead, small amounts of iron, cadmium,
arsenic, antimony, tin, and copper. In order to determine
these impurities it is necessary to dissolve from 20 to 100
grms. of the metal, according to its degree of purity. The
weighed sample in the form of borings or small pieces is
placed in an Erlenmeyer flask provided with a funnel tube
and a gas-delivery tube, and, after the addition of warm
water and dilute sulphuric acid, the solution of the zinc is
effected at a gentle heat.

The gas that is given off on solution is passed through
hydrochloric acid containing bromine, or through a solution
of hydrogen peroxide in order to absorb the arseniuretted
hydrogen that it may contain. Since zinc is a strongly
positive metal, and separates all other metals from their
salt solutions, all the metals present in the zinc as
impurities will appear in the flask as a metal sponge, which
will not be attacked by the acid until all the zinc has
passed into solution. This spongy precipitate is therefore
filtered off before the last traces of the zinc have dissolved.
It will contain all the metals named as impurities of
commercial zinc with the exception of the arsenic, which
will be found in the absorbing solution through which the
evolved gas was passed. The arsenic is separated by
evaporating the solution, and by precipitating with sul-
phuretted hydrogen gas. The mass of spongy metal is
dissolved in aqua regia, and, after evaporating off the
nitric acid, sulphuretted hydrogen gas is conducted through
the hydrochloric acid solution of the metals. The precipi-
tate is digested with sodium sulphide, the solution of the
antimony and tin sulpho-salts and the insoluble residue,
which may con tain lead, copper, cadmium, and bismuth, being
treated further as already described under 'Antimony -Tin'
(see p. 201) and 'Refined Soft Lead' (see p. 232) respectively.

The filtrate from the sulphuretted hydrogen precipitate
may contain iron, zinc, and possibly manganese.

These are separated as described under * Refined Soft
Lead.' ^^^^AS^

/& Of THB *\

I UNIVERSITY I

236 THE ELECTROLYTIC PROCEDURE

It is advisable to test the solution of the zinc in excess
of sodium hydrate, for iron, by means of a few drops of
permanganate.

The arsenic, antimony, and tin may also be determined
in a special sample of the zinc by dissolving in aqua regia,
evaporating to dryness, taking up with hydrochloric acid,
precipitating with sulphuretted hydrogen gas, and digesting
the sulphides with sodium sulphide solution. The solution
of the sulpho-salts of these three metals thus obtained is
then treated further by the method described under
1 Britannia Metal ' on p. 231.

Black Copper, Raw Copper. Black copper is not a
pure smelting product, but is an alloy of copper with small
amounts of iron, nickel, cobalt, zinc, antimony, arsenic,
silver, gold, and bismuth. The total of these impurities in
raw copper amounts to - 40 to '70 per cent.

In order to determine the impurities it is most con-
venient to dissolve separately two 25-grm. pieces of the
bright and clean sample in a mixture of 200 c.cms. water,
and 175 to 180 c.cms. nitric acid of 1'20 sp. gr. To the
clear solution thus obtained 25 c.cms. cone, sulphuric acid
are added, the whole is evaporated to dryness, and the free
sulphuric acid is expelled.

The residue is taken up with 20 c.cms. nitric acid and
350 c.cms. water, and the silver, the amount of which has
been previously determined, is precipitated as chloride in
this solution by the addition of the exact volume required of a
standardised hydrochloric acid solution. The precipitate
produced, which may contain, in addition to silver, lead and
antimony, is allowed to settle, and is then filtered off. The
copper is separated from the nitric acid solution by electro-
lytic deposition upon a platinum cone of large size, which
must be frequently changed. The deposition is stopped
when che solution has become colourless, in order to prevent
the separation of arsenic and antimony at the kathode that
would otherwise occur. The solution of the second 25-
grm. portion of the copper sample is treated in a similar

PRACTICAL EXAMPLES 237

manner, and the two solutions that remain after the
deposition of the copper are then mixed, evaporated to
dryness, taken up with hydrochloric acid, filtered, and
sulphuretted hydrogen gas passed through the hot solu-
tion until a complete precipitation of the arsenic has been
effected. The nitrate from this precipitate may contain
iron, cobalt, nickel, and zinc, and these metals are separated
in the manner described under ' Refined Soft Lead.'

The precipitate may contain, in addition to arsenic,
antimony lead silver copper and bismuth, all as sulphides.
Both this precipitate and that first obtained with hydro-
chloric acid are digested with sodium sulphide.

The solutions of the sulpho-salts thus obtained are
mixed, and are then treated further as described under
'Britannia Metal' on p. 231. The residue of metallic
sulphides, insoluble in sodium sulphide, is dissolved in
nitric acid, the silver is precipitated by means of hydro-
chloric acid, the bismuth by ammonium carbonate, and the
lead and copper are separated by the method described
under ' Copper- Lead ' on p. 178. The bismuth will be
found to have partly separated with the copper first de-
posited, and it is therefore necessary to dissolve this
copper in nitric acid, to boil this solution with excess of
cone, hydrochloric acid until all the nitric acid has been
displaced, to remove the excess of hydrochloric acid by
further boiling, and to precipitate the bismuth and portion
of the copper present as basic chlorides by the addition of a
large volume of boiling water.

After allowing the precipitates of oxychlorides to settle
completely, they are filtered off, dissolved in nitric acid, and
the two metals separated by means of ammonium carbonate.

Electrolytic copper is analysed in the same manner, but
in this case, since the impurities are less in amount, a
greater weight of copper must be employed in the analysis.

Refined Copper. This grade of copper contains
cuprous oxide in addition to the impurities found in black
copper. The determination of this is effected in a separate

238 THE ELECTKOLYTIC PROCEDUEE

sample as follows. A few grams of the finely divided
sample is shaken with from 100 to 150 times the weight
of water, containing rather more silver nitrate than is
theoretically required for the amount of cuprous oxide
assumed to be present.

The reaction between the oxide and the silver nitrate
results in the formation of silver, basic copper nitrate, and
neutral copper nitrate, the first two of which separate in
the solution as a precipitate. This is filtered off, dissolved
in nitric acid, and the silver separated from the copper by
the method described under 'Copper-Silver' on p. 181.
The calculation of the results is based on the relationship
expressed by 2Ag=Cu 2 0.

Commercial Tin, Tin-foil. Commercial tin always
contains antimony, arsenic, lead, iron, and copper as
impurities. In order to estimate these, a weighed portion
of the sample is dissolved in aqua regia, the solution is
evaporated to dry ness, and, after taking up the residue with
hydrochloric acid and water, sulphuretted hydrogen gas is
passed through the resulting solution. The precipitate is
treated with sodium sulphide solution, and the three metals,
arsenic, antimony, and tin, which pass into solution, are
separated as described under ' Britannia Metal 'on p. 231.
In this case it is necessary to dissolve the first deposit of
antimony, and to redeposit the metal from the solution
thus obtained, since, if the electrolysis continues for any
length of time, the first deposit of antimony will be found
to contain tin. The sulphides of lead and copper in the
insoluble residue from the sodium sulphide digestion are
dissolved in nitric acid, and these two metals separated in
the usual manner (see p. 178). The iron is determined
in the filtrate from the sulphuretted hydrogen precipitate.

Cast-iron, Steel, Iron Ores. Electrolytic methods are
only in use for the determination of two constituents of
raw iron or iron ores lead and copper. In order to effect
the determination of these 5 to 10 grins, of the sample are
dissolved in hydrochloric acid, the solution is evaporated to

PRACTICAL EXAMPLES 239

dryness, taken up with hydrochloric acid and water, filtered
hot, and sulphuretted hydrogen gas passed through. The
precipitate of lead and copper sulphide thus obtained is
dissolved in nitric acid, and the two metals are electro-
lytically separated by the method described fully on p. 178.

Cube-Nickel. This commercial product contains, in
addition to its chief impurity copper, arsenic, antimony,
iron, cobalt, carbon, and sulphur.

The sample of metal is dissolved in aqua regia, the
solution is evaporated to dryness, taken up with hydro-
chloric acid, the diluted solution filtered, and sulphuretted
hydrogen gas is conducted through the clear filtrate. The
treatment of the precipitate of sulphides thus obtained is
carried out as described under some of the preceding alloys.

The filtrate is evaporated to a small bulk, and is
oxidised with bromine water. After treatment with dilute
sulphuric acid, if the iron present is only small in amount,
an addition of ammonium hydrate is made in order to pre-
cipitate it. If the amount of iron is considerable, the
solution is neutralised with sodium hydrate, acetic acid
is added, and the iron is precipitated as basic acetate.

The iron may be determined in these precipitates either
gravimetrically or by the electrolytic method described on
p. 102. The nickel and cobalt contained in the filtrate
from the precipitate of iron are determined together,
according to the method given in detail on p. 106. The
separation of the two latter metals must then be carried
out by the gravimetric methods of analysis.

Nickel-Speiss, Raw Nickel. Nickel-speiss consists
mainly of a compound of nickel, iron, copper, and cobalt,
with arsenic and sulphur.

In order to analyse this mineral, 1 grm. of the finely
ground sample is dissolved in aqua regia or in forming nitric
acid, and the solution thus obtained is evaporated to dry-
ness. The residue is taken up with hydrochloric acid and
water, the solution is heated, and sulphuretted hydrogen
gas is then conducted through it until cold. The precipi-

240 THE ELECTEOLYTIC PKOCEDURE

tate of copper and arsenic quickly settles ; it is filtered off
and dissolved in a small quantity of nitric acid. This
solution may be converted by chemical means into an am-
moniacal one, and the copper then separated from the
arsenic by the method described under ' Copper- Arsenic ' on
p. 186. It is, however, better to treat the nitric acid
solution with sodium hydrate and sodium sulphide. The
antimony and arsenic which pass into solution are separated
according to the method given on p. 201. The copper
remains undissolved as copper sulphide, and is electro-
lytically deposited from its nitric acid solution as described
under ' Copper ' on p. 93. The filtrate from the precipitate
obtained with sulphuretted hydrogen is evaporated to dry-
ness after addition of a small quantity of potassium
chlorate. If the amount of iron present is under 4 per
cent., the residue is taken up with sulphuric acid, and
ammonium hydrate added in excess. The precipitate of
iron as hydroxide is filtered off, and the metal determined
either by the gravimetric or electrolytic method. The
filtrate from the ferric hydrate is electrolysed in order to
separate the nickel, as described under 'Nickel' on p. 106.

If the amount of iron present exceeds 4 per cent., the
residue that remains after evaporation is dissolved in a
small amount of hydrochloric acid, the solution is made
slightly alkaline with sodium hydrate, and the iron, is pre-
cipitated as basic acetate by the addition of acetic acid and
by boiling. The further treatment for the determination
of the nickel and the iron is then carried out as described
above.

Raw nickel from the lead-smelting works contains the
same constituents as nickel- speiss, and it may therefore
be analysed by the method described for the latter. Since,
however, there is as a rule no necessity to determine any
constituents beyond the copper and nickel, a shorter
method may be employed.

The solution is prepared by dissolving 1 grm. of the
finely powdered sample in hydrochloric acid containing

PEACTICAL EXAMPLES 241

bromine, and is evaporated to dry ness in order to drive off
the arsenic. This evaporation is repeated many times with
fresh amounts of the acid. The residue is then taken up with
a few cubic centimetres dilute sulphuric acid, and the solu-
tion is again evaporated until white fumes appear. Water is
then added, sulphuretted hydrogen gas is conducted through
the solution, and the precipitate of sulphides is filtered off.

This precipitate is then ignited in order to drive off any
remaining traces of arsenic or antimony, and, after cooling,
the copper oxide is dissolved in nitric acid, and the metal
deposited electrolytically.

The nitrate from the sulphides precipitate is oxidised
with bromine water, the iron is precipitated with am-
monium hydrate and filtered off (this requires repeating,
after re- solution of the first precipitate), and the filtrate
is evaporated to dryness. The residue is ignited with
ammonium chloride, in order to remove the zinc as zinc
ammonium chloride.

The remaining salt is brought into solution with dilute
sulphuric acid and water, and, after addition of an excess
of ammonium hydrate, the nickel is electrolytically de-
posited under the conditions given on p. 106.

Nickeliferous Magnetic Pyrites ; Arsenical Cobalt and
Nickel Ores ; Roasted Cobalt Slimes from Colour Works ;
and other Nickel and Cobalt Smelting Products. These
products and substances are treated by methods exactly
similar to those described for nickel-speiss and raw nickel.

Copper Regulus and Lead Matte. Copper regulus and
lead matte contain, in addition to copper and lead, much
iron, sulphur, and silica. It is sufficient for most practical
purposes to know the percentage of copper and lead. In
order to effect the determination of these two constituents,
1 grm. of the finely powdered substance is dissolved in
30 c.cms. nitric acid, and the solution after boiling is diluted
with hot water and filtered. The lead is then separated
from this solution as peroxide at the anode by electrolysis.
When the lead deposition is completed, the kathode bearing

242 THE ELECTROLYTIC PROCEDURE

the greater portion of the copper is dipped into the
remaining acid electrolyte, and the whole of the copper is
allowed to pass again into solution. The solution is
evaporated to dryness with sulphuric acid, the residue is
taken up with water, and the copper is precipitated by
boiling with sodium hyposulphite or by conducting sul-
phuretted hydrogen through the solution. The precipitate
of sulphides is filtered off, ignited, and the oxide dissolved
in nitric acid ; from this solution the copper is electro-
lytically deposited. It is not possible to effect a direct
separation of the copper, since the traces of arsenic, anti-
mony, and silver present in these products would be
deposited at the kathode with the copper, and the large
amount of iron present would exercise a disturbing
influence.

Cupriferous Pyrites ; Burnt Pyrites. In cupriferous
pyrites, and in lixiviated or unlixiviated roasted pro-
ducts, it is chiefly necessary to know the percentage of
copper.

About 5 grms. of the finely powdered sample is treated
with hydrochloric acid, to which later some nitric acid is
added, and the solution of the soluble portion of the ore
is effected with the aid of gentle heat. The liquid is then
evaporated nearly to dryness to remove the excess of
nitric acid, the diluted hydrochloric acid solution is filtered,
and sulphuretted hydrogen is passed through the filtrate.
The precipitate will contain lead, copper, and arsenic as
sulphides, and is treated for their separation as described
under the last product.

Copper Ashes, Copper Matte, Copper Slags, Flue
Lust. The copper in cupriferous ashes and some other
furnace by-products may be extracted by simple digestion
with nitric acid. The copper is then separated from this
acid solution by electrolytic deposition.

Those products which do not dissolve in nitric acid
require treating by the method described under ' Copper
Regulus.'

PEACTICAL EXAMPLES 243

Galena. This natural ore of lead contains principally
lead sulphide, but small amounts of copper, iron, silver,
antimony, arsenic, and zinc are always present with the
lead.

If the amount of antimony is not great, the finely
ground sample of the ore is treated with cone, nitric acid,
and after the oxidation is completed the solution is diluted,
filtered from the gangue, and the filtrate evaporated to
dryness.

The residue is taken up with hydrochloric acid and hot
water, and sulphuretted hydrogen gas is then passed
through the hot solution. The liquid is allowed some time
to settle, and is then filtered. The precipitate is treated
with ammonium carbonate solution in order to extract the
arsenic, with sodium sulphide solution to remove the
antimony, and the residue (part of it only, if more than 1
grm. ore has been used) is then dissolved in nitric acid, and
the copper separated from the lead by the electrolytic
method.

If the galena be one containing exceptionally large
amounts of antimony, a separate portion of the sample
weighing a few grams is treated with nitric and tartaric
acids, and the liquid is then made up to a definite volume.
A measured portion of this is withdrawn, the lead is pre-
cipitated as lead sulphate by means of sulphuric acid, and
the filtrate is treated with excess of sodium hydrate and
with sodium sulphide.

The solution of the sulpho-salt of antimony thus ob-
tained is then used for the electrolytic deposition of that
metal as described under 'Antimony' (see p. 146). The
filtrate from the first precipitate with sulphuretted hy-
drogen contains zinc and iron. It is oxidised by means
of bromine water, the iron is precipitated by ammonium
hydrate, and the zinc is determined either volu metrically
or by one of the electrolytic methods given under ' Zinc '
on p. 114.

The silver is most accurately determined in galena by

R 2

244 THE ELECTEOLYTIC PROCEDURE

the dry method. Very frequently it will be found that the
analysis is most successfully performed by determining the
individual metals in different test-samples of the ore. In
all cases the lead and the gangue are first removed, by dis-
solving the ore in nitric acid, evaporating with sulphuric
acid to dryness, and filtering off the insoluble portion and
the lead sulphate.

Roasted galena is treated in exactly the same manner
as galena, with the exception of the preparation of the
solution of the ore. This is effected by direct treatment
with aqua regia, repeated evaporation to dryness with
hydrochloric acid, and final solution of the residue in
hydrochloric acid and water.

Fahl Ores ; Tetrahedrite. The different fahl ores con-
tain arsenic, antimony, lead, zinc, iron, and silver. If no
arsenic be present in the ore, 1 grm. of the finely ground
sample is dissolved in 15 c.cms. aqua regia, the solution is
evaporated to dryness, the residue brought into solution by
means of hydrochloric acid and water, and the separation
of the individual metals then effected by the methods
described under ' Galena.' The silver will be found in the
residue from the first solution of the ore ; its amount is
most satisfactorily determined by operating by the dry
method upon a larger amount of the ore.

If arsenic be present, the ore is opened up by means of
10 c.cms. nitric acid in place of aqua regia.

Tin-stone. This ore of tin consists principally of tin
and iron oxides. In order to open up the ore, the very
finely ground sample is fused with 3 parts sodium hydrate
and 3 parts sulphur. The melt when cold is lixiviated
with water, an excess of sodium sulphide solution is added,
and the further conduct of the analysis follows that
described under * Commercial Tin ' on p. 238.

The residue from the melt and lixiviation contains the
iron and the other impurities of the ore. These are sepa-
rated by the methods already described.

Stibnite. This ore of antimony is a sulphide of the

PRACTICAL EXAMPLES 245

metal, containing iron, lead, copper, and arsenic as im-
purities. The opening up of the finely ground sample of
ore for analysis is achieved by fusing with sodium hydrate
and sulphur as in the case of tin-stone.

The melt is lixiviated with water, an excess of sodium
sulphide is added to the solution, and the antimony is
electrolytically determined in the filtered liquid by the
method given under * Antimony ' on p. 146. The arsenic
is determined in the remaining electrolyte.

The residue that remains after lixiviation of the melt
contains the lead, copper, and iron. It is dissolved in
nitric acid, and the two former metals are electrolytically
separated in this solution by the method described fully
under ' Copper- Lead ' on p. 178. The iron remains in
solution, and, after conversion of the nitrate into sulphate,
it is determined by electrolytic deposition.

The above series of practical examples does not contain
references to all the cases in which electrolytic methods
are now being employed, or in which they might be
employed, for the analysis of alloys, furnace by-products,
metals, or ores. These examples are given chiefly as a
guide to the various ways in which electrolysis may be used
as an aid to the ordinary methods of analysis. It will always
be found most convenient to combine the chemical and
electrolytic methods of separation ; and, rightly used, the
latter will take an independent place by the side of the
gravimetric and volumetric methods which have hitherto
been solely employed in analytical work.

246

APPENDIX

Theoretical Percentage of the Metallic Elements in certain
Metallic Salts.

No.

Name of Salt

Chemical Formula

Percentage

f Antimonyl Tartrate

OC..-IA qu

t (Tartar-emetic) .

f C> 4 rl 4 (bD<J)K.U 6 + 3*1 2 U

t>O J.T: OU

Bismuth Nitrate .

Bi(NO,), + 5H..O 42-91 Bi

Cadmium Sulphate

CdS0 4 + 4H,0 i 40-00 Cd

Cobalt Sulphate .

CoS0 4 + 7H.,0 20-92 Co

Cobalt Chloride .
Copper Sulphate .

CoCl 2 +6H 2 6
CuS0 4 + 5H.,0

24-71 Co
25 33 Cu

Copper Chloride .

CuCl 2 + 2H 2 6

37-06 Cu

Ferrous Sulphate .

FeS0 4 + 7H 2

20-14 Fe

f Ferrous Ammo- )
1 nium Sulphate >

FeS0 4 (NH 4 ) 2 S0 4 + 6H 2

14-28 Fe

Gold Chloride

AuCl 3 + 2H 2

57-98 Au

Lead Nitrate .

Pb(N0 3 ) 2

j 62-54 Pb
(72-21 Pb0 2

f Manganese Sul- 1
phate .

MnS0 4 + 7H 2

r 19-85 Mn
1 31-40 Mn0 2

Manganese Nitrate

Mn(N0 3 ) 2 +6H 2

(19-16 Mn
(30-31 MnO,

Mercuric Chloride.

HgCl 2

73-80 Hg

Nickel Sulphate .

NiS0 4 + 7H..O

20-94 Ni

Nickel Chloride .

NiCL+6H,6

24-72 Ni

Platinic Chloride

PtCl 4 + 5H 2

45-56 Pt

f Potassium Chloro- )
1 platinate .

K.PtCl,,

40-00 Pt

j Potassium Auric )
1 Chloride . /

AuCl 3 .KCl + 3H 2

42-05 Au

[ Potassium Ferric j

Sulphate (Iron \

Fe(S0 4 ) 3 .K 2 S0 4 + 24H 2

11-12 Fe

( Alum) . )

i Potassium Ferric )
1 Oxalate . 1

Fe 2 (C 2 4 ) 3 .3K 2 C 2 4 + 6H 2

11-40 Fe

Silver Nitrate

AgN0 3

63-52 Ag

Stannous Chloride

SnCl 2 + 2H 2

52-04 Sn

Zinc Sulphate

ZnS0 4 + 7H 2

22-68 Zn

NAME INDEX

ARBHENIUS, 22, 25, 30

BECQUEBEL, 2, 127, 130

Beilstein and Jawein, 115, 122

Bersoz, 153

Berzelius, 7, 8

Bloxam, 2

Bourgoin, 14

Brand, 101, 105, 121, 127, 131,

137, 159, 212, 217
Bunsen, 14, 38, 39

CLAKKE, 140

Classen, 80, 109, 117, 130, 141,

142, 145, 169, 209, 211, 212,

217

Classen and Bongartz, 100
Classen and Eliasberg, 159
Classen and Halberstadt, 154
Classen and Ludwig, 140, 145,

188
Classen and V. Eeiss, 100, 102,

109, 117, 124, 127, 130, 145,

148, 149, 159
Clausius, 21, 24

DANIELL, 7, 8

Davy, 2

de la Escosura, 141, 142

de la Kive, 2, 152

Drossbach, 97, 185

Dulong and Petit, 19

ELBS, 72
Eliasberg, 206

Elkington, 2, 152
Engels, 132, 151, 212

FABADAY, 5, 16

Foregger, Von, 110, 112, 216

Fresenius and Bergmann, 106,

135
Freudenberg, 37, 165, 177, 182,

205, 207

GALVANI, 1

Gibbs, 2, 96, 97, 106, 174
Gore and Sanderson, 145
Groger, 133
Grotthiiss, 24

HELMHOLTZ, Von, 16
Heydenreich, 101, 123, 174, 188
Hittorf, 9, 12, 25, 26, 27
Hotf, Van't, 30

JACOBI, 1

Jahn, 14

Joly and LeidiS, 155, 156

Jordis, 118

KILIANI, 127, 165, 182
Kiliani and V. Foregger, 120
Klobukow, Von, 80
Kohlrausch, 19, 20, 28, 53, 58
Kriitwig, 137

248

NAME INDEX

LE BLANC, 44, 46

Luckow, 2, 96, 97, 99, '105, 106,
109, 110, 115, 123, 127, 128, 130,
135, 138, 139, 145, 148, 152, 160,
188

Liipke, 17

MAC KAY, 97, 185

Millot, 114, 115, 120

Moore, 99, 105, 121, 125, 130, 148

Morton, 2

Miiller, Von, and Kiliani, 105, 117,

125, 137, 154
Mylius and Fromm, 41

NEUMANN, 53, 157, 189, 190
Nicholson and Avery, 105, 110,
121

OBEKBECK, 47
Oettel, 97, 108, 185
Ohl, 109, 110
Ostwald, 4, 10, 34

PARRODI AND MASCAZZINT, 102, 113,
119, 127, 145

EAYLEIGH, 20

Reinhardt and Ihle, 113, 114, 117
Riche, 106, 114, 119, 130
Eiidorfif, 98, 114, 119, 130, 133,

140, 154, 159
Euolz, 2, 99, 152

SCHIFF, 127

Schmucker, 142, 159, 187

Schucht, 127, 130, 155, 156, 157,

158

Schweder, 109, 110, 172
Smith, 101, 105, 123, 137, 142,

153. 154, 156, 177, 184, 192, 195,

196, 206

Smith and Cauley, 141
Smith and Frankel, 130, 141, 159,

183, 196, 205, 207, 208
Smith and Keller, 155
Smith and Knerr, 139, 158, 206
Smith and Moore, 125, 152
Smith and Moyer, 140, 188
Smith and Muhr, 109, 198
Smith and Saltar, 158
Smith and Spencer, 183

THOMAS AND SMITH, 158, 159

Volta, 1

Vortmann, 80, 114, 120, 127, 142,
148, 159, 160, 211, 214, 215

WAHL, 155

Wallace and Smith, 122, 141, 153,

198, 207

Warwick, 101, 123, 125, 130
Weber and Kbhlrausch, 19
Weil, 127
Wieland, 158
Winkler, 107
Wrightson, 109, 110

YVER, 206

SUBJECT INDEX

ACCUMULATORS, use of, 69

Acid solutions, separations in, 40

Acids, decomposition of organic,

13,14
Alloys, analysis of,

copper -aluminium, 228
copper-gold, 228
copper-nickel, 222
copper-nickel-zinc, 222
copper-nickel-zinc-silver, 225
copper-silver, 221
copper-tin, 226
copper-tin-zinc, 227
copper-tin - zinc, - phosphorus,

227

copper-zinc, 220
lead-antimony, 230
lead-tin, 228
lead-tin-bismuth, 229
lead - tin - bismuth - cadmium,

229

tin-antimony-arsenic, 230
tin-mercury, 230
tin-zinc, 228
Aluminium,

Separation from cobalt, 218
from copper, 177
from iron, 213
from nickel, 218
from zinc, 218
Aluminium bronze, analysis of,

228

Amalgams, 160
Ampere, definition, 20
Ampere meters ; or ammeters, 57
Anions, definition of, 5, 9
Anodes, definition of, 5

Antimony, electrolytic deposition
of, 144

Separation from arsenic, 200
from cadmium, 208
from copper, 186
from gold, 199
from mercury, 194
from silver, 191
from. tin, 201
Antimony, amalgam, 164
Arsenic, electrolytic deposition of,
148

Separation from antimony,
200

from bismuth, 200
from cadmium, 208
from copper, 185
from gold, 198
from lead, 189
from mercury, 194
from platinum, 199
from silver, 191

BASINS, for electrolytic operations,

Bismuth, electrolytic deposition
of, 157

Separation from arsenic, 200
from cadmium, 200
from cobalt, 200
from copper, 185
from iron, 200
from lead, 188
from mercury, 194
from nickel, 200
from silver 191

250

SUBJECT INDEX

Bismuth, separation from tin,
200

from zinc, 200

Bismuth amalgam, 162

Brass, analysis of, 220

Britannia metal, analysis of,
231

Bronze, analysis of, 226

aluminium, analysis of, 228
phosphor, analysis of, 227

Burnt pyrites, analysis of, 242

CADMIUM, electrolytic deposition
of, 122

Separation from alkalies, 208
from arsenic 208
from bismuth, 200
from chromium, 208
from cobalt, 206
from copper, 174
from iron, 207
from lead, 190
from manganese, 208
from mercury, 196
from molybdenum, 208
from nickel, 206
from osmium, 208
from silver, 192
from tin, 208
from tungsten, 208
from zinc, 205
Cadmium amalgam, 161
Charges of the ions, 19, 20, 23
China silver, analysis of, 225
Cobalt, electrolytic deposition of,
111

Separation from aluminum,
218

from bismuth, 200
from cadmium, 206
from chromium, 218
from copper, 172
from iron, 209
from lead, 190
from manganese, 217
from mercury, 196
from nickel, 214
from silver, 192
from zinc, 215
Cobalt, ores of, analysis, 241

Cobalt, regulus, analysis, 241
Cobalt smelting products, analysis,

241

Complex salts, 10, 11
Conductivity of the electrolyte,
9, 21, 72

of water, 8, 31
maximum, 27, 28

molecular, 27
relative, 28
specific, 27

Copper, electrolytic deposition of,
92

Separation from aluminium,
177

from antimony, 186
from arsenic, 185
from barium, 177
from bismuth, 185
from cadmium, 174
from chromium, 177
from cobalt, 172
from iron, 169
from lead, 177
from magnesium, 177
from manganese, 179
from mercury, 184
from nickel, 172
from potassium, 177
from silver, 181
from tin, 186
from zinc, 167

Copper ashes, analysis of, 242
Copper, black, analysis of, 236
Copper, German mint, analysis of,

227

Copper matte, analysis of, 242
Copper pyrites, 242
Copper, raw, analysis of, 236
Copper, refined, analysis of, 237
Copper regulus, analysis of, 241
Copper slag, analysis of, 242
Copper voltameter, 52
Coulomb, definition of, 20
Coupling cells and accumulators,
mixed, 68
parallel, 66
series, 66

Cupriferous flue dust, analysis of,
242
pyrites, analysis of, 242

UNIVERSITY

SUBJECT INDEX

251

Current conduction, 67

regulation, 49, 63, 87
Current density, calculation of, 38,
86

definition of, 37

normal, 37, 87

influence upon deposits, 38
Current strength, diminution of, 74

increase of, 65

measurement of, 50, 60, 87

unit of, 20

DECOMPOSITION of acetic acid, 14
formic acid, 13
lead chloride, 6, 34
potassium sulphate, 8, 34
typical complex salts, 12, 13
zinc chloride, 6, 39, 41

Decompositions, primary, 6
secondary, 8, 11

Decomposition values, 45, 46

Deposits, drying the, 89
washing the, 89

Detonating gas voltameter, 52, 53

Dissociation, theory of, 22, 23, 25
of water, 24, 31, 39

Double salts, 10, 11

ELECTEOCHEMISTKY, development

of, 1

Electrochemical equivalents, 19
Electrodes, basin, 80
fork-shaped, 108
forms of, 80, 81, 82
holders for, 84, 85
jacket, 83
materials for, 79
saucer, 82
spiral, 83

Electrolysis, conduct of analytic,
86

phenomena of, 6, 33
Electrolytes, conductivity of, 21,
72,88

constitution of, 21
Electrolytic procedure, general re-
marks, 79, 86

Electro-motive force, calculation
of, 46

Electro-motive force, influence upon

decompositions, 36
measurement of, 60, 61, 63,

Electrotyping, earliest examples
of, 1

FAHL ores, analysis of, 244
Faraday's laws, 16, 17
Flue dust, analysis of, 242

GALENA, analysis of, 242
Galvanometers, general remarks,
50

tangent, 54
torsion, 56

Gas couple, example of, 44
German silver, analysis of, 222
Gold, analysis of mint-, 228

electrolytic deposition of,

152

separation from silver, 191
mercury, 195
other metals, 198

HAFTINTENSITAT, Le Blanc's theory
of, 44

IONS, absolute velocity of, 29
definitions of, 5, 9
electro-static charges of, 19,

20,23

energy carried by, 19, 20, 23,
migration of, 24
relative velocities of, 25, 26,

valency of, 9, 23
Ionic dissociation, 31
Ionic reactions, 10
Iridium, electrolytic deposition of,

156

Iron, electrolytic deposition of,
102

separation from aluminium,
213

from bismuth, 200
from cadmium, 207

252

SUBJECT INDEX

Iron, separation from chromium, i
214

from cobalt, 209

from copper, 169

from gold, 198

from lead, 190

from manganese, 211

from mercury, 196

from nickel, 209

from silver, 192

from zinc, 211
Iron, analysis of cast, 238
analysis of ores of, 238

JACKET electrodes, 83
Joule, definition of, 20

KATHODES, definition, 5
Kations, definitions of, 5, 9
Kohlrausch's laws, 28

LEAD amalgam, 161
Lead, electrolytic deposition of,
126

separation from arsenic, 189
from bismuth, 188
from cadmium, 190
from cobalt, 190
from copper, 177
from iron, 190
from manganese, 189
from mercury, 188
from nickel, 190
from silver, 187
from zinc, 190
Lead trees, 42

Lead, argentiferous, analysis of,
234

hard, analysis of, 230
matte, analysis of, 241
refined, analysis of, 232
soft, analysis of, 232

MANGANESE, electrolytic deposition
of, 130

separation from cadmium,
208

Manganese, separation from cobalt,
217

from copper, 179
from iron, 211
from lead, 189
from mercury, 196
from nickel, 217
from zinc, 218

Mercury, electrolytic deposition of,
138

separation from antimony,
194

from arsenic, 194
from bismuth, 194
from cadmium, 196
from cobalt, 196
from copper, 184
from gold, 195
from iron, 196
from lead, 188
from manganese, 196
from nickel, 196
from osmium, 196
from palladium, 196
from platinum, 196
from silver, 191
from tin, 195
from zinc, 196
Metals incapable of deposition,

list of, 159
Migration of the ions, theory of,

24
Migration velocities, absolute, 29

relative, 25, 26, 29
Mixed coupling, 68
Mixed salt solutions, decomposition
of, 35

NICKEL, electrolytic deposition of,
106

separation from aluminium,
218

from bismuth, 200
from cadmium, 206
from chromium, 218
from cobalt, 214
from copper, 172
from iron, 209
from lead, 190
from manganese, 217

SUBJECT INDEX

253

Nickel, separation from mercury,
196

from silver, 192
from zinc, 215

Nickel, cube-, analysis of, 239
mint-, analysis of, 222
mint-, old Swiss, analysis of,

225

ores, analysis of, 241
raw, analysis of, 239
smelting products, analysis of,

241

Speiss, analysis of, 239
Normal current density, definition
of, 37

OHM, definition of, 20

Organic acids, decomposition of,

13, 14
Osmotic pressure, law of, 30

PALLADIUM, electrolytic deposition

of, 155

Parallel coupling, definition of, 67
Peroxide formation, conditions of,

Phosphor-bronze, analysis of, 227
Platinum, electrolytic deposition

of, 154

separations from other metals,

199

Polarisation, causes of, 48
Products of electrolysis, primary,

33, 35

secondary, 33, 35
Pyrites, analysis of roasted, 242

REACTIONS, ionic, 10
primary, 6, 8
secondary, 8. 11
Kegulating the current, 63, 87
Eesistance, definition of circuit, 67
Resistance of metallic conductors,

67, 72, 74

Resistance-boxes, general remarks,
74,75

with mercury contacts, 76
with plug contacts, 75

Eesistances, forms of adjustable

wire, 77, 78
Eesults of electrolysis, recording

the, 90
Ehodium, electrolytic deposition

of, 156
Eose's metal, analysis of, 229

SALT solutions, decomposition of

mixed, 35

Salts, complex, 10, 11
double, 10, 11
metallic, percentage of metals

in, 246
Secondary cells or batteries, use

of, 69
Separations of several metals,

methods used, 165, 219
Separations in acid solutions, 40

by varying E.M.F., 37
Series coupling, definition of, 66
Shunt circuit, definition of, 55

use of, 57

Silver, electrolytic deposition of,
134

separation from antimony, 191
from arsenic, 191
from bismuth, 191
from cadmium, 192
from cobalt, 192
from copper, 181
from gold, 191
from iron, 192
from lead, 187
from mercury, 191
from nickel, 192
from palladium, 192
from platinum, 192
from zinc, 192

Silver, China, analysis of, 225
German, analysis of, 222
leaf, analysis of counterfeit,

228

mint-, analysis of, 221
Silver tree, formation of, 42
Silver voltameter, 51
Solder, analysis of, 228
Spongy deposits, cause of, 41
Steel, analysis of, 238
Stibnite, analysis of, 244

254

SUBJECT INDEX

Supports for electrodes, 84, 85
Switch-board for cells and accumu-
lators, 69

TANGENT galvanometer, 54
Temperature, influence of, 88
Tetrahedrite, analysis of, 244
Thallium, electrolytic deposition

of, 157

Thermo-battery, use of, 71
Tin, electrolytic deposition of, 148
separation from antimony, 201
from bismuth, 200
from cadmium, 208
from copper, 186
from gold, 198
from mercury, 195
Tin amalgam, analysis of, 230
Tin, commercial, analysis of, 238
Tin-foil, analysis of, 238
Tin-stone, analysis of, 244
Tombac, analysis of, 220
Torsion galvanometer, 56
Tree formations, cause of, 42
Type metal, analysis of, 230

VOLT, definition of, 20

Voltaic series of metals, 35
Voltameters, general remarks, 50

copper, 52

detonating gas, 52, 53

silver, 51
Voltmeters, technical forms, 62, 63

WOOD'S metal, analysis of, 229

ZINC, electrolytic deposition of,
113

separation from aluminium,
218

from bismuth, 200
from cadmium, 205
from chromium, 218
from cobalt, 215
from copper, 167
from gold, 198
from iron, 211
from lead, 190
from manganese, 218
from mercury, 196
from nickel, 215
from silver, 192

Zinc amalgam, 160

Zinc, analysis of commercial, 234

riUNTEI) BiT

3POTTISWOO]).: AND CO., NEW-STREET SQUAUJC
LOHDOH

WHITTAKEE & CO.'S
TECHNOLOGICAL AND SCIENTIFIC LIST.

TRANSFORMERS FOR SINGLE AND MULTIPHASE

CURRENTS : a Treatise on their Theory, Construction, and Use. By G-ISBERT
KAPP, M.Inst.C.E., M.InstE.E. Translated from the German by the Author.
With 133 Illustrations. 254 pages, with Index. Crown 8vo. 6*.
' Is the most complete and practical book we have seen on the subject.'

ELECTRICAL ENGINEER.

ELECTRIC TRANSMISSION OF ENERGY, and its Trans-

formation, Subdivision, and Distribution. A Practical Handbook. By GISBERT
KAPP, C.E., Member of the Institution of Civil Engineers, Member of the Institution
of Electrical Engineers. With 166 Illustrations. Fourth Edition, mostly re-
written. 455 pages. Crown 8vo. 10*. 6d.

'This book is one which must of necessity be found in the hands of every one who
desires to become acquainted with the best and latest information on the subject.'

ELECTRICAL ENGINEER.

THE DYNAMO: its Theory, Design, and Manufacture." With

190 Illustrations, mostly from original drawings. By 0. C. HAWKINS, M.A.,

A.I.E.E., and F. WALLIS, A.I.E.E. Second Edition, revised. 530 pages. 10s. Gd.

1 A work of no mean ability. One valuable feature thrdughout the book is the

excellence and number of the illustrations.' ELECTRICAL ENGINEER.

' We welcome this book as a thoroughly trustworthy and useful work.' ELECTRICIAN

ALTERNATING CURRENTS IN PRACTICE. A Practical

Treatise upon their Application to Industries. Authorised Translation from the
French of LOPPE and BOUQUET by F. J. MOFFETT, AJnst.E.E., Electrical
Engineer to the Colony of Lagos, W. Africa. With over 300 Illustrations.
Crown 8vo. 15.

CONTENTS : Alternators, Motors, Transformers and Condensers, Direct Transforma-
tion of a Current of one system into one of another system, Distributing Mains, Distri-
bution of Current, Commercial Measurement of Alternate Currents.

ALTERNATING CURRENTS OF ELECTRICITY AND

THE THEORY OF TRANSFORMER. By ALFRED STILL.

THE THEORY AND PRACTICE OF ANALYTICAL ELEC-

TROLYSIS OF METALS. Translated from the German of Dr. BERNARD
NEUMANN by J. B. C. KEESHAW, F.I.O. With 36 Illustrations. Crown 8vo. 10s. 6rf.
By THOMAS H. BLAKESLEY, M.A., M.Inst.C.E., Hon. Sec. of the Physical Society.

ALTERNATING CURRENTS OF ELECTRICITY. Third

Edition. Enlarged. 5s.
' It is written with great clearness and compactness of statement.' ELECTRICIAN.

ELECTRIC LIGHT INSTALLATIONS. By Sir D. SALOMONS,

Vice-President of the Institution of Electrical Engineers. A.I.C.E., M.AmerJ.E.E.,
M.P.S. Seventh Edition, thoroughly Revised and Enlarged.

Vol. I. ACCUMULATORS. With 33 Illustrations. 5*.
' The best work on the subject.' ENGLISH MECHANIC.

Vol. II. APPARATUS. With 305 Illustrations. 7*. 6rf.
Vol. III. APPLICATION. Witli 32 Illustrations. 5.
' A valuable addition to the literature of the subject.' ENGINEER.

CENTRAL STATION ELECTRICITY SUPPLY. By ALBERT

GAT, M.IJE.E., Chief Engineer Islington Central Station, and 0. H. TEAMAN,
Chief Assistant Engineer, Islington Central Station. With many Illustrations.

Whittaker's Technological and Science List.

PRACTICAL ELECTRICAL MEASUREMENTS. An Intro-

ductory Course in Practical Physics for Students and Young Engineers. By
E. H. CRAPPER, Lecturer in Physics and Electrical Engineering, Sheffield
Technical School. With 56 Illustrations. Crown 8vo. 2.i. 6d.

' The book not only furnishes a course in electrical testing but may also be profitably
ueed by advanced studems in technical schools.' NATURE.

THE ALTERNATING CURRENT CIRCUIT: an Introductory

and Non-Mathematical Book for Engineers and Students. By W. PERREN
MAYCOCK, M.I.E.E. With 51 Illustrations. Small Crown 8vo. cloth, 2s. 6a.

ELECTRICAL INFLUENCE MACHINES : containing a full

account of their Historical "Development, their Modern Forms, and their Practical
Construction. By J. GRAY, B.Sc. With 90 Illustrations. 4s. Gd.
' This excellent book.' ELECTRICAL REVIEW.

In Small Crown 8vo. cloth, Illustrated, 5s. per Volume.

ELECTRO-TECHNICAL SERIES. By EDWIN J. HOUSTON,

Ph.D., and A. E. KENNELLY, Sc.D.

Alternating Electric Currents.

Electric Heating.

Eleetromagnetism.

Electricity in Electro-Therapeutics.

Electric Incandescent Lighting.
Electric Motor.
Electric Street Railways.
Electric Telephony.

Electric Are Lighting. Electric Telegraphy.

ELECTRICITY IN OUR HOMES AND WORKSHOPS. A

Practical Treatise on Auxiliary Electrical Apparatus. With 143 Illustrations.
By SYDNEY F. WALKER, M.I.B.E., A.MJnst.C.E. Third Edition, Kevised and
Enlarged. Wide Crown 8vo. 65.
The work is eminently a practical one, and deserving of considerable praiee.'

_ - ELECTRICAL REVIEW.

WORKS BY S. R. BOTTONE.

A GUIDE TO ELECTRIC LIGHTING. For Householders

and Amateurs. With many Illustrations. 20th Thousand. Is.
' Accurate, lucid, and suitable for the purpose.' ELECTRICIAN.

HOW TO MANAGE A DYNAMO. Second Edition, Kevised

and Enlarged. Illustrated. Cloth, pocket size, Is.
' This little book will be very useful.' ELECTRICAL ENGINEER.

ELECTRICAL INSTRUMENT MAKING for AMATEURS.

With 71 Illustrations. Sixth Edition, Revised and Enlarged. 3s.
' To those about to study electricity and its application, this book will form a very
useful companion.' MECHANICAL WORLD.

ELECTRIC BELLS, AND ALL ABOUT THEM. With more

than 100 Illustrations. Fifth Edition, Revised. 3s.

'Anyone desirous of undertaking the practical work of electrical bell-fitting will find
everything, or nearly everything, he wants to know.' ELECTRICIAN.

ELECTRO-MOTORS: HOW MADE AND HOW USED.

With 86 Illustrations. Third Edition, Revised and Enlarged. 3*.
' We are certain that the knowledge gained in constructing machines snch as
described in this book will be of great value to the worker.' ELECTRICAL ENGINEER.

By Louia BELL, Ph.D., M.Arner.Inst.E.E.
ELECTRICAL POWER TRANSMISSION : a Practical Treatise

for Practical Men. VHh 229 Illustrations. Medium 8vo. cloth, 10*. 6d.
By CHARLES PROTEUS S EINMETZ, with the assistance of ERNST J. BERG.
THE THEORY AND CALCULATION OF ALTERNATING

CURRENT PHENOMENA. With 184 Illustrations. Medium 8vo. cloth, 10*. M.

Whittaker's Technological and Scientific List. 3

By ALFRED E. WEINER, M.A.mer.Inst.E.E.
PRACTICAL CALCULATION OF DYNAMO-ELECTRIC

MACHINES. A Manual for Electrical and Mechanical Engineers, and Text-book
for Students of Electro-Technics. Medium 8vo. cloth. Illustrated. 10s. Qd.

By Professor ERIC GERARD, Director of L'Institut Electrotechnique Monteflore,
University of Liege, Belgium.

LESSONS IN ELECTRICITY AND MAGNETISM. Trans-

lated under the direction of Loins DUNCAN, Ph.D., John Hopkins University.
With Additions as follows : A Chapter on the Rotary Field, by Dr. Louis
DUNCAN ; a Chapter on Hysteresis, by CHARLES PROTEUS STEINMETZ ; a Chapter
on Impedance, by A. E. KENNELLY ; a Chapter on Units, by Dr. CARY T.
HUTCHIJ.SON. With 112 Illustrations. Medium 8vo. cloth, 10s. 6d.
By Dr. FREDK. BEDELL and Dr. ALBERT C. CREHORE, of Cornell University.
ALTERNATING CURRENTS. An Analytical and Graphical

Treatment for Students and Engineers. 325 pages. With 112 Illustrations.
Second Edition. Medium 8vo. cloth, 10s. Qd.

By FRANK B. Cox, B.S.

CONTINUOUS CURRENT DYMAMOS AND MOTORS:

their Theory, Design and Testing. With Sections on Indicator Diagrams,
Properties of Saturated Steam, Belting Calculations, &c. An Elementary 1 reatise
for Students. Cloth. 271 pp. 83 Illustrations. 7s. 6d.
By CARL HERING.

ELECTRIC RAILWAYS, RECENT PROGRESS IN. About

400 pages and 120 Illustrations. Price 5s.

By Lieut. 0. D. PARKHUI;ST, Assoc.Mem.Am.Inst.E.E.

DYNAMO AND MOTOR BUILDING FOR AMATEURS.

With Working Drawings. With 22 Illustrations. 4s. 6d.
By GISBERT KAPP.

ALTERNATING CURRENTS OF ELECTRICITY: their

Generation, Measurement, Distribution, and Application. With' 37 illustrations
and 2 Plates. 4s. Qd.

By PHILIP ATKINSON, A.M., Ph.D.
ELEMENTS OF STATIC ELECTRICITY. WithfullDescrip-

tion of the Holtz and Tbpler Machines, and their Mode of Operating. Second
Edition, Revised. With 64 Illustrations. 6s. Qd.

By E. J. HOUSTON, A.M., Professor of Natural Philosophy in the Central High School,
Philadelphia ; Professor of Physics in the Franklin Institute of Philadelphia ;
Electrician of the International Electrical Exhibition.

DICTIONARY OF ELECTRICAL WORDS, TERMS, AND

PHRASES. Third Edition. With an Appendix. 667 pages, 582 Illustrations. 21s.
' Fills a very large gap that previously existed in electrical literature.'

A. E. KEXNELLY (Edison Laboratory).
' The name of the author is a sufficient guarantee of the excellence of the work.'

ELECTRICAL REVIEW.

By ECGENE 0. PARHAM, B.S., Electrician Steel Motor Company, Pennsjlvania, and
JOHN C. SHEDD, M.S. (Cornell), Professor of Physics. Marietta College, Ohio.

DYNAMOS AND MOTORS, SHOP AND ROAD TESTING

OF. Illustrated wifch many Diagrams and Cuts. fevo. cloth, 8s. 6rf.

By NIKOLA TESLA.
EXPERIMENTS WITH ALTERNATE CURRENTS OF

HIGH POTENTIAL AND HIGH FREQUENCY. With a Portrait and 35
Illustrations, and Biographical Sketch of the Author. 4s. Qd.
By E. A. MERRILL.

COMPLKTiS RULES FOR THE SAFE INSTALLATION OF ELECTRICAL PLANT.

ELECTRIC LIGHTING SPECIFICATIONS. For the Use of

Engineers and Architects. With the Phoenix Firlfc Rules. Second Edition,
Revised. 6s.

LONDON: WHITTAKER & CO., PATERNOSTER SQUARE, E.G.

WHITTAKEE'S LIBRARY

Illustrated. In Square Crown 8vo. cloth.

'Messrs. Whittaker's valuable series of practical manuals.'

Electrical Review,

THE PRINCIPLES OF PATTERN MAKING. Written

specially for Apprentices, and for Students in Technical Schools. By J. HORNER,

A.M.I.M.E. Illustrated with 101 Engravings, and containing a Glossary of the

Common Terms employed in Pattern Making and Moulding. 3*. d.

1 The book is well illustrated, and for its size will be found one of the best of its kind.'

INDUSTRIES.
'"By W. PERKEN MAYCOCK, M.T.E.E.

FIRST BOOK OF ELECTRICITY AND MAGNETISM.

Sec nd Edition, Revised and Enlarged, with 107 Illustrations. 2s. 6d.
'As a first book for such stud j nts as have to pass examination*?, it is admirable.'

ELECTRICAL ENGINEER.
By W. PERREN MAYCOCK, M.I.E.E.

THE ALTERNATING CURRENT CIRCUIT : an Introductory
and Non-Mathematical Book for Engineers and Students. With 51 Illustrations,
and Ruled Paper for Notes*. 2s. 6<t.

By the SAME AUTHOR.
ELECTRIC LIGHTING AND POWER DISTRIBUTIpN.

An Elementary Manual for Students preparing for the Preliminary and Ordinary
Grade Examination of the Oity and Guilds of London Institute. Written in
accordance with the new Syllabus. Fourth Edition. In 2 vols. Vol. I. with 231
Illustrations, 6s. Vol. II. preparing.

' The work will no doubt become a standard text- book for schools and classes on this
subject ; as such it has few equals.' ELECTRICAL REVIEW.

METAL TURNING. By J. HORNER, A.M.I.M.E. With 81 Illus-

trations. Second Edition. 4s.

' The book does well what it professes to do, its aim being to explain and illustrate
the practice of plain hand turning and slide-rest turning as performed in engineers'
workshops.' INDUSTRIES.

FITTING, THE PRINCIPLES OF. For Engineer Students.

By J. HORNER, A.M.I.M.E. Illustrated with about 250 Engravings, and contain-

ing an Appendix of Useful Shop Notes and Memoranda. 5s.

' Calculated to aid and encourage the most useful get of handicraftsmen we have
amongst us.' DAILY CHRONICLE.

PRACTICAL IRONFOUNDING. By J. HORNER, A.M.I.M.E.,

' a Foreman Pattern Maker.' Illustrated with over 100 Engravings. Second
Edition. 45.

' Every pupil and apprentice would find it, we think, an assistance to obtaining a
thorough knowledge of his work. The book, however, is not intended mtrely for the
student, but contains much useful information for practical men.' INDUSTRIES.

By G. E. BONNEY.
ELECTRICAL EXPERIMENTS. With 144 Illustrations. 2s. Gd.

This is an excellent book for boys.' ELECTRICAL REVIEW.
ENGINEER DRAUGHTSMEN'S WORK. By a Practical

Draughtsman. With 80 Illustrations. Is. 6<L
' Will be found of practical value to the beginner in the drawing office.' ENGINEER.

By G. E. BONNET.

INDUCTION COILS. A Practical Manual for Amateur Coil-
makers. With a new chapter on Radiography for those wishing to experiment
with Rbntgen's X Rays. With 101 Illustrations. Ss.

' In Mr. Bonney's useful book every part of the coil is described minutely in detail,
and the methods and materials required in insulating and winding the wire are fully
considered.' ELECTRICAL REVIEW.

Whi Maker's Library of Arts, Sciences, dc., 5

By Gr. B. BONXEY.

THE ELECTRO-PLATERS' HANDBOOK. A Practical Manual

for Amateurs and Young Students in Electro Metallurgy. With full Index and
61 Illustrations. Second Edition, Revised and Enlarged, with an Appendix on
ELECTROTYPING. 3.?.
'It contains a large amount of sound information.' NATURE.

By H. ORFORD.
LENS WORK FOR AMATEURS. With numerous Illustrations.

Small Crown 8vo. 3*.

' The book is a trustworthy guide to the manufacturer of lenses, suitable alike for
the amateur and the young workman." NATURE.

By the same Author.

MODERN OPTICAL INSTRUMENTS. With 88 Illustrations.

2s. Gd.
' Clearly and concisely written.' BRITISH JOURNAL OF PHOTOGRAPHY.

By J. TRAILL TAYLOR, late Editor of 'The British Journal of Photography.'

THEOPTICS OF PHOTOGRAPHY AND PHOTOGRAPHIC

LENSES. With 63 Illustrations. 3s. 6d.
' An excellent guide, of great practical use.' NATURE.

By E. H. CRAPPER, Lecturer in Physics and Electrical Engineering, Sheffield Technical

School.

PRACTICAL ELECTRICAL MEASUREMENTS. An Intro-
ductory Course in Practical Physics for Students and Young Engineers. With
56 Illustrations. 2s. Gd.

By SYDNEY P. WALKER, M.I.E.E., M.I.M.E., A.M.I.C.E.;

ELECTRICITY IN OUR HOMES AND WORKSHOPS. A

Practical Treatise on Auxiliary Electrical Apparatus. Third Edition. Revised
and Enlarged. With 143 Illustrations. 6s.

' It would be difficult to find a more painstaking writer when he is describing the
conditions of practical success in a field which he has himself thoroughly explored.'

ELECTRICIAN-.

By JOSEPH POOLE, A.I.E.E. (Wh. Sc. 1875), Chief Electrician to the New Telephone
Company, Manchester.

THE PRACTICAL TELEPHONE HANDBOOK. With

228 Illustrations. Second Edition. Revised and considerably Enlarged. 5s.
' This essentially practical book is published at an opportune moment.' ELECTRICIAN.

By D. DENNING.
THE ART AND CRAFT OF CABINET-MAKING. A Practical

Handbook to the Construction of Cabinet Furniture, the Use of Tools, Forma-
tion of Joints, Hints on Designing and Setting Out Work. Veneering, &c. With
219 Illustrations. 5s.
' We heartily commend it.' CABINET MAKER.

By J. HOPKINSON, D.Sc., F.R.S.

DYNAMO MACHINERY, ORIGINAL PAPERS ON. With

98 Illustrations. 5s.

'Must prove of great value to the student and young engineer.' ELECTRICAL REVIEW.
' A most valuable work.' ENGLISH MECHANIC.

By SYDXET F. WALKER, M.I.E.E., M.I.M.E., A.M.I.C.E.
ELECTRIC LIGHTING FOR MARINE ENGINEERS or,

How to Light a Ship by Electric Light, and How to Keep the Apparatus in Order.
Second Edition. With 103 Illustrations. Crown 8vo. 5*.

By S. R. BOTTONE.

HOW TO MANAGE A DYNAMO. Illustrated. Second Edition.

Revised and Enlarged. Pott 8vo. cloth. Pocket size. 1*.
' This little book will be very useful.' ELECTRICAL ENGINEER.
'The book should prove extremely useful.' ELECTRICAL REVIEW.

6 Whittaker's Library of Arts, Sciences, &c.

By S. II. BOTTONE.

ELECTRICAL INSTRUMENT MAKING for AMATEURS.

A Practical Handbook. With 78 Illustrations. Sixth Edition, Revised and
Enlarged. 3s.

' To those about to study electricity and its application this book will form a very
useful companion.' MECHANICAL WORLD.

By P. C. ALLSOP, Author of ' Telephones, their Construction,' &c.
PRACTICAL ELECTRIC-LIGHT FITTING. A Treatise ou

the Wiring and Pitting up of Buildings deriving Current from Central Station
Mains, and the Laying down of Private Installations, including the latest edition
of the Phoenix Fire Office Rules. With 224 Illustrations. Second Edit., Revised. 5.*.
' A book we have every confidence in recommending." DAILY CHRONICLE.

By S. R. BOTTONE.

ELECTRO-MOTORS : how Made and how Used. A Handbook for
Amateurs and Practical Men. With 70 Illustrations. Third Edition. Revised
and Enlarged. 3s.
1 Mr. Bottone has the faculty of writing so as to be understood by amateurs.'

INDUSTRIES.
By S. R. BOTTONE.

ELECTRIC BELLS, AND ALL ABOUT THEM. A Practical

Book for Practical Men. With lucre than 100 Illustrations. Fifth Edition,

Revised and Enlarged. 3s.

' Anyone desirous of undertaking the practical work of electric bell-fitting will find
everything, or nearly everything, he wants to know.' ELECTRICIAN.
' No bell-fitter should be without it.' BUILDING NEWS.

By S. R. BOTTONE.
RADIOGRAPHY: its Theory, Practice, and Applications. With

47 Illustrations. 5s.

By J. GRAY, B.Sc.

ELECTRICAL INFLUENCE MACHINES : containing a full
Account of their Historical Development, their Modern Forms, and their Practical
Construction. 4*. 6d.

' This excellent book.' ELECTHICAL ENGINEER.

' We recommend the book strongly to all electricians.' ELECTRICAL PLANT.
By EDWIN J. HOUSTON, A.M., Professor of Natural Philosophy ami Physical Geography
in the Central High School of Philadelphia, Professor of Physics in the Franklin
Institute of Pennsylvania, <fec.

ADVANCED PRIMERS OF ELECTRICITY.

Yol. L ELEC IRICITY AND MAGNETIS vf. 3s. 6d.
Vol. II.-ELECTRICAL TRANSMISSION OF INTELLIGENCE. 5s.
JT6L III. ELECTRICAL MEASURfcMENM S. 5s.

By Sir DAVID SALOMONS, Bart., M.A., Vice-President of the Institution of Electrical
Engineers, &c.

ELECTRIC LIGHT INSTALLATIONS, AND THE

MANAGEMENT OF ACOUMULA'l ORS : a Practical Eandbook. Sixth Edition,
Revised and Enlarged. With numerous Illustrations. G*.
' We advise every man who has to do with installation work to study this work.'

ELEOTKICAL ENGINEER.

MINERALOGY : the Characters of Minerals, their Classification

and Description. By F. H. HATCH, Ph D. With 115 Illustrations. 2s. 6rf.
'Dr. Hatch has adm'rably united brevity and clearness in his treatment of the
crystallographical and phjsical characters of minerals.' NATURE.

ELECTRICITY AND MAGNETISM. By S. BOTTONE. With

103 Illustrations. 2s. Qd.

GEOLOGY. By A. J. JUKES BROWNE, F.G.S. With 95 Illustrations.

2s. 6d.
' An excellent guide to the rudiments of the science.' ATHEN^UM.

LONDON: WHITTAKER & CO., PATERNOSTER SQUARE, E.G.

THIS BOOK IS DUE ON THfc LAST
STAMPED BELOW

AN INITIAL FINE OF 25 CENTS

OVERDUE.

LD 2l-50m-8,-32