QD
5 5 Tt ~
2 * .ECTROCHEMICAL
EXPERIMENTS
^1 OETTEL
UC-NI
277 7M7
GAR R SMITH
LIBRARY
OF THE
UNIVERSITY OF CALIFORNIA.
Class
\
INTRODUCTION
TO
ELECTROCHEMICAL
EXPERIMENTS
BY
DR. FELIX OETTEL
TRANSLATED
(WITH THE AUTHOR'S SANCTION)
BY
EDGAR F. SMITH
)F CHEMISTRY IN THE UNIVERSITY OF PENNSYLVANIA
-DIVERSITY
OF
J^Smb {TwentE*sii Illustrations
PHILADELPHIA
P. BLAKISTON, SON & CO.
1012 WALNUT STREET
I8 9 7
Q'D
COPYRIGHT, 1897, BY P. BLAKISTON, SON & Co.
PRESS OF WM. F. FELL &. CO.,
I22O-24 SANSOM ST.,
PHILADELPHIA.
PREFACE.
THE purpose of this little volume is to furnish tech-
nical chemists and all persons interested in the ap-
plications of electricity to chemical manufacture with
a concise guide, containing in a compact form all
that is essential for the comprehension and solution
of problems arising in this comparatively new field of
chemical investigation. That it has in a measure
fulfilled its mission is evidenced by the hearty recep-
tion and favorable criticism accorded it in its German
home, and by its translation into Russian. That
it may prove equally helpful to all its English-speak-
ing readers is the hope of the translator, to whom it
is a pleasure to acknowledge indebtedness to Prof.
George F. Barker for advice and assistance in the
preparation of the text and correction of the proof
sheets.
TABLE OF CONTENTS.
PAGE
A. SOURCE, MEASUREMENT, AND^REGULATION OF CURRENT, . 9
Sources of Current.
Galvanic Batteries Primary Batteries 10
Arrangement of Batteries, 14
Storage Cells Accumulators, 20
Thermopiles, 25
Dynamos, 27
Rules for Arrangement and Working of a Small Dynamo, 34
Current Measurement, 37
Silver Voltameter, 38
Copper Voltameter, . . 39
Oxyhydrogen Voltameter, 4
Tangent Galvanometer, 43
Torsion Galvanometer, 47
Technical Measuring Apparatus, 49
Ampere-hour'Meter, 5 1
Measurement of Pressure, . . 53
Regulation of Current, 57
B. ARRANGEMENT OF EXPERIMENTS.
Vessels, 63
Diaphragms, 65
Electrodes, 68
Conductors, 7 1
Electrolyte, 73
Sketch of Arrangement of Experiment, 75
vii
Vlll TABLE OF CONTENTS.
C. PHENOMENA OBSERVED IN ELECTROLYSIS. PAGE
Decomposition Pressure. Polarization Current, 77
Law of Faraday. Current Efficiency, 88
Transference of Ions, . 92
Current Density, 94
Working Pressure, 96
D. PRELIMINARY EXPERIMENTS OF AN ELECTROLYTIC PRO-
CESS, 98
E. .CALCULATION OF NECESSARY POWER. CHOICE OF DY-
NAMO, 100
F. PRACTICAL PART.
1. Construction and Calibration of a Tangent Galvanometer, 104
2. Calibration of a Galvanometer by Means of a Shunt, . . 107
3. Construction of Simple Instrument for Measuring Pres-
sure, ... Ill
4. Calculation for and Construction of a Regulating Resist-
ance, 115
5. Working of an Arsenical Copper Liquor, 118
6. Arrangement for Electrochemical Analysis, 134
G. TABLES.
I. Electrochemical Equivalents of the More Important
Elements, 141
II. Thermochemical Data, 142
III. Wire Resistances, 144
INTRODUCTION
TO
Electrochemical Experiments.
A. SOURCE, MEASUREMENT, AND REGU-
LATION OF THE CURRENT.
SOURCES OF THE CURRENT.
Four sources may be considered : ordinary galvanic
batteries (primary batteries), storage cells or accumu-
lators, thermopiles, and dynamo machines. The first
are most readily obtained, but constitute a rather in-
complete adjunct. Storage cells are decidedly more
advantageous, so far as regards constancy of current
and cleanliness in handling. Those who have em-
ployed this source of electric energy return to the
ordinary battery only in cases of absolute necessity.
Storage cells may be charged either by dynamos or
by thermopiles. The latter were, for a time, deemed
worthless, but since their improvement in construc-
tion have again come into favor. They are to be
recommended for experimental work on a small scale,
* 9
IO ELECTROCHEMICAL EXPERIMENTS.
and for analytical purposes, but are only available
when illuminating gas can be had. A small dynamo
is the best source of current for various practical
experiments.
In undertaking an electrochemical investigation, the
most rational beginning is the selection of the proper
working conditions for the experiment, aside from
batteries or accumulators. With the data and experi-
ence thus gained, the next step is the arrangement of
a small plant provided with a dynamo. Difficulties,
as a rule, now appear; these are generally of a con-
structive nature. When they have been surmounted,
and the miniature experimental plant serves its
purpose properly, calculations for some definite pro-
cess may next engage the attention of the experi-
menter.
After this brief enumeration of the different sources
of electric energy and their general application, a
more detailed description of them may be given.
PRIMARY BATTERIES (GALVANIC CELLS).
Cells of this class, designed for the execution of
electrochemical experiments, should furnish a strong
and constant current. Many forms have been de-
vised, but there is not one which fully meets these
conditions, hence there can be no purpose in entering
into a detailed description of each variety. Two may
be mentioned the Bunsen cell and the Daniell cell.
The Bunsen cell is the zinc-carbon combination.
SOURCES OF THE CURRENT. I I
It consists, usually, of a glass jar in which there
is a heavy, amalgamated zinc cylinder ( pole).
Within the latter stands a porous cup. This con-
tains concentrated nitric acid, in which is immersed
a bar of hard gas-carbon. The glass jar contains
dilute sulphuric acid (1:20). The action of the
current causes a reduction of the nitric acid, and
fumes of nitrogen dioxide arise, making it necessary
to keep the batteries in a good draught chamber.
This inconvenience may be partly obviated by drop-
ping into the cup, from time to time, chromic acid or
potassium bichromate. The electromotive force of
the cell is 1.8 V. At first the current from it is
very strong, but it grows considerably less in the
course of a few hours. This form of battery is very
well adapted for experiments requiring a strong cur-
rent for a relatively short time, and where high pres-
sures are necessary. It is not suited for experiments
extending through a period of days.
The following points should be observed when
using this battery. The zinc must be well amalga-
mated, otherwise a tumultuous evolution of hydro-
gen will occur, leading to a rapid consumption of the
zinc. In amalgamating, first dip the zinc cylinder
into very dilute sulphuric acid, then pour mercury
over it, and distribute the latter with a brush. The
clay cup should not be so porous that much nitric
acid can reach the zinc. In arranging the battery,
the sulphuric acid is first introduced, and when the
12 ELECTROCHEMICAL EXPERIMENTS.
porous cup has become thoroughly permeated with
it, the nitric acid is introduced into the cup. As re-
gards the carbon bars, it may be said that those from
natural retort carbon are superior to those made
from pressed carbon. To prevent them from absorb-
ing nitric acid, which would eventually reach and
destroy the metallic binding-screw attachments at
their exposed ends, the bars should be heated or
dried thoroughly and the ends then immersed in
melted paraffin, the excess of the latter being re-
moved with a brush. The binding screws, in union
with the battery poles, should be clean and bright ;
the other parts may be covered with an asphalt paint,
and in this way be protected from acid vapors. The
current may be conducted from the battery according
to the experiment by a stout copper wire (not less than
one mm. in diameter). This is sometimes rolled into
spirals, although there is really no necessity for so
doing.
The Daniell cell possesses less electromotive force
but greater constancy than the Bunsen cell. It con-
sists of amalgamated zinc in dilute sulphuric acid,
and copper in a saturated copper sulphate solution,
the two liquids being separated by a porous cup.
The sheet of copper serving as the positive pole is
cylindrical in shape, and perforated. A copper wire
is welded to it. In order to maintain a concentrated
solution, a perforated bottom is sometimes placed in
the upper portion of the jar. Crystals of copper
SOURCES OF THE CURRENT. 13
sulphate are, from time to time, placed upon it. If
zinc be on the exterior and copper be placed in the
porous cup, the combination will prove more energetic
than with the reverse condition. The pressure of
such a cell will be about 1.05 V. The nature of the
porous cup affects the efficiency of a battery very
much. If it be too dense, the internal resistance of
the cell will be too great. If it be too porous, then
the copper solution will penetrate to the zinc and
the latter will rapidly become covered with a film of
copper, in consequence of which the action of the
cell will be much diminished.
In testing a porous cup, fill it, when perfectly dry,
with water. It should be wet throughout within a
few minutes, but the water should pass through it very
slowly. It is a good rule to reject all porous cups
which, shortly after they have been moistened, show
drops of water on their external surface.
Another form of Daniell cell, arranged for con-
tinuous use, consists of a hollow zinc cylinder, not
amalgamated, standing in a concentrated zinc sul-
phate solution. In the porous cup there is a copper
cylinder immersed in a solution of copper sulphate.
By use, the latter loses color, indicating that it is
necessary to add crystals of blue vitriol. From time
to time the zinc solution is siphoned off, an equal
volume of water being added to prevent the separation
of crystals of zinc sulphate. Once or twice every
week the zinc cylinder should be taken out and
14 ELECTROCHEMICAL EXPERIMENTS.
cleaned. An arrangement of this sort consumes
very little zinc, but has a greater internal resistance.
Its pressure does not exceed 0.9-0.95 V.
The disagreeable deposit of salts on the edges and
over the sides of the cells can be lessened by employ-
ing porous cups with glazed edges, or by paraffining
the portion of the cup extending beyond the liquid.
It can not, however, be wholly overcome. When the
battery is disconnected, the carbons and the porous
cups should be thoroughly washed, and put away
when dry. If the washing be omitted, the cups will
be cracked or at least be damaged to a marked
decree by the salts which crystallize out.
When a battery or cell is to be again put together,
the cup must first be thoroughly saturated with the
zinc sulphate solution, and the blue vitriol solution
then be introduced into it. If the cup be not dry,
but moist with water, it will, in consequence, yield but a
feeble current until its walls are filled by diffusion
with the better conducting vitriol solution.
ARRANGEMENT OF CELLS.
Having a number of cells, it is possible to arrange
them in three different ways :
(a) Parallel, when all the zinc poles are con-
nected with one another, and the copper poles in like
manner (Fig. i).
(b) In scries, when each zinc pole is connected with
the copper pole of the adjacent cell (Fig. 2).
SOURCES OF THE CURRENT. 15
(c) In groups (inixed arrangement] ; an equal number
of cells are united into groups and the latter then
arranged as individual cells. The arrangement will
uu/
FIG.
be different, depending upon whether that of (a)
Fig. 3 or (<) Fig. 4 is observed within the group.
The first is preferable, because slight defects in the
individual cells are less disturbing.
FIG. 2.
When should the one or the other combination be
chosen ? In answering this question, the following
points should be remembered :
i6
ELECTROCHEMICAL EXPERIMENTS.
(1) The maximum work of a battery is obtained
when the resistance in the outer circuit is equal to
the total resistance of the battery.
(2) In the parallel arrangement of cells, the electro-
motive force of the battery is not altered. The inter-
FIG. 3.
FIG.
nal resistance is, however, diminished directly accord-
ing to the number of cells.
(3) When cells are arranged in series, both the
pressure and the resistance of the individual cells are
increased in the sum total.
If the electromotive force of a cell be represented
SOURCES OF THE CURRENT. I/
by e, and its internal resistance by w, then a battery
of n cells arranged parallel would have
the electromotive force e
and
the internal resistance .
n
A battery with cells in series would, on the con-
trary, have
the electromotive force ;/. e
and
the internal resistance n. w.
With a battery of n cells in series, composed of a
elements parallel, each group would have
the electromotive force e
and
the internal resistance ,
so that in the entire battery of n such groups there
would be
the electromotive force n. e
and
the internal resistance n.
a
According to Ohm's law, the current strength in a
circuit is equal to the electromotive force divided by
the total resistance. The latter equals the sum of the
internal resistance Wj of the battery and the resistance
I 8 ELECTROCHEMICAL EXPERIMENTS.
W a of the external circuit, so that the current strength
I may be expressed by the formula
E
= Wi + W a '
This formula gives two possibilities for the increase
of current: either by increase of the numerator or the
diminution of the denominator. The first follows
from the arrangement of cells in series (increase of
electromotive force), the second from their parallel
arrangement (reduction of Wi). Which course should
be pursued depends upon whether W a is large or
small in proportion to W { . Several examples will
serve to demonstrate that in cases of great external
resistance a series arrangement of the cells is the
proper arrangement, while with low external resistance
the parallel arrangement should be chosen.
Examples :
The E. M. F. of a cell is 1.05 V.
Internal resistance of a cell is 0.5 $.
(a) External resistance is 10 Q.
Current strength with various combinations :
1.05
1 cell, I = - --=0.10 amp.
0.5 + 10
2. I.O5
2 cells in series, ....!= =0.19 amp.
2. 0.5 f 10
. 4. 1.05
4 cells in series, . . . . I = = o. 15 amp.
4. 0.5 -f 10
SOURCES OF THE CURRENT.
Reverse :
1.05
2 cells in parallel, . . . I = - = o. IO2 amp.
1.05
4 cells in parallel, . . . I = = o. 104 amp.
*-+ 10
(b) External resistance .-= o.i Q.
Current strength with various combinations :
1.05
1 cell, ........ I = =1.75 amp.
0.5 + o.i
2. I.O5
2 cells in series, ....! = 1.91 amp.
2. 0.5 -j- o.i
4- I -5
4 cells in series, ....!=- - = 2.0 amp.
4- 0.5 + o.i
Reverse :
1.05
2 cells in parallel, . . , I = = 3.0 amp.
1.05
4 cells in series, ....! = = 4.67 amp.
0-5
1+0.1
4
After some experience in electrochemical work it
will be easy to determine whether one is confronted
in an experiment with a high or low resistance, and
the manner of cell arrangement will accordingly
follow. Until such experience has been acquired, the
safest, wisest course will be to experiment ! Intro-
2O ELECTROCHEMICAL EXPERIMENTS.
duce a measuring instrument between the battery and
the experimental cell, and vary the arrangement of
the cells until the maximum current strength is ob-
tained. The mixed arrangement will be found prefer-
able if the cells employed, either in consequence of
their small size, or from any other causes, show a high
internal resistance. Several cells arranged in parallel
will give the same result as one large form of like type.
Meidinger cells are frequently met with in labora-
tories. They are only suitable for analytical opera-
tions in which great current strength is not required.
For electrochemical experiments of any other sort
they are worthless, because too many of them must
be arranged in groups to get currents of any great
degree of intensity.
STORAGE CELLS, ACCUMULATORS,* SECONDARY
BATTERIES.
While all primary batteries are more or less incon-
venient to handle, and, as a rule, furnish currents which
are not very strong, storage cells or secondary bat-
teries are excellent sources of electric energy, and
serve for the most varied electrochemical experiments.
Accumulators consist of a number of lead plates
which are covered with a so-called "active mass."
*A clear, concise description of the action and care of secondary
batteries will be found in Elbs' " Die Accumulatoren," Leipzig, 1893.
ffoppe, " Die Accumulatoren," 2te Aufl., Berlin, gives a more ex-
haustive and historical account of their development.
SOURCES OF THE CURRENT. 21
On the anode (positive) plate this is lead superoxide,
while on the kathode (negative) plate it is spongy
lead. The liquid is pure, dilute sulphuric acid.
Secondary batteries are, therefore, galvanic batteries,
which, in consequence of lack of diaphragm, have
a vanishingly, low, internal resistance. When ex-
hausted, it is not necessary to take them apart ; for
by contact with a more powerful source of energy
they can be directly charged /. e., they can be again
restored to their normal, useful state.
The various modifications found in trade are prac-
tically the same. Nearly all are guaranteed for a
longer or shorter period. The Tudor and Correns *
types are among the best. There are both stationary
and transportable varieties ; the latter are closed, so
that no loss of acid occurs in transportation from one
place to another.
The capacity of a storage cell is given in ampere-
hours. The current strength varies with the individual
types, and so long as it is not exceeded, it is immaterial
how the electric energy is applied /. e., whether a
powerful current is used for a brief period or a feeble
current for a prolonged period. A storage cell with
a capacity of 100 ampere-hours with a maximum cur-
rent of 10 amp. may be discharged at the rate of
10 amperes for a period of 10 hours, or
5 " " " 20 "
i " " " " 100 " etc.
* The chloride accumulator is most widely known in this country.
TR.
22 ELECTROCHEMICAL EXPERIMENTS.
Most factories give instructions as to the manner
in which their secondary batteries shall be handled.
General rules may, however, be given here. The
smaller types are usually mounted. The glass jars
should be thoroughly cleaned without removing the
lead plates. The latter are then completely immersed
in pure, dilute, cold sulphuric acid of 1.15 sp. gravity.
The purity of the acid is important. The presence of
arsenic or nitric acid in it is harmful. If the acid at
hand is not sufficiently pure, conduct hydrogen sul-
phide through it. Filter out the precipitated sul-
phides, and expel the hydrogen sulphide gas by means
of an air current. When the glass jars have been
filled, begin " charging." Connect the brown -f- plates
with the + conducting wire of a dynamo (or a ther-
mopile) and the gray plates with the pole. The
current recommended for the cell, by its manufacturer,
is next conducted into it. As a rule, this first charg-
ing will extend through a day or more. It is in this
way that the plates are first brought into normal con-
dition. The current will, apparently, be taken up
completely by the accumulator. The -f- plates grad-
ually change in color to a brownish black, while the
plates become a light gray. Eventually, a strong
evolution of gas will be observed, first on the positive
and later on the negative plates ("the acid boils").
When the evolution of gas on all the plates remains
nearly the same for an hour, the first " charge " may
SOURCES OF THE CURRENT. 23
be regarded as finished. The pressure for each cell
will have increased to 2.5 V. ; the specific gravity of
the acid will vary from 1:15 to 1.18-1.20. All of the
active mass upon the positive plates will now be com-
pletely oxidized, and that upon the negative plates
will be fully reduced. The first discharging can next
follow. It should be done with the current strength
previously mentioned. The pressure will fall rapidly
to 2.0 V. and then remain constant for a long period.
As soon as it decreases to 1.85 V., the discharging
should be interrupted. The cells should then be
charged a second time until the pressure reaches 2.5
V. and equal gas evolution is observable on all plates.
The cell is now ready for further use.
To insure long life to the cell, certain precautions
should be observed :
1. It must be preserved from " short-circuiting."
2. The maximum strength of the discharging cur-
rent must not exceed that given by the manufacturer
of the cell.
3. Do not discharge below 1.85 V.
4. It should not continue long in a discharged con-
dition. If not wanted for use, it should be held in a
charged state.
5. It is well occasionally to overcharge /. e., to con-
tinue charging for some hours with a feeble current
even after the " boiling" has begun.
6. Should it become necessary at any time to re-
move a plate from the acid, it must not be allowed to
24 ELECTROCHEMICAL EXPERIMENTS.
become air-dried. It should be immediately im-
mersed in dilute acid.
7. Avoid heavy blows : they loosen and throw out
the " active masses."
By observing these rules, storage cells can be kept
in satisfactory condition for some time, especially if
the maximum effect be not constantly aimed at. It is
better to charge too frequently than not enough. In
time the surface of the liquid will sink below the
upper edge of the plates. Pure water should then be
added. The specific gravity of the acid should be
taken after charging and discharging. The use of the
hydrometer will furnish a means of ascertaining how
far the charge has been consumed. If, in time, it is
noticed that, after charging, the specific gravity of the
liquid is not the same as was observed at first, when
the cell was in this condition, it is evidence that acid
has been lost by spattering or in some other way.
Dilute acid should be added several times for the water
that evaporates until the normal condition is restored.
Storage cells answer well, both for analytical and
experimental purposes. The current and pressure of
any source of electricity can, by this means, be
altered in various ways. For example, a small
dynamo of 5 V. and 30 amperes, together with six
accumulators, each having a maximum discharge of
ten amperes, are at the disposal of the operator. As
the machine is only capable of charging ---- = 2 sec-
SOURCES OF THE CURRENT. 2$
ondary batteries in series, groups of three cells each
in parallel are formed, and these then connected in
series to the machine. When the charging is finished,
the following groupings may be made :
I to 6 in series will yield 2-12 V. and 10 amp.
I " 6 parallel " " 2V." 10-60 amp.
2X3 " " 4 V. " 30 amp.
3X2 6 v. " 20 amp.
Accumulators have found their way into labora-
tories slowly because dynamos with which to charge
them were rarely present. Recently, thermopiles *
have been used successfully for this work, hence
there now remains no good reason for their non-
adoption generally in electrolytic work.
THERMOPILES.
Thermopiles, which transform heat into electricity,
would be the most convenient sources of electric
energy for laboratory purposes if they could be built
in durable forms. In this respect the older modifica-
tions of Noe and Clamond were sadly lacking. They
required much attention and a constant gas pressure,
and, in spite of all care, rarely lasted for any great
length of time. The new modifications of Giilcher f
show decided improvement over the early types, and
the common verdict in regard to them is very favor-
able. Julius Pintsch, Berlin, O., Germany, makes
* Elbs, Chem. Ztg., 1893, 66. f D - R - p -> N <>- 44^6-
c
26
ELECTROCHEMICAL EXPERIMENTS.
three varieties of this thermopile (Fig. 5). The
largest model, consuming 170 liters of gas per hour,
develops an electromotive force of 4 V. with an
internal resistance of 0.6 to 0.7 Q. The price of this
form is about $48. The smaller models consume
130-70 liters of gas, and have a pressure of from 3-1.5
FIG. 5.
V. If it be desired to use the larger form for charg-
ing storage cells, the latter should be arranged
parallel. This will result in the production of a cur-
rent of from 2-3 amperes. For ordinary purposes,
therefore, a thermopile of this description will be
ample. At present they are used in laboratories
SOURCES OF THE CURRENT. 2?
chiefly for electrolytic analyses, but combined with
secondary batteries can be more widely applied.
DYNAMO-MACHINES.
The dynamo is the proper source of electric energy
in all experiments requiring a powerful current for an
extended period.
The action of such machines is dependent upon the
electric current induced in a coil of wire when brought
into a magnetic field i. e., when it is rotated between
the poles of a magnet. The portion that is rotated
is called the armature. Externally it has the form of
a flat ring or cylinder. The magnets about which
the armature rotates are not permanent, but are
electromagnets excited by a part of the current pro-
duced by the machine. The several current impulses
induced in the armature are collected in the commu-
tator, or collector, and are given up by the brushes
to the external conductors.
Two large classes of dynamos exist: direct current
machines, and those producing alternating currents.
In the first class, two successive current impulses have
the same direction, while in the second class they
proceed in opposite directions. For electrochemical
purposes, direct current machines are alone of conse-
quence.
The manner of winding also causes a division into
series machines, shunt machines, and compound
machines. In the first, the current produced in the
28 ELECTROCHEMICAL EXPERIMENTS.
armature proceeds immediately about the electro-
magnets, then through the external circuit back to
the armature. In the second, the current divides
itself in the armature. The smaller portion of it
proceeds about the electromagnets, while the major
portion passes through the external circuit. In the
mixed or compound machines, a part of the winding lies
in the shunt circuit, the remainder in the main circuit.
Shunt machines alone interest us. They have this
important advantage, that in consequence of any
disturbances the poles can not reverse. They will,
therefore, be somewhat more closely considered. As
an example, or type, the Schuckert * flat-ring machine
(Fig. 6) may be mentioned.
The current circulating in the external circuit is the
main current, that about the magnets is the shunt cir-
cuit of the machine. The latter is interrupted at one
point for the introduction of the shunt-regulator, N.
This serves to change the pressure at the terminals
of the machine. If resistance be introduced by
means of this regulator into the magnet winding, the
current passing through it will be less than before ;
consequently the magnetism and the magnetic field
of the machine will be reduced and the pressure will
fall. By suitable winding of the shunt regulator the
pressure may be varied within wide limits. Another
means of altering the pressure consists in changing
the velocity. When the latter is increased, the pres-
* In this country the Edison machine is generally used. TR.
SOURCES OF THE CURRENT.
2 9
FIG. 6.
3<D ELECTROCHEMICAL EXPERIMENTS.
sure increases, and vice versa. This course is resorted
to only when extraordinary conditions prevail,
otherwise the velocity allowed by the builder of the
machine must be taken as normal. A dynamo with
an external resistance of 0.068 Q will yield
125 Amp. 8.5 V. with 500 revolutions.
215 " 14.7 V. " 800 "
280 " 19. o V. " 1000 "
Dynamos are constructed for the most varied cur-
rent strengths and pressures. In chemical operations,
strong currents are of prime importance the pres-
sures are relatively low. Chemical dynamos vary, as
a rule, from 4-20 volts and 100-500 amperes. For
electric lighting the customary pressures vary from
65-110 V., while the current strength falls below 100
amperes.
The product obtained by multiplying together the
voltage and the amperage represents the output of a
dynamo. The number of volt-amperes is, therefore,
a direct means of measuring the mechanical power of
a dynamo in each moment of its activity.
A chemical dynamo " running empty," i. e., when
it is not connected with a current circuit, rapidly at-
tains the pressure allowed by its prevailing velocity.
This is called the open-circuit pressure. It is always
greater than that observed when the machine is doing
work, or than the pressure on closed circuit. A wire
resistance connected with the two terminals at once
SOURCES OF THE CURRENT. 3!
shows a current, which maybe calculated from Ohm's
law. According to the latter, the current strength
I is
E. M. F.
Sum of the resistances.
The electromotive force is the pressure of the ma-
chine. The resistance is on the outside the resist-
ance W, while within the machine there is the resist-
ance of the armature A and the resistance of the mag-
net coils M. These two latter resistances are arranged
parallel because the dynamo is a shunt machine. To-
A 1VI
gether they equal -^ ; hence the equation for the
current strength is
I = ___E_
A + M
By lowering the external resistance, the current in-
creases and the pressure falls. The product of the
pressure and current strength the output of the
dynamo approaches the maximum. By further re-
duction of the external resistance the pressure will
fall still further ; the current will increase for a short
period, but will then decrease until, with no external
resistance (by short circuit of the dynamo), it becomes
zero. The machine then furnishes no current. The
voltage and amperage also rapidly drop from their
maximum to zero.
32 ELECTROCHEMICAL EXPERIMENTS.
This action is related, too, with the flow of current
toward the limbs of the magnet. So long as the ex-
ternal resistance is great in comparison with the
resistance of the armature of the magnet, so long will
the greater portion of the current follow the more
convenient path about the electromagnets. It will
powerfully excite these, and acquire, in consequence, a
high pressure. As the external resistance gradually
grows less, the current which flows about the limbs
of the magnet will be diminished, the magnetic field
of the machine will become less powerful, also its
efficiency, so that, eventually, when the external
resistance has, by short circuit, fallen to zero, a cur-
rent will no longer flow about the magnets, and the
machine will cease to work.
The maximum work of a dynamo is attained when
the external resistance is slightly greater than its
internal resistance. This condition is the basis for
the normal work given by the manufacturer of the
machine.
Smaller machines, intended for experimental pur-
poses, models, are constructed with two separate
circuits. These carry a double coil upon the arma-
ture. The main coil, with low resistance, gives up its
current through the main brushes to the external cir-
cuit; the other coil has greater resistance, and the
current developed therein is delivered to the magnets
altogether in the side brushes. There are also
machines with separate magnet excitation. Should
SOURCES OF THE CURRENT. 33
short circuiting be produced in such machines by
carelessness, the only resistance in the entire circuit
would be the main armature, and all the energy of the
machine would be consumed in bringing this resist-
ance to such a state that it would ignite. In other
words, the armature would be burnt out and the
machine be ruined. Direct current machines and
compound dynamos are damaged in the same manner.
It is only the true shunt machine that is non-sensitive.
The natural desire on the part of any one consider-
ing the introduction of a small dynamo, is that it shall
be suitable for every possible purpose. This wish
can not, of course, be wholly realized. It can not be
expected that one and the same machine will serve
both for electrolysis and fusion purposes, because, in
the first instance, low pressure and high current are
required, while in the second case the requirements
are directly the reverse high pressure and moderate
current. It is always well to have sufficient pressure,
as it can be easily reduced, but it is increased with
difficulty. But few electrolytic operations require
more than 5 V. pressure, so that this, in most cases,
would be amply sufficient ; yet, to render the machine
as widely useful as possible, it is well to double this
power for the following reason.
In all technical operations, this question must be
constantly kept in view : How can this work be done
on a technical scale ? The work of a dynamo can,
however, only be doubled by increasing the current
34 ELECTROCHEMICAL EXPERIMENTS.
strength or by increasing its pressure. Reasons, to
be given later, will show that the latter course is pre-
ferable. However, to consume or exhaust the highest
pressure the electrolytic baths must be arranged
in series, as in the case of primary batteries. But
with this arrangement, phenomena frequently present
themselves which are not noticeable in a single bath.
If the machine, then, has a direct pressure of 10 V., it is
even possible, with a counter bath pressure of 5 V., to
try series arrangements. With baths of lower pressure,
it is, of course, understood that there can be more
baths arranged in series.
A machine having a pressure of 10 V. and a cur-
rent of 70-100 amperes will satisfy the greatest de-
mands which can be made upon a dynamo intended
for electrolytic experiments. The energy consump-
tion will, therefore, equal from iJ^-2 H. P.
RULES FOR THE ARRANGEMENT AND RUNNING OF
SMALL DYNAMOS.
The machine should stand upon a solid foundation,
which can not be easily shaken. When possible, this
should be a pier about J^ m. in height. It will render
the observation of and attention to the brushes much
easier. It would be very convenient and practicable
if the dynamo stood upon rails which would permit
of its movement in the direction of the driving belts.
Such a shifting is the simplest and most convenient
means of stretching a belt that has become loose and
SOURCES OF THE CURRENT. 35
flabby ; cutting and sewing anew would otherwise
be necessary. An arrangement permitting this can
be furnished for every machine upon mere request to
the maker.
The transmission is best provided with a triple cone
pulley, so that the middle step affords the normal
number of revolutions, and the other two a somewhat
higher or a lower number. Avoid covered driving
belts.
The bed of the machine should be kept well lubri-
cated. The " self-oilers " should always be full. The
brushes should fit lightly but closely. When the
collector is pressed too closely it is unnecessarily
abraded and heated. To insure long life to the col-
lector, it should be constructed from hard bronze,
while the brushes should consist of soft copper. The
writer's experience warrants the statement that brushes
from sheet metal are superior to straight or twisted
wire. They consist of rolled sheets, thin as paper
and soft as feathers, which slide along the entire width
of the collector. In consequence of their great elas-
ticity and the absence of any points, such brushes do
not spark, and the collector continues smooth for
quite a long period.
New brushes are brought in contact with the col-
lector when the dynamo is running empty. Project-
ing points produced by wear should be cut off, because
they rapidly damage the machine. The brushes in
action require careful attention. When sparking sets
36 ELECTROCHEMICAL EXPERIMENTS.
in, the binding screws should be eased and the brushes
withdrawn until the evil is corrected. In this adjust-
ment care must be taken not to raise the brushes from
the collector during the revolution of the latter. The
consequence of such an act would be not only the
production of a blinding spark, but, in all probability, a
very severe shock. In disconnecting the main current
avoid interrupting the conductors with both hands,
thus introducing the body into the circuit. This
always induces heavy shocks. In other respects the
various parts of a dynamo can be handled during its
action without experiencing any unpleasant sensation.
When the collector has become rough and worn,
the brushes are removed, and it is polished with
emery paper. When great inequalities exist, a smooth
file may be used, or the collector may be made to
revolve around a sharp file.
The abrasion of the brushes and of the collector
gives rise to a fine metallic dust which deposits on
all parts of the machine. It must be removed at
intervals. This is done, in the case of portions more
difficult of access, by use of a bellows. Gentle tap-
ping displaces it from the brushes. Copper dust
should not be allowed to accumulate on the collector
or its connections with the armature ; it gives rise to
short circuits in individual segments.
The current is best conducted to the baths by flat
metallic sheets of ample section. Branches are more
conveniently made from these than when rods of
CURRENT MEASUREMENT. 37
metal are used. Their connection with the machine
is made with strips of copper cord, so arranged that
the machine is still movable on its foundation.
CURRENT MEASUREMENT.
Two courses maybe followed in measuring currents.
The chemical action of the electric current (volta-
meter) or its magnetic action upon a conductor
through which it passes (galvanometer) may be used
for this purpose. To determine the current strength
in a circuit, i. e., in an experiment, it is well to insert
one of the instruments just described into the circuit,
which consists of: battery experiment measuring
instrument battery. It would be wholly wrong to
have the following succession : battery instrument
battery, and then assume that the current strength
thus observed was identical with that from the arrange-
ment : battery experiment battery.
Electromagnetic measuring apparatus is calibrated
in an entirely empirical way by means of voltameters,
therefore the latter will be the first to receive a brief
description. Voltameters are based upon Faraday's
law, which reads : " The same current in equal units
of time decomposes equivalent quantities of chemical
compounds." As representatives of the latter, we
may choose silver nitrate, copper sulphate, or dilute
sulphuric acid, and then determine the quantities of
metallic copper, metallic silver, or electrolytic gas sep-
38 ELECTROCHEMICAL EXPERIMENTS.
arated in a definite period of time. Consequently, we
distinguish silver voltameters, copper voltameters, and
the oxyhydrogen voltameters. Certain precautions
must be observed with each'to obtain accurate results.
SILVER VOLTAMETER.
This is regarded as the most accurate voltameter.
Secondary reactions do not occur in it. The high
equivalent of silver, and the fact that very considerable
quantities of metal are precipitated by comparatively
feeble currents, reduce the error in weighing to a min-
imum. A disadvantage which must be recognized is
that silver is greatly inclined to separate in crystals,
which are loosely attached to the kathode. Conse-
quently, stronger currents can not be sent through
the apparatus without the possibility of some of the
deposit becoming detached. In measuring strong
currents with this instrument, electrodes with a suffi-
ciently large surface must be provided, besides which
the current should not be allowed to act for more
than one to two minutes.
The voltameter usually consists of a silver or plati-
num dish serving as a kathode, in which there is a
moderately concentrated neutral silver nitrate solu-
tion. The anode, of pure silver, is a pencil dipping into
the liquid. This anode is surrounded by a muslin
bag serving to collect any particles of silver which may
separate, thus preventing them from falling upon the
dish and so falsely increasing the weight of the latter.
CURRENT MEASUREMENT. 39
A more convenient form of this voltameter consists
of a small beaker glass containing the silver solution.
A sheet of pure silver serves as the anode, while the
kathode is a plate of platinum. The liquid should
be agitated with a glass rod during the action of the
current. Finally, the platinum plate is removed,
rinsed with water, dried, and weighed. If, after sev-
eral experiments, the crystals have become so large
that they threaten to fall off, clean the platinum
plate by immersion in nitric acid.
A current of one ampere precipitates
0.001118 gram Ag in one second, or
0.06708 " " " " minute.
COPPER VOLTAMETER.
In this instrument the electrodes are two copper
plates of equal surface. The thicker of the two
serves as the anode, the thinner plate as the kathode.
They dip into a solution of copper sulphate. In this
instance, also, the liquid should be thoroughly agi-
tated during the action of the current. The precipitated
copper should have a bright red color and be perfectly
adherent. It is first dipped into water, then into
alcohol, after which it is dried over a flame. When
strong currents are used anodes are placed on both
sides of the kathode.
Early data indicate that, as a filling liquid, an
almost saturated and perfectly neutral copper sulphate
solution answers best. It, however, increases the
4O ELECTROCHEMICAL EXPERIMENTS.
resistance of the voltameter, in consequence of which
a relatively high pressure will be required. Further-
more, low results arise when feeble currents are em-
ployed, because then the copper which separates con-
tains cuprous oxide. It has been shown* that per-
fectly exact values, agreeing with those obtained
by the silver voltameter, have resulted by using cur-
rent densities varying from 0.06 to 1.5 ampere for a
sq. dm. of kathode surface. This was done by the
employment of the following solution :
15 grams of crystallized copper sulphate.
5 " " concentrated sulphuric acid.
5 c.c. " alcohol.
100 " " water.
The pressure (o. I o. 5V.) is only half as large as
that required for the neutral liquid.
Such an arrangement of the copper voltameter
is well adapted for most practical requirements. Its
results are correct. It is inexpensive and easily
made, and, in addition, can be thrown into a circuit
for almost any length of time. It can also be used
with advantage as an ampere-hour meter (see p. 5 i).
One ampere of current precipitates 0.0197 gram of
copper in a minute, or 1.181 gram of copper per hour.
THE OXYHYDROGEN VOLTAMETER.
In this voltameter dilute sulphuric acid (sp. gr.
1.1-51.2) is decomposed between two platinum
* Chemiker Zeitung, 1893, 543.
CURRENT MEASUREMENT. 41
plates. The resulting electrolytic gas is measured.
An ampere liberates 10.44 c - c - f electrolytic gas (at
o and 760 mm. pressure) per minute. This kind of
voltameter is convenient, because it does away with
all weighings. The reduction of the volume of gas
FIG. 7.
can be taken from tables, and in a few minutes the
apparatus will again be ready for a new measurement.
This convenience, however, is offset by the disad-
vantage that the pressure required by the voltameter
is high. It varies from 1.7 to 2.5 V. according to the
42 ELECTROCHEMICAL EXPERIMENTS.
CURRENT MEASUREMENT. 43
size of the instrument. Therefore, to ascertain the
current strength in an experiment which demands a
pressure of 4 V., it would be necessary to employ
some source of electric energy with a pressure of
about 6 volts !
If an oxyhydrogen voltameter is to be used, the
modification made by Kohlrausch (Fig. 7) is preferable.
The eudiometer vessel is filled by merely inverting
the apparatus. What is known as Schmitt's nitrogen
eudiometer (Fig. 8) can readily be converted into a
practical electrolytic voltameter. The electrodes are
introduced through rubber stoppers. The measuring
burette, during use, is pushed somewhat above the
rubber stopper. In reading, it is placed on the latter
and brought to equal pressure by the adjustable level-
ing tube.
Voltameters of this description are only in rare cases
adapted for current measurement by introduction into
the circuit. In order to read them the. experiment
must generally be interrupted, and they augment the
resistance of the circuit to a marked degree. These
inconveniences may be avoided by the use of measur-
ing apparatus based upon electromagnetic principles.
These will now be described. The
TANGENT GALVANOMETER
is the simplest example of its class. It consists of
a circular heavy copper wire. A magnetic needle,
attached to an unspun silk fiber, oscillates in this ring
44 ELECTROCHEMICAL EXPERIMENTS.
over a graduated circle. The plane of the ring is
placed in the meridian in the direction of the needle.
The latter is short, compared with the diameter of the
ring (not more than one-sixth). The magnetic strength
of the needle does not affect the measurement. When
the needle is short, a more accurate reading can be ob-
tained, even when there is a large divided circle, by
attaching to the needle a lighter and longer indicator.
When the needle is deflected out of the meridian,
the current strength will be
1 = c, tang. o.
Here, c indicates a constant value for each instru-
ment.
To determine this constant c of the galvanometer,
introduce the latter, together with one of the volta-
meters, just described, into the circuit of some con-
stant source of electric energy (the order being : bat-
tery voltameter galvanometer battery). Close the
circuit for a few minutes, note the average deflection,
and from the reading of the voltameter calculate
the current in amperes. If, with a deflection of a,
the current strength was found to be I amperes, the
constant sought will be
I
c = '
tang, a
Several such experiments are made with varying
currents, and from the mean thus found the value for
c is taken. A table is then calculated once for all to
CURRENT MEASUREMENT. 45
serve for the different deflections. To alter the cur-
rent strength, an additional cell is added, or a German
silver wire is introduced into the circuit as resist-
ance.
Gaugain has made a modification of the tangent
galvanometer in which the magnetic needle does not
swing in the center of the ring, but is shifted from the
circular plane one-fourth of its diameter. The length
of the needle can now be increased without produc-
ing a variation from the law of tangents.
Having constructed a table for the galvanometer, it
will soon be observed that the measurements between
6o-9O are uncertain, because for each degree of
deflection the increase in current will become greater.
It is not well to exceed 65. In measuring larger
currents, two courses are open : either a second gal-
vanometer is used, which, with equal length of needle,
has a larger ring diameter, or, a so-called shunt is in-
troduced.
When two points in a circuit are connected, not by
a single wire, but by two wires of different resistances,
the current strength in the two wires will be inversely
as the resistances. For example, when the current I
between the two points A and B (Fig. 9) meets the
two wires I and II with the resistances W\ and W 2
respectively, it divides itself into the two currents
ii and i 2 , between which the following relations exist :
i. I* + 1 = 1.
2. ij : i 2 = W 2 : W r
46 ELECTROCHEMICAL EXPERIMENTS.
If, for example, the resistance W T is one-third that of
W 2 , then three times as much current will flow
through the former as through the latter, or, when
referred to I, it would be: ij == ^ I and i 2 == J^ I.
On introducing, therefore, a galvanometer into a
circuit and connecting its terminals by means of a
wire, it will be possible, through the resistance of
the selected wire, to deflect the major portion of the
entire current through this shunt and to convey any
desired fraction of it through the galvanometer. In
FIG. 9.
this manner the capacity of the latter is correspond-
ingly increased. If the shunt has a resistance equal
to that of the galvanometer, this would receive but
one-half of the total current, hence its readings
should be doubled. If, in general, the resistance of
the shunt is -^ of that of the galvanometer, the read-
ings must be multiplied by n -\- i. When, on the
other hand, it is desired to conduct ~ of the total cur-
rent through the galvanometer, a shunt having ~- t of
CURRENT MEASUREMENT. 47
its resistance must be introduced. If the resistance
of the shunt is not accurately known, the correct
readings of the galvanometer can be ascertained by
a re-calibration with a voltameter. The wire used as
shunt must not be stretched out straight, otherwise
the current passing through it will influence the
needle. It should be doubled and then coiled up.
In adjusting a galvanometer, care must be taken that
adjacent currents do not influence it. Consequently,
it is connected to the main circuit by two long wires,
lying closely together, so that their inductive action
can be disregarded.
By the aid of a shunt it is possible to employ very
sensitive galvanometers, having many turns of wire
(multipliers or galvanometers), for the measurement
of current strength. The readings of such an instru-
ment can not be deduced directly from a simple
formula, but the instrument itself must be calibrated
throughout its entire range with a voltameter (p. 106).
TORSION GALVANOMETER.
The torsion galvanometer of Siemens and Halske
(Fig. 10) is an exceedingly delicate instrument, and
for exact measurements is almost indispensable. A
bell magnet, provided with copper damping, is sur-
rounded by a coil of many turns, and attached to a
spring, by whose rotation the deflection of the magnet
is compensated and restored to its original position.
The magnitude of the angle of torsion, therefore,
48 ELECTROCHEMICAL EXPERIMENTS.
serves as a means of measuring the current passing
through the instrument. The divisions are so selected
that the wire coils have a resistance exactly equal
to I 8, and one degree of deflection corresponds to
O.OOI amp. With a shunt of ^ Q the reading must be
FIG. 10.
multiplied by 10; i. e., i corresponds to o.oi amp.;
analogously, with a shunt of
fo 2 : i = o.i ampere,
lg 12 : 1 = I ampere.
As the greatest torsion angle equals 170, it is possi-
ble, with such an instrument and the three shunt
resistances, which have been mentioned, to measure
CURRENT MEASUREMENT. 49
currents ranging from o.ooi-i/o.o amp., which is,
obviously, a very wide range.
MEASURING INSTRUMENTS FOR TECHNICAL PURPOSES.
These instruments carry an empirical scale, which
gives direct readings in amperes. The instruments
themselves can be constructed for all current strengths,
but their range can not be read with the same
degree of accuracy throughout. The first divisions
are usually contracted; hence their readings are less
reliable. Technical instruments of this class, called
amperemeters, are indispensable for the control of
work in a manufacturing establishment. They give a
reading of current strength in amperes at any moment.
They should always be introduced into the main cir-
cuit leading from the source of the current to the
electrolytes. As most amperemeters are entirely
independent of the direction of the current passing
through them, it is immaterial whether they are con-
nected with the negative or positive conducting wire.
Two phenomena, as a rule, are utilized in their con-
struction : (i) A solenoid, through which the current
passes, tends to draw an iron rod into its center ; or,
(2) a solenoid attracts an eccentrically arranged strip
of sheet-iron toward the periphery. The two attrac-
tive forces are opposed, either by gravity or by the
action of a spring. The modifications of this appa-
ratus are almost innumerable. Two typical forms
will be described :
5O ELECTROCHEMICAL EXPERIMENTS.
The spring- galvanometer of Kohlrausch (Fig. 11)
consists of a vertical solenoid. In it there is sus-
pended a hollow sheet-iron cylinder, to which a spiral
spring is attached. When the current passes through
FIG. ii.
it, the iron cylinder is drawn into the solenoid until
equilibrium is established by the extension of the
spring. The latter then causes an indicator to move
CURRENT MEASUREMENT. 51
over a vertical scale. The compact form of this
instrument recommends it.
The amperemeter of Hummel (Fig. 12), though one
of the earliest constructions, is yet one of the best.
The solenoid in it lies in a horizontal position. It
carries eccentrically a thin sheet of iron also in hori-
zontal position, bent and fixed at certain points. It
is provided with a long index, vibrating before a
scale. The index nearly balances the weight of the
iron strip, so that the sensibility of the instrument is
FIG. 12.
rather great. The absence of magnets and springs,
as well as the convenient and accurate arrangement
of the apparatus on the zero-point, when it is mounted,
contribute much to making this a trustworthy am-
peremeter. Its calibration is generally reliable.
AMPERE-HOUR METER. .
The electrolytic processes, proceeding quantita-
tively according to any single reaction, are few in
52 ELECTROCHEMICAL EXPERIMENTS.
number. The majority are accompanied by secondary
changes which represent more or less loss in work.
Hence a prime factor in every electrolytic process is
the determination of the current efficiency ; i. e., the
ratio of the current active in the desired direction to
the total current passing through the experimental cell.
This demands an arrangement allowing the measure-
ment of the amount of current which passes through
the circuit in a definite period, and which is indepen-
dent of the fact, whether the current strength has
varied during the period of observation, or whether
periods of cessation have actually occurred.
This essential can be realized in two ways : By
use of the electromagnetic and the chemical action of
the current. Aron applies the first means in his reg-
istering amperemeter. Two synchronous clocks are
used : one has an ordinary pendulum, while that of
the second is controlled by an electromagnet. Con-
sequently, during the passage of the current, a time
difference will be noticed in the reading of the two
clocks, from which the amount of current, which has
passed through the instrument, can be calculated.
This apparatus is extensively employed in measuring
the current consumed in electric light plants.
The amperemeter of Kohlrausch can be re-arranged
so as to serve for the purpose of an ampere-hour
meter. This may be accomplished by attaching a
colored pencil to the indicator and allowing it to
write upon a strip of paper regulated in its move-
CURRENT MEASUREMENT. 53
ments by clockwork. All the variations in the cur-
rent are recorded in this way. The surface traversed
by the pencil represents the ampere-hours.
The copper voltameter described on page 39 can,
without much expense, be changed into an am-
pere-hour meter. It should be retained continuously
in the circuit. A glass or earthenware vessel will
serve to contain the electrolyte. The anodes should
be copper plates varying from 3 to 10 mm. in thick-
ness. A thin copper plate will answer for the ka-
thode. It should be weighed before and after the
experiment upon an ordinary balance. One ampere-
hour corresponds to 1.18 grams of copper. If the
electrolyte is well agitated during the experiment, as
much as 2*/2 amperes can be allowed for 100 sq. cm.
of smooth kathode surface. The liquid may be agi-
tated either by a small stirrer or by conducting a
stream of hydrogen gas through it. It is not advis-
able to blow air through, because sulphuric acid con-
taining air oxidizes copper, and the results will, conse-
quently, be inaccurate.
MEASUREMENT OF PRESSURE.
Ohm's law is thus expressed : E = I . w. From
this it is evident that every pressure measurement can
be referred to a current measurement. To ascertain
the pressure E, prevailing at the points A and B of a
circuit, a shunt with a resistance w, should be at-
54 ELECTROCHEMICAL EXPERIMENTS.
tached to them, and the current in this branch be
then measured. The chief condition is that the
quantity of current passing through the shunt should
be an exceedingly small fraction of the main current,
hence w must be very large in proportion to the
resistance of the main current between the points
A and B. The reason for this is very evident. By
the attachment of the shunt, the resistance of the
conductors between the points A and B is reduced,
because the current has now two outlets. The smaller
the resistance of the shunt, the more will the total re-
sistance A B be diminished. To produce the original
current strength in the now smaller resistance, a lower
pressure than at first will be required ; hence, by the
introduction of the shunt, the pressure is changed, and
is, indeed, lowered; or, in other words, low pressures
are obtained with the aid of a shunt of relatively less
resistance. The greater the resistance of the shunt,
the less will the total resistance between A and B be
altered, and, therefore, the more accurately will the
pressure be determined.
The instrument for the measurement of pressure
the voltmeter is, therefore, an amperemeter for
very low currents. It possess a high resistance in
itself, or a great resistance is inserted before it. Its
divisions, empirically deduced, do not correspond to
the current strengths which the individual deflections
produce, but they are the product of these current
strengths and the total resistance of the instrument;
CURRENT MEASUREMENT.
/'. e. t I . w, which represents the pressure according
to Ohm's law, expressed in volts.
Every sensitive galvanometer may be calibrated to
do the work of a voltmeter by the insertion of a
sufficiently large resistance (p. 1 1 1). The capacity of
every voltmeter can be doubled by the insertion of a
resistance equal to that of the instrument itself; and
when the resistance is twice as great, the capacity is
trebled, etc., etc.
The methods of pressure measurement, which are
determined by the comparison of an unknown electro-
motive force with that of a normal element, can be
omitted here, as they are all rather inconvenient.
The chemist, engaged in electrochemical work, must
be able to know, in a very few minutes, both the
pressure and current strength, either by deflection of
a needle or by some other indicator. It is only by
this means that he can rapidly attain his purpose, and
have full oversight of his experiments at any moment.
The torsion galvanometer is not only an excellent
aid in determining current strength, but it can also
be applied in measuring pressure. This may be
accomplished by inserting resistances of varying
degree. This galvanometer has an internal resistance
of one ohm, and i of deflection represents o.ooi
ampere. When pressure is to be measured, i=
o.ooi X i o.ooi V. By the insertion of 9 Q t the
total resistance of the apparatus becomes 10 Q\ that
is, it is ten times greater than at first, and, in conse-
56 ELECTROCHEMICAL EXPERIMENTS.
quence, 1 deflection corresponds to ten times its
original value, which also follows from the formula
of Ohm : E = o.ooi X 10 o.oi V.
The insertion of 99 Q raises the total resistance to
100 @, and 1 deflection corresponds to o.ooi X 100
= 0.1 V.
The insertion of 999 Q increases the entire resist-
ance of the apparatus to 1000 @, and 1 deflection
corresponds to o.ooi X 1000 == I V.
For convenience, the three resistances, 9, 99, and
999 $, are introduced into a box, and can be -inserted
in the circuit by means of a plug. To avoid injuring
the apparatus, when ascertaining unknown pressures,
it is best to throw in the greatest resistance at first,
then proceed downward to the lowest. The poles
should be so arranged that the magnetic needle is
deflected in a direction opposite to the graduation.
By turning the torsion screw in the line of graduation,
the indicator is again brought back to the zero mark,
and the angle of torsion may then be read. If this
be too small, the torsion spring is released by turn-
ing it back to the zero mark, and a plug is inserted at
the next highest point of resistance. This is repeated
until the angle of deflection is sufficiently large.
The principles of construction in all amperemeters
now in use permit of the application of the latter as
voltmeters, hence these occur in the greatest varieties
and with the greatest differences in capacity. Ex-
ternally, they resemble the amperemeters, but differ
CURRENT MEASUREMENT. 57
from them in having many more coils of wire and
in their high resistance.
Amperemeters and voltmeters containing spirals or
springs should be tested, from time to time, as to their
accuracy, particularly if they are to be used con-
tinuously in a circuit.
REGULATION OF CURRENT.
The previous chapters have dealt with the produc-
tion of the current and its measurement, both as to
intensity and pressure. The methods by which it
can be altered within any desired limits will be next
considered.
p
It follows from Ohm's law: I = , that I can
lw
be altered in two ways : either by alteration of pres-
sure, the total resistance remaining the same, or by
changing the total resistance, the pressure remaining
the same.
A. ALTERATION OF CURRENT STRENGTH BY CHANGE
IN PRESSURE.
With primary or secondary batteries, the pressure
may be varied by arranging a smaller or larger num-
ber in series. It has already been stated that, in the
case of batteries with high resistance, the introduc-
tion of each cell into the circuit is accompanied by
an appreciable change in the expression Jw.
With dynamos it is possible to alter the pressure,
58 ELECTROCHEMICAL EXPERIMENTS.
without disturbing the construction of the machine,
by changing the velocity or by altering the resistance
in the magnet coils. An example was given on page
30, showing how, by increasing the velocity, the pres-
sure on open circuit was increased, and mention was
also made that in setting up a dynamo for experi-
mental purposes it is well to provide it with a cone-
pulley, so that the middle pulley would cause the
normal velocity and the other two either a higher or
a lower speed. If the variations, above or below,
are maintained within 20 per cent, of the maximum
or minimum, there is no danger of doing harm to
the machine. This is furthermore true, because, in a
machine used in an experimental plant, the require-
ments are not continuous, but only temporary, and
for conditions that are extraordinary.
The second method of altering the pressure con-
sists in inserting a resistance, by means of the regu-
lating shunt, in the limb of the magnet (p. 28). This,
however, merely lowers the pressure. If the regulator
supplied by the manufacturer allows only a moderate
reduction, it can be connected with larger resistances.
B. ALTERATION OF CURRENT STRENGTH BY ALTERA-
TION IN TOTAL RESISTANCE.
Two distinct conditions may occur here. Increase
in current strength may be achieved
I. By reduction of the internal resistance of the
source of the current. This refers especially to bat-
CURRENT MEASUREMENT. 59
teries. Their internal resistance may be diminished
by placing several of them parallel, or by the use of
a larger form of the same kind of battery. This is
also true of storage cells, but it must be observed
that such conditions rarely obtain, because this source
of current has but slight internal resistance. It can
only happen should the current required for an
experiment be greater than the heaviest discharge
allowed for this type of battery.
The internal resistance of dynamos can not be
diminished.
2. By reduction of the external resistance. Under
this head the conducting wires must be considered.
Those used in electrolytic experiments should be as
short as possible and not too light in weight. For
the minor experiments, good copper wires of 2 mm.
diameter will be sufficient. It is customary to allow
about from 2 to 3 amperes for a copper wire with a
cross-section of one sq. mm.
The resistance of the experimental cell, or " bath,"
as it is usually termed, may be reduced
(1) by increasing the electrode surfaces,
(2) by diminishing their distance from one another,
(3) by increasing the conductivity of the electrolyte,
(4) by using a thinner or more porous diaphragm,
if such is required by the bath.
It often happens, in using some source of electricity,
e.g, a dynamo or an aggregation of storage cells,
60 ELECTROCHEMICAL EXPERIMENTS.
where there is abundant pressure and but slight inter-
nal resistance, that the current and pressure must be
reduced to some definite value. This can be accom-
plished by throwing resistances into the circuit.
German silver wires (or nickelin or rheotan), of vary-
ing diameters, arranged in spiral or zigzag form upon
a frame of wood, answer well for this purpose. At
suitable distances solder on short, thick copper wires,
which dip into mercury cups. When such a resist-
ance is introduced into the circuit, the current is com-
pelled to pass through all the wire divisions ; and in
order that the resistance may be lowered, it is usual to
throw out one or several of the divisions. This is
generally done by connecting the corresponding mer-
cury cups with a copper wire having this shape | 1.
To screw the individual wires to metal blocks, and
throw out the several divisions by introducing plugs,
is less worthy of recommendation, because usually
the metallic blocks soon become worn and the plugs
do not fit well. Furthermore, such plug contacts do
not long remain bright and clean in a laboratory at-
mosphere. The adjustable rheostats (Fig. 13) are
better. In them the individual resistance divisions
are directed to metal buttons ; over these passes the
movable arm.
The resistances of the several divisions of wire are
numbered as is done in the case of a box of weights :
I, 2, 2, 5, IO, 20, 20, 50, etc., U ; or, by doubling each
CURRENT MEASUREMENT.
6l
value, i, 2, 4, 8, 16, Q. By the latter plan more is
accomplished with the same number of resistances,
but the arrangement is less convenient.
The resistance of a wire is greater, the thinner it is ;
hence, it might be thought that it would be more
advantageous to use a very light wire, for then the
FIG. 13.
minimum quantity of wire would be consumed. This
conclusion is, however, false. The electric energy
annihilated by a wire is transformed into heat. On
sending a current of moderate strength through a
very thin wire, the increase in temperature could rise
to the ignition point, or even to the point effusion of
62 ELECTROCHEMICAL EXPERIMENTS.
the wire. In making calculation for a current regu-
lator, the intensity of the current passing through
each division or section must be considered. The
following table gives results obtained in experiment-
ing with nickelin wires; it may prove of assistance in
making the calculation for current regulators :
DIAMETER IN
RESISTANCE FOR
IGNITION BEGINS
MM.
i M. LENGTH IN Q.
WITH AMPERES.
O.2
13
i-7
0.4
3-2
4
0.6
1.41
7
0.8
0.79
9 l /2
I.O
0.51
14/2
1-25
0.33
20
1.50
0.23
32
i-75
0.16
40
2.O
0.13
45
The current strength given here should not be
actually reached, because ignition always proves
destructive to resistance wires (p. 1 16). With power-
ful currents, such as dynamos give, wires will no
longer answer, because the cooling of a wire by in-
creasing its thickness is always unfavorable. Hence,
it is preferable and better to take short strips of thin
sheet nickelin. These, with a section equal to that
of a wire, can carry much heavier currents and yet
not ignite.
THE VESSELS. 63
STRIPS OF NlCKELIN, 0.3 MM. IN THICKNESS.
WIDTH IN MM. RESISTANCE FOR MAXIMUM CHARGE
i M. LENGTH IN U. IN AMPERES.
IO
0.133
40
15
0.0889
60
20
0.0667
80
25
0-0533
95
30
0.0444
no
35
0.0381
130
40
0.0333
145
45
0.0296
160
5o
0.0267
175
The maximum charges given above are so measured
that the sheets of metal, under the ordinary methods
of cooling, never reach the ignition point. They will
not fuse through until the current is doubled or
trebled. Several metal sheets can be screwed or sol-
dered together.
B. ARRANGEMENT OF EXPERIMENTS.
THE VESSELS.
In the initial experiments, when electrolysis is to
be employed, it is best, where possible, to use glass
vessels, simply because the latter permit the eye to
observe many of the changes occurring. For exam-
ple, by transmitted light, alterations in concentration
at the electrodes may be detected by the formation
04 ELECTROCHEMICAL EXPERIMENTS.
of currents in the liquid; alterations in color, the in-
tensity of the gas evolution at different points on the
electrodes, the separate stages in the production of
precipitates at the kathode, and the solution of the
anode, as well as other points of interest, can be noted.
The general introduction of storage cells of many
sizes has made it possible to obtain glass jars of the
FIG. 14.
most varying dimensions. Smaller vessels may be
prepared by painting cigar-boxes, etc., on the interior
and exterior with paraffin. Such baths answer for
both acid and alkaline liquids. They will last for
several weeks at least.
With experiments conducted on a larger scale,
earthenware troughs or boxes, such as are now
used in galvanizing establishments, are much em-
ployed. They are, however, being gradually sup-
DIAPHRAGMS. 65
planted by wooden vessels lined with lead (Fig. 14).
The latter are made of stout planks, provided with
tongue and groove, with addition of a tarred string,
and are tightly bound together and screwed up with
bolts. The inner surface of these receptacles is cov-
ered with tar or pitch, or, what is better, with sheet-
lead. The separate lead sheets are then welded
together at their edges with an oxyhydrogen flame,
so that the receptacle is really a water-tight lead box,
resting in a frame of wood.
DIAPHRAGMS.
When, in an electrolytic process, substances are
produced at one or at both electrodes, which prove
to be soluble in water, it is impossible, unless suitable
means are adopted, to prevent the separated material
from mixing. Substances thus separating and again
mixing destroy one another, or react upon one another
in a very undesirable way. To obviate this, a porous
diaphragm is introduced between the electrodes. It
should, first of all, prevent the products, appearing at
the electrodes, from mixing. Next, it should offer
little resistance to the passage of the current, and it
should, from a mechanical standpoint, be firm and
durable, and not affected either by the chemicals
present in the bath, or by any which may be pro-
duced in it. The material composing it should be
easily obtained and inexpensive. These requirements
66 ELECTROCHEMICAL EXPERIMENTS.
are fulfilled with difficulty, hence anything approxi-
mating the chief requirements must suffice. The
preparation of a diaphragm, satisfactory in every par-
ticular, is still a problem of the future. Wide fields
will be opened up to electrolysis when once this de-
sideratum is found. The discovery of a diaphragm
suitable for acid and also for alkaline liquids, would
certainly bring a high pecuniary reward. Recently,
efforts have been made to conduct, on a technical
scale, operations requiring diaphragms, but without
success, and the consequence has been that manufact-
urers have turned again to those processes and ma-
terials in which these disturbing factors (diaphragms)
are not present.
Bunsen, in preparing metallic magnesium from the
fused chloride, brought into notice the simplest form
of diaphragm. He introduced a partition from above
into the fused mass ; two communicating chambers
were thus formed, and into them he introduced the
electrodes. Chlorine can not remain dissolved in the
fused mass; it rises immediately to the surface, where
it distributes itself and escapes with much foaming.
The diaphragm prevents it from passing into the
second chamber, where the magnesium collects in
little globules.
In galvanic batteries we noticed that the porous cup
acted really as a diaphragm. These cups answer well
or poorly for the work for which they are constructed,
depending upon the material and the temperature at
DIAPHRAGMS. 67
which they have been baked. A method for testing
them has already been given on p. 13, and it is not
necessary to repeat the same here. For acid liquids
porous cups answer well enough, but they are useless
with alkaline liquids, because the latter decompose
and soon destroy them. When experimenting on a
small scale, the porous cups are satisfactory enough as
diaphragms, but in technical operations their compara-
tively high price, and their short life, due ofttimes to
their fragility, prevent their general adoption. When
the products sought are costly, the use of diaphragms
is to be recommended. The cylindrical form of por-
ous cup is usually quite inconvenient in the construc-
tion of apparatus, hence it will be found to be more
practical to use plates of porous clay ; these should
be placed in the baths as diaphragms. The formation
of such chambers as would result in this way must
produce no insurmountable difficulties.
Quite frequently the sole purpose of the diaphragm
is to prevent pulverulent substances separated at one
electrode from passing to the other electrode. In such
instances the diaphragm is really nothing more than
a filter, so that a bag of silk or muslin can be attached
to the respective electrode (see old form of silver vol-
tameter, p. 38).
The material used in technical operations, at least
in an experimental way, for diaphragm purposes, has
been parchment paper, felt, or asbestos.
68 ELECTROCHEMICAL EXPERIMENTS.
THE ELECTRODES.
To save space, electrodes are generally in plate or
sheet form. These are hung parallel in the bath.
This arrangement allows the use of both sides of the
electrodes. When the purpose is to throw a metal
out of solution, a sheet kathode of the same metal
should be used to get rid of impurities. Otherwise
the choice of kathode material is merely dependent
upon the nature of the liquid which it is proposed to
decompose. A favorable circumstance which maybe
mentioned in this connection is, that the kathode is
protected from corrosion under the liquid, by the re-
ducing action of the current. ' In acid liquids lead or
copper is employed, while iron is used in alkaline
liquids. The electrodes should dip far into the liquid,
first, in order that the material may be used, second,
because the salts of the solution creep up and strongly
corrode that portion of the electrode which is not im-
mersed. Particular attention should be directed to
the points of contact, otherwise great resistance may
be introduced, and, indeed, the current can eventually
be entirely interrupted. Exposed points should be
protected by a layer of varnish.
Anodes, according to their deportment toward a
current, are divided into two groups. The first group
comprises those anodes which decompose and are dis-
solved the soluble anodes. In the second group are
the insoluble anodes. Soluble anodes should be used
THE ELECTRODES. 69
if it can possibly be done in the process under study.
The insoluble anodes are platinum, lead, and carbon
for acid and neutral solutions, while iron is used in al-
kaline liquids.
At the ordinary temperatures platinum anodes are
not attacked in solutions of any kind, not even by the
halogens ; hence they are the most satisfactory ma-
terial for experimental purposes. Their high price
prevents their general adoption in technical work.
The attempt has been made to substitute platinum
coated sheets for plates of the pure metal, without,
however, any degree of success. The platinum coated
material is not sufficiently dense and adherent to pre-
vent the solution of the metal beneath it. With
platinum anodes, the union between the plate and the
wire is always effected by rivets or by welding, but not
by soldering, as the gold used in soldering is not only
dissolved by the halogens, but also by the oxygen
acids during the electrolytic process.
Sheets of lead form very satisfactory anodes in
solutions of pure sulphuric acid. They soon become
coated with a brownish-black, protecting layer of
lead dioxide (PbO 2 ), otherwise oxygen appears upon
them just as it does with platinum. (The author has
observed that the oxygen liberated upon a lead anode
is free from ozone.) The lead dioxide slowly sepa-
rates in leafy scales, which are increased by interrup-
tion of the current. The consumption of lead, how-
ever, is in no sense more appreciable. In the presence
70 ELECTROCHEMICAL EXPERIMENTS.
of halogens and nitric acid, the lead is attacked more
rapidly, but irregularly, with the production of white
layers of chloride or sulphate (p. 87).
When there is an evolution of chlorine, the third
substance, carbon, is stable, but it is strongly attacked
in all cases where oxygen is evolved. Carbon anodes
serve admirably in hydrochloric acid solutions, but in
sulphuric acid they are visibly affected, large black
scales falling off, while a brown dye-stuff dissolves.
The resistance of carbon being so much greater than
that of the metals, the carbon anodes must, conse-
quently, be of much larger cross-section. The con-
ductivity and the power of resisting the action of
chemicals are greater, the harder and denser the car-
bon is; therefore, cut retort-carbon is better than the
artificial and compressed carbon. The creeping of
solutions on the porous carbon plates is disagreeable
and troublesome, causing destruction of the contact
points. It may be overcome by paraffining the carbons
(p. 12).
Thus far no material has been discovered which
will answer as a durable anode in electrolytic pro-
cesses where mixtures of chlorine and oxygen are
produced. Platinum is too expensive and carbon is
consumed too rapidly. A process of this class a
process of eminent importance in which this diffi-
culty is keenly felt, is the electrolysis of sodium
chloride for the manufacture of chlorine and caustic
soda.
THE CONDUCTORS. /I
THE CONDUCTORS.
Soft copper is invariably used to conduct the cur-
rent. In minor experiments, the ordinary, refined cop-
per with a conductivity of from 52 to 56 will suffice.
In technical operations, electrolytic copper is used.
Its conductivity varies from 58.5 to 60. If, at any time,
it is necessary to throw in a flexible piece, experi-
ments on a small scale will require only a thin spiral,
but in the larger operations a piece of copper cable will
be the most suitable. The contacts must all be kept
bright. When they become dull, as happens at the
electrodes, it should be the care of the operator to
inspect them frequently and brighten them up. The
cross-section is so chosen that in conductors of mod-
erate length, say up to 20 meters, not more than 3
amperes will fall to I sq. mm. It follows from this
that in using powerful currents large cross-sections
are requisite, and consequently that conducting rods
of very considerable cost are necessary. It is best,
in general, not to use currents exceeding 500 amperes.
To increase the production, and yet use a single
machine, it is better to increase the pressure than to
increase the current strength. To conduct or dis-
tribute the current from the main line, screw clamps
of the most varied construction may be used (Fig.
is).
The screw a is used to connect two wires ; it is
pushed over the one wire, b and c are very conveni-
72 ELECTROCHEMICAL EXPERIMENTS.
ent modifications for the construction of apparatus.
They are either screwed directly into wood (b), or,
like the table connector c, are fastened to a table by
means of two screws, d serves to connect a sheet
and wire conductor, e is used to connect larger con-
ducting rods. The hinge arrangement permits of its
attachment from without.
Cylindrical rods are not well suited for conducting
purposes, because the borings in the binding screws
FIG. 15.
are only intended for a single wire. A much more
convenient arrangement is the flat conducting wire.
This can then be attached to the machine, and there
being a larger contact surface, one can be assured of
a more complete distribution of the current. The
" rider " binding screw / can be clamped to this.
The binding screw g facilitates conduction by means
of the round wire.
THE ELECTROLYTE. 73
An essential for every electric conductor is the
" key " or " switch." At the completion of every ex-
periment, the main current must be interrupted. This
is necessary in order to prevent a reverse discharge
of the experimental cell. In minor experiments the
wires are merely withdrawn from the binding posts or
screws. In larger or technical operations an adjust-
able rheostat is used. The contact of the latter
breaks the entire current. To renew the electrolysis,
the dynamo is permitted to run until it has gained
its maximum speed and pressure ; then the crank or
arm of the switch is slowly moved over the individual
resistances until the point is reached at which full
current again sets in. The current and energy con-
sumption are gradually raised to the maximum; a
damaging loading of the machine, by jerks, is thus
avoided ; for this would happen were the full current
turned on at once.
THE ELECTROLYTE.
Not much in general is to be said concerning the
electrolyte. It may be either crude material to be
changed by the electrolysis, and then its properties
are regarded as assumed in the purchase ; or, it is a
means to an end e.g., the purification or refinement of
metals. Then the effort is made to have its chemical
properties such that they will answer for the require-
ments of the designed process. In the refinement of
74 ELECTROCHEMICAL EXPERIMENTS.
copper, the electrolyte is the sulphate, because lead,
antimony, tin, and gold dissolve in it with difficulty. If
a chloride solution should be employed, the metals
mentioned would first pass into solution, and later they
would reappear in the precipitate.
The electrolyte should be as good a conductor as
possible, so that little energy will be required to
overcome its resistance. Conductivity increases
with increasing concentration, by acidulation, and
by the application of heat. A maximum conduc-
tivity is imparted to many substances by increas-
ing their concentration. The application of heat to
the electrolyte is permissible when no undesirable
secondary reactions and no appreciable expense are
incurred. As a rule, the liquors are sensibly heated
by the current itself.
In experiments with organic substances, it may be
added that they must, in all cases, be dissolved before
they are subjected to electrolysis. When they are
insoluble in water, they should be brought into a
suitable condition for the action of the current by
means of acids, alkalies, etc.
An advantage in many electrolytic processes is
found in maintaining a thorough agitation of the elec-
trolyte; this can be effected by circulation or by
mechanical means. It is in electrometallurgy par-
ticularly that a more or less complete circulation of
the liquor plays an important role.
THE ARRANGEMENT OF EXPERIMENTS. 75
THE ARRANGEMENT OF EXPERIMENTS.
FIG. 16.
The regulating resistance R is first thrown into the
circuit. It is followed by A an apparatus intended
to measure the current strength. This is either a
technical amperemeter, a galvanometer, or the shunt
of a galvanometer. If the experiment is to be car-
ried out quantitatively, the ampere-hour meter is next
inserted; it is the copper voltameter K. The current
is then introduced and passes through the experi-
mental cell V, the screw-clamp U, used to interrupt
the current, and then back to the battery. To ascer-
tain the pressure in the experimental cell the instru-
ment S is attached to the two electrodes by means of
a thin copper wire. This instrument is either a volt-
76 ELECTROCHEMICAL EXPERIMENTS.
N
FIG. 17.
FIG. 18.
PRESSURE OF DECOMPOSITION. 77
meter or a torsion galvanometer, provided with a series
resistance. The latter instrument can be used both
for the measurement of current and of pressure, by at-
taching its conducting wires to the shunt-resistance
N or to the series resistance W, in connection with
the experimental cell (Figs. 17 and 18).
All such aids as are used in a chemical examina-
tion of the precipitates, solutions, and gases arising
in an electrolytic experiment, are to be considered as
a part of the necessary outfit.
C. PHENOMENA ARISING IN ELECTRO-
LYSIS.
PRESSURE OF DECOMPOSITION. POLARIZING
CURRENTS.
When water acidified with sulphuric acid is elec-
trolysed with insoluble anodes and a varying electro-
motive force, it will be observed that its decomposi-
tion begins when the pressure =1.5 V. Water
can not be resolved into its components hydrogen
and oxygen with a lower pressure. The higher
this factor rises, the more energetic will the current
be. As in the case of water, so, too, in every other
chemical compound, there is a certain minimum pres-
sure, below which decomposition does not occur.
When hydrogen and oxygen combine to yield water,
78 ELECTROCHEMICAL EXPERIMENTS.
heat is evolved, energy is dissipated. The water
molecule has less energy than its components pos-
sessed when they were free and before they united.
If, then, the water molecule is to be decomposed, this
will only be possible by applying an amount of en-
ergy equal to that which is lost in its formation.
This energy is supplied by the electromotive force of
the current. The law of the conservation of energy
would lead to the hope that the decomposition-
pressure might be calculated. This is, indeed,
possible.
The heat modulus (thermal value) is the number of
calories, in grams, set free in the formation of a mole-
cule of a chemical compound referred to one gram of
hydrogen as a unit. The thermal value of water is,
therefore, -f- 68, 360 c. : /. e., in the union of 2 grams of
hydrogen with 16 grams of oxygen, forming 18 grams
of water, heat is evolved sufficient to raise 68,360
grams of water i C. This is expressed symbolically
by placing a comma between the symbols of the
several elements and enclosing the whole in brackets :
(H 2 ,0) = + 68,360.
Water of solution is represented by " aq." This is
always very large in proportion to the dissolved sub-
stance. (KCl aq) represents the aqueous solution
of a molecule of potassium chloride.
Knowing the thermal value (heat modulus) attached
to an electrolytic process, the decomposition-pressure
POLARIZING CURRENTS. 79
may be calculated in the following manner : * Divide
the thermal value (heat modulus) W by the product
of 23,067 multiplied by the number of valences dis-
solved by the current:
Examples :
1. In the case of (H 2 ,O):
W = 68,360.
n = 2,
68,360
therefore Z = - r = 1.48 V.
2 . 23,067
2. In the decomposition of copper sulphate (Cu
SO 4 , aq), with insoluble anodes, into copper, oxygen,
and sulphuric acid (Cu,O,SO 3 aq) we find
W = 55,960
n = 2,
55,960
consequently Z = - r = 1.21 V.
* 2 . 23,067
3. In the decomposition of (AgCl) into (Ag,Cl):
W =-. 29,380
n = I,
hence Z = 1.27 V.
If the current supply in an electrolysis be inter-
rupted and a galvanometer be connected with the
electrodes, it will be observed, from the deflection,
* The deduction of this formula is given in any complete volume on
Physics.
8O ELECTROCHEMICAL EXPERIMENTS.
that a current proceeds from the electrodes, which,
as regards direction, is the opposite of that which
was originally sent through the electrodes. At first
it will be quite strong, but it will gradually grow
less. This phenomenon has long been known under
the name of "current polarization." It is attribu-
table or due to the ions, separated at the electrodes,
seeking to re-unite to the original compound.
The current produced in this way must manifestly
be opposed to the original current. If the products
of electrolysis are not removed, should they remain
in the solid or liquid condition in the vicinity of the
two electrodes, the polarization current can, depend-
ing upon its quantity, enrich the original primary
current very materially (storage cells). If gas evo-
lution has occurred at one or both of the electrodes,
the polarization current can carry only a fraction of
the primary current. At first, so long as the elec-
trodes are covered by a gaseous layer, this current
is quite powerful, but it diminishes rapidly. The
polarization current is also responsible for another
phenomenon. If primary batteries are used in carry-
ing on an electrolytic process, a process well adapted
from the preceding statements for the production of a
continuous polarization current, it may occur in time
that the electromotive force of the elements will be
exhausted and become equal to that of the polariza-
tion current Then, equilibrium would be established
between the battery and the experimental cell, with
POLARIZING CURRENTS. 8 1
the consequent cessation of current. If, for any
reasons, the pressure of the battery continues to fall
even lower, the direction of current will be reversed ;
the experimental cell will discharge itself, like a stor-
age cell, into the battery. A like occurrence, dis-
charge of the experimental cell, with annihilation of
all that has already been accomplished, will be ob-
served, if, when a dynamojs employed for the source
of current, the circuit is not interrupted at the close
of the work. The armature then acts like a short
circuit. The polarization current can, under certain
conditions, become so powerful that it will be strong
enough to again set the inactive machine into motion.
The decomposition-pressure previously mentioned,
is also the maximum electromotive force of the polari-
zation current. It can usually be determined by permit-
ting the electrolysis to proceed for some time, then in-
terrupting the source of current and as quickly as
possible attaching the torsion galvanometer. In this
manner it may also be demonstrated that the electro-
motive force of the polarization current gradually
grows less.
When soluble metallic anodes are employed in
an electrolysis, the conditions become complicated.
The anion the constituent deposited at the anode
is not liberated as such, but combines in statu nas-
cendi with the substance of the anode. This union
develops heat which is favorable to the process.
Consequently, the current is not called on to furnish
82 ELECTROCHEMICAL EXPERIMENTS.
all the energy required : it is only necessary that this
should equal the difference between the two thermal
values (heat moduli), i. e., the heat of decompo-
sition of the electrolyte minus the heat of formation
of the new compound produced at the anode. If
the anode consists of the same metal as that which
is to be deposited from a solution, e. g., as in a
copper or silver voltameter, then the pressure of
decomposition becomes zero, for the energy con-
sumed at the kathode in the decomposition of copper
sulphate into Cu -f- SO 4 is just as great as that pro-
duced at the anode by the union of SO 4 with Cu.
When the pressure of decomposition equals zero,
naturally there can be no polarization current; hence
it is said of such combinations that " the polarization
is removed by the choice of soluble anodes." In prac-
tice, the decomposition-pressure never entirely disap-
pears, but is only more or less reduced, hence the
polarization current, arising from the reaction, may
be constantly detected.
Attention may here be called to a misunderstand-
ing which the author has frequently encountered.
We read, and sometimes hear, that the polarization
may be removed by blowing a current of air into the
experimental cell. If a gas is developed in a pro-
cess the scheme proposed can accomplish merely this,
viz. that after the short circuit of the cell only a very
small fraction of the current first introduced reap-
pears as polarizing current. This is because the one
POLARIZING CURRENTS. 83
product of the electrolysis the gas is removed,
therefore the material for the prolongation of the po-
larizing current is absent. It is a mistake, however,
to suppose that by the mere blowing-in of air it is
possible to obtain a lower decomposition-pressure.
The chemical reaction which the current completes,
is, as has been observed, ample for this purpose, and
this is in no sense influenced by blowing in air.
A second special case arises in the use of soluble
anodes, consisting, not of metal, but of some chem-
ical compound. As a typical case, the process of
Marchese* may be here introduced. In it plates
of copper, serving as kathodes, are hung in a
solution of blue vitriol, while the anodes consist of
plates made from copper-matte. The latter is mainly
cuprous sulphide (Cu 2 S), with a small amount of
iron sulphide (FeS). The following reactions occur :
Copper, from the blue vitriol solution, separates at
the kathode. It has the thermal value (Cu,SO 4 aq).
Sulphur appears at the anode, while copper dis-
solves as sulphate CuSO 4 . The course of the
reaction may be imagined to progress in this way :
At first the copper sulphide breaks down into
sulphur and copper; and for this change a quantity
of energy corresponding to the thermal value (Cu 2 ,S)
will be requisite. After this the liberated copper will
be converted into copper sulphate, and in so doing
*D. R-P., Nr. 22429.
84 ELECTROCHEMICAL EXPERIMENTS.
produces energy corresponding to the thermal value
(Cu,SO 4 aq). Since the thermal value of the electro-
lysis is equal to the algebraic sum of the thermal
values developed at the two electrodes, we then have,
if the heat consumption be represented by , and
the heat production by +,
KATHODE ANODE
(Cu,S0 4 aq) (Cu,S) + (Cu,SO 4 aq).
The first and third members fall away, so that in
the process the actual energy-quantity is that corre-
sponding to the decomposition of the copper sul-
phide. As (Cu 2 ,S) equals 20,270 c, the decomposition-
pressure will be
..
2 . 23067
This diminution in the pressure of decomposition
only occurs when the material to be dissolved is a
conductor of electricity and serves, at the same time,
as anode, so that the solution occurs in consequence
of the direct action of the current. For example,
this end would not be attained if insoluble anodes
were used, and the ore were to be suspended in the
bath by means of baskets. A solution of the ore
occurring in this way, if indeed it did happen, would
be a secondary reaction, which would have absolutely
nothing to do with the electrolysis, hence would not
affect the required pressure.
Recently several suggestions have been made for
obtaining metals electrolytically which seek to reduce
POLARIZING CURRENTS. 85
the pressure of decomposition by a very peculiar
method. Thus, insoluble anodes have been used and
these are hung in the solution of some readily oxi-
dizable substance. The electrodes are, therefore,
separated from one another by a diaphragm. Indeed,
Siemens (D. R.-P., 42243 and 48959) electrolyzed a
mixed solution of ferrous sulphate and copper sul-
phate by allowing the solution first to pass through
the kathode chamber, and then through the anode
compartment. In this way copper was deposited at
the kathode, after which the solution of ferrous sul-
phate passed to the anode where it took up the
atomic group SO 4 the anion separated from the
copper sulphate and became ferric sulphate. The
latter was next digested with finely divided chalco-
pyrite or copper-matte. Sulphur separated, the ferric
salt was reduced, and copper sulphide dissolved, thus
regenerating the original liquid, which in turn was
again subjected to electrolysis.
The thermochemical changes were :
ANODE KATHODE
+ (2FeS0 4 aq,S0 4 ) (Cu,SO 4 aq).
The thermal values needed here are not exactly
known. The difference in question can, however, be
calculated by writing the reactions differently :
+ (2FeS0 4 aq,0,S0 3 aq) - (Cu,O,SO 3 aq).
The heat of oxidation resulting from the conver-
sion of ferrous into ferric sulphate may be ascertained
86 ELECTROCHEMICAL EXPERIMENTS.
by subtracting the heat of formation of two molecules
of ferrous sulphate from the thermal value (heat
modulus) of ferric sulphate :
(Fe 2 ,O 3 ,3SO 3 aq) = 224,880 c.
2(Fe,O,SO 3 aq) = 186,460 c.
(2FeSO 4 aq,O,SO 3 aq) = 38,4800.
Hence the heat consumption at the kathode (Cu,O,-
SO 3 aq) equals 55,960, the heat produced at the anode
38480, and the difference would be 17,480 c, which
would correspond to a pressure of 0.38 V.
In fact, the author, experimenting on a small scale
and observing the precautions laid down on p. 80,
was able to confirm a pressure of decomposition
equal to 0.380.41 V. by using platinum anodes, and
on substituting anodes of carbon found 0.37-0.40 V.
A sheet of lead, used as anode, gave an evolution of
oxygen and a decomposition pressure of 1.26 V.
This also occurred when the experiments were varied
in every possible way. Therefore Siemens' process
is only practicable with platinum or carbon anodes,
but not with lead anodes ! There is no explanation
of this rather singular deportment.
Hopfner (D. R-P., 53782), using cuprous chloride,
and Seegall (D. R-P., 53196) employing ferrous chlo-
ride, pursued an idea similar to that of Siemens. The
suggestion that oxidizable salts should be used in
electrolysis is deserving of great consideration.
As observed in the Siemens process, the nature of
POLARIZING CURRENTS. 8/
the electrodes also exercises an influence upon the
pressure. An example will show this. When a
copper sulphate solution is electrolyzed with plati-
num anodes the pressure of decomposition will be
observed to be 1.26 V. On substituting lead anodes
these will gradually become coated with dioxide
(PbO 2 ) and on making a test, the pressure of decom-
position will be found to be 1.55-1.60 V. This is
due to the reactions which give rise to the polariza-
tion current. The reverse electrolysis takes place
with platinum anodes, while with lead anodes there
comes into play the . formation of lead sulphate
(PbSO 4 ) from the oxide (PbO) and the sulphuric acid
(H 2 SO 4 ). The dioxide at the anode first loses an
atom of oxygen and passes into oxide (PbO), which
unites with the free sulphuric acid to form lead sul-
phate and water. In fact, the heat, resulting from the
formation of the lead dioxide (PbO 2 ) from lead (Pb)
and oxygen (O 2 ) should tend to diminish the pressure
of the primary electrolysis; yet nothing of this kind
is observed, because the main reaction evolves oxy-
gen, and but little of it is consumed in the formation
of PbO 2 . With the polarizing current, however, the
gradually accumulating dioxide immediately comes
into evidence with its entire mass.
The magnitude of the pressure of decomposition
and its eventual diminution by means of aid-reactions,
are of the greatest importance for the commercial
success of a process or method. In the case of a
88 ELECTROCHEMICAL EXPERIMENTS.
metallic conductor the pressure E, which is necessary
to preserve a current of I ampere in a resistance w,
equals :
E = I. w.
In the electrolysis there is added to this the pres-
sure of decomposition Z, so that for every electro-
lysis the necessary pressure is
E=: I. W + Z.
In this expression Z is a constant, determined only
by the nature of the electrolytic process and wholly
independent of the dimensions of the bath. The
value I. w., however, is most intimately connected
with the dimensions of the bath and is smaller, the
lower the resistance w of the bath. Consequently Z
is usually greater than I. w. and will, in large meas-
ure, determine the pressure necessary for carrying on
the work of the bath. The higher the pressure, how-
ever, the greater will the energy consumption for a
definite production be. Hence, in every electrolytic
process, care must be exercised to have the pressure of
decomposition as low as possible, i. e., use soluble
anodes whenever it is possible, or if this be not prac-
ticable, endeavor to facilitate the electrolytic process
by other aid-reactions.
FARADAY'S LAW. CURRENT EFFICIENCY.
When a current is passed through different solu-
lutions in series, the products separated in equal peri-
FARADAY'S LAW. CURRENT EFFICIENCY. 89
ods of time are in proportion to their equivalent
weights (law of Faraday).
For example, the same current in an equal period
of time, decomposes
I molecule of HC1, AgNO 3 , KOH ;
y, " ZnCl 2 , CuSO 4 , H 2 O;
y z " AuQ 3 , NH 3 , etc.
As a current of one ampere precipitates 4.02 grams
of silver per hour, it is easy to calculate the number
of grams of any substance which will be decomposed
or deposited in a definite period of time.
Example. How much blue vitriol can be decom-
posed daily, in a bath, using a current of 50 amperes?
How much copper will be deposited ?
In accordance with the law just stated above, for
every atom, or 107.6 parts by weight of silver, one-
half molecule = 124.6 parts of CuSO 4 ,5H 2 O will
be decomposed, and consequently j Cu = 31.6
parts by weight of copper will be deposited. One
ampere-hour corresponds to 4.02 grams of silver,
hence 4.02 . ' = 4.65 grams of blue vitriol and
4.02 ' = 1. 1 8 grams of copper; so that in
24 hours, with a current of 50 amperes : 24 . 50 . 4.65
55^ grams of CuSO 4 ,5 aq will be decomposed
and there will separate 24 . 50 . 1.18 = 1416 grams of
copper.
When a metal enters its compounds with varying
9O ELECTROCHEMICAL EXPERIMENTS.
valence, the quantities of that metal deposited by
the current will naturally vary according to the va-
lence. With the same current, Cu 2 Cl 2 will yield twice
as much copper as CuSO 4 . In most instances the
reductions first proceed to the lowest state of oxida-
tion before the metal begins to separate.
Very few electrolytic processes proceed in a simple
way. In most cases secondary reactions play an
important role. There is a double possibility for this :
unintentional oxidations can occur at the anode or
reductions at the kathode. In such instances the
law of Faraday serves for the sum of the reactions
occasioned by the current. In the decomposition of
copper nitrate, metallic copper appears at the kathode,
while a portion of the nitric acid is reduced to lower
oxides of nitrogen, or even to ammonia. Oxygen is
liberated at the anode, and is partially consumed in
the reoxidation to nitric acid of the nitrogen oxides
formed at the kathode. When ferric chloride is
electrolyzed, ferrous chloride and metallic iron occur
at the kathode, which is, in turn, converted at the
anode into ferric chloride. A similar phenomenon
explains why manganese can not be quantitatively
precipitated at the anode from a liquid containing
iron, because the salts, in a lower state of oxidation,
which have been simultaneously formed, favor the
re-solution of portions of the manganese.
It is of the utmost importance to know the quan-
titative course of an electrolytic process. That por-
FARADAY'S LAW. CURRENT EFFICIENCY. 91
tion of the total current sent into a bath, which is
active in the direction desired, represents the current
efficiency, while the remainder, consumed in undesired
secondary reactions, is the current loss. In every
electrolytic process the secondary reactions should
be determined both qualitatively and quantitatively,
and by alteration of conditions the effort should be
made to eliminate these disturbing factors. The
character of the side reactions may be discovered by
a qualitative examination of the electrolyte after the
completion of the experiment, and the amount of
loss by a direct determination of the current yield
(efficiency).
To this end insert an ampere-hour meter (a copper
voltameter) in the circuit in series with the experi-
mental cell (see p. 52). Fill a large beaker or glass
jar with copper sulphate (p. 40), observing- the ar-
rangement of apparatus and the size of the kathode,
as indicated on p. 53. When the experiment is
finished, determine the increase in weight of the
kathode, and by Faraday's law calculate how much
of the desired product should be present in the ex-
perimental cell. The actual amount is learned by
weighing, or by titration, and its percentage may
then be calculated.
Example. In the electrolysis of a solution of salt,
after a certain time, 650 c.c. of liquor, containing 4
per cent. NaOH, were obtained. During this same
period 30.4 grams of copper had been deposited in
92 ELECTROCHEMICAL EXPERIMENTS.
the copper voltameter. Calculate the current yield
(efficiency).
According to Faraday's law, 2 molecules, or So
parts by weight of caustic soda, result for each atom,
or 63.2 parts by weight of copper; hence, for the
30.4 grams of copper obtained in the experiment,
38.5 grams of NaOH would be produced (63.2 : 80 =
30.4 : x = 38.48). The actual yield was
650 . = 26.0 grams.
100
Hence the current efficiency would be
26
== 67.5 per cent.
3-5
The rest of the current, equaling 32.5 per cent, was
therefore expended upon other reactions.
ION TRANSFERENCE.
In electrolysis the components of a compound are
not only liberated from their molecular union, but
they'undergo a rearrangement. If the movement of
the liquid particles is not interfered with, this phe-
nomenon will not be noticed, but if it is restricted
by a diaphragm, alterations in concentration will be
observed at the electrodes.
This interesting phenomenon, which can not be
considered theoretically at this time, may be best
understood by an example. Decompose dilute sul-
ION TRANSFERENCE. 93
phuric acid (56.7 grams acid to the liter) between two
semicylindrical sheets of lead, by a current of one
ampere. Electrolytic gas will be set free, and at the
expiration of the experiment the acid will show the
same concentration which it first had. A porous cup
is next introduced between the electrodes. In three
hours the acid in the cup will contain 63.25 grams
of sulphuric acid per liter, while the external liquid
will contain 55.47 grams per liter of the same acid.
After two additional hours the contents will be found
to be 68.77 grams and 52.86 grams respectively.
Hence a transference has occurred on the part of the
sulphuric acid, toward the anode ; and it has been
withdrawn from the vicinity of the kathode. By re-
versing the experiment, it will be observed that this
phenomenon is independent of the varying sizes of
the electrode surfaces. On mixing the two portions
of acid there will result an acid containing 57.1 grams
per liter. Its slight difference from the original content
is due to the acid of unknown concentration absorbed
by the porous cup. Next, take the smaller sheet of
lead, which has been hung in the porous cup, and
let it serve as kathode while the larger is made the
anode. In three hours the concentration will be as
follows:
in the porous cup ( ) 50.65 grams H 2 SO 4 per liter
outside the porous cup (-(-) 60.04 " " " "
Again, the acid in the anode chamber has been en-
riched. Similar phenomena invariably show them-
94 ELECTROCHEMICAL EXPERIMENTS.
selves if diaphragms are used in the work. They
must be considered as a part of the investigation
because they influence the concentration of the
liquors either favorably or unfavorably.
CURRENT DENSITY.
The quantity of the products formed in any elec-
trolytic process accords with the law of Faraday and
the current strength; the quality, however, as well
as the nature of the reactions, depends in many in-
stances chiefly upon the size of the electrode surfaces.
Current density is the current strength for a unit of
surface of an electrode. In technical operations the
unit of surface is the sq. m y but in scientific researches
it is ordinarily the sq. dm. Whether a metal is de-
posited electrolytically in a spongy, crystalline, or
amorphous form depends very much upon the cur-
rent density prevailing at the kathode. The smaller
the electrode surface the more energetically must the
action of the current show itself. Thus, Bunsen,
by using exceedingly high current densities, actually
succeeded in precipitating metallic chromium from an
aqueous solution.
In working with soluble anodes, the influence of
current density will also find expression. When the
current density is low, the individual constituents of
the anode will dissolve one after the other. Metals
in solution will be precipitated one after the other, in
CURRENT DENSITY. 95
accordance with the law that the metal which devel-
ops the greatest energy in its solution will first dis-
solve, and that one which for its deposition requires
the consumption of the least quantity of energy will
be first precipitated. As the density increases, these
reactions take place almost simultaneously. From a
neutral solution containjjig copper and zinc, a current
of low density will first throw out the copper, while
with higher density an alloy of copper and zinc, i. e.,
brass, will be deposited.
In an electrolytic process in which metals are not
precipitated, but where gases or substances soluble
in water are separated, it might be assumed that the
current density was of no moment. This, however,
is not the case. If there is a possibility of producing
different stages in the oxidations or reductions occur-
ring at the electrodes, as, for example, is often the
case with organic bodies, the higher density will
always produce a more far-reaching oxidation or re-
duction than the current with low density.
In addition to the physical character and the chem-
ical nature of the depositions which are formed, the
current density further exercises an influence on the
pressure of the bath. If, in arranging for an experi-
ment, the size and distance of the electrodes from
one another, as well as the nature of the electrolyte,
remain unchanged and the current density alone be
varied, it will be observed that the bath pressure will
increase simultaneously with the density of the cur-
96 ELECTROCHEMICAL EXPERIMENTS.
rent. This phenomenon is not singular or strange
because, inasmuch as the resistance of the bath is
not altered, a higher current-strength can, according
to Ohm's law, only be attained by a higher pressure.
THE WORKING PRESSURE.
Although the dependence of the pressure upon
various circumstances has been touched upon at
many points in the preceding chapter, these relations
must again be considered from a common point of
view.
The working pressure depends :
(1) Upon the decomposition-pressure developed by
the thermal value of the process under consideration.
(2) Upon the sum of all the resistances of the
bath.
(3) Upon the current density which must be main-
tained.
Economy demands that the working pressure shall
be low. Let us see how this can be accomplished.
(a) The decomposition-pressure of a process is
dependent upon the chemical changes producing it:
hence it can not be changed at will. However, as
indicated upon p. 82, it is possible in many instances
to bring in a reaction which will aid the electrolysis
one which will produce heat and thereby dimin-
ish the consumption of external energy in the form
of electricity. The use of soluble anodes and the
THE WORKING PRESSURE. 97
continuous addition of oxidizable substances to in-
soluble anodes may also be mentioned in this connec-
tion.
(b) The resistance of the bath may be reduced
by greater concentration of solution, the addition of
a free acid, if this is advisable or allowable, by heat-
ing the electrolyte, and by bringing the electrodes
nearer together. In arranging the electrodes care
should be exercised that short-circuiting between ad-
jacent electrodes is not only avoided but is impossi-
ble. The resistance in a liquid can be diminished
not only by bringing the electrodes nearer together,
but also by increasing their surface, because thereby
the liquid layer to be traversed by the current will
acqVire a larger cross-section. This is, however,
scarcely allowable because the size of the electrode
surfaces has already been fixed by the current density
most favorable for the operation. A slight advantage
can be obtained by having the electrodes of unequal
size. The occasion will be rare when the current
density at both electrodes must have a definite value.
This is necessary usually for only one electrode,
either the negative or positive, depending upon the
nature of the process. Therefore, the other electrode
may be made larger, and in this way the resistance in
the liquid will be diminished. It is only possible to
make an essential gain in this manner when the two
electrodes show decided differences in size.
(c) When the density and composition of the so-
98 ELECTROCHEMICAL EXPERIMENTS.
lution in an electrolytic process are given, the pres-
sure value is at once fixed and only the minor details
can now be considered, such as may be produced by a
greater or less distance between the electrodes and a
higher or lower bath temperature. With such provi-
sions the pressure can no longer be varied at will with-
out changing both the current density and the separa-
tion of the electrodes. With unchanged solution and
unchanged distances between the electrodes, there is of
necessity a lower density for a lower pressure, and
vice versa.
D. THE PRELIMINARY EXPERIMENTS
NECESSARY FOR AN ELECTRO-
CHEMICAL PROCESS.
If electricity is to be used in carrying out a process,
answers should be given to the following questions :
1. What are the most favorable conditions for the
experiment?
2. What expense will attend the process ?
The experiment should not be conducted on too
contracted a scale. The selected electrodes should
have I oo sq. cm. surface per side. This is particularly
desirable where metal precipitations are concerned.
The metallic deposits on the edges of the electrodes
are generally different from those appearing on the
middle of its surface, so that a false judgment might
PRELIMINARY EXPERIMENTS. 99
easily follow if the electrode had, so to speak, only
an edge surface.
As electrolyte choose a solution apparently suit-
able for the purpose, and at first vary the current
density. Note the pressures which are produced,
and observe the physical and chemical nature of the
deposits ; examine the secondary reactions both
qualitatively and quantitatively. Next alter the com-
position of the solution let it be more dilute, more
concentrated, neutral, acid, alkaline. Change the salts
until a suitable solution and proper current density
have been found. When the most favorable condi-
tions have been discovered, make a few preliminary
experiments upon the current efficiency, and when it
is possible let this be done with extremes in tempera-
ture (high and low) which are likely to figure in the
practical application. During the course of the ex-
periment it will no doubt become patent that the
construction of the apparatus must be altered. In
all such changes consider the question : Will this
arrangement prove satisfactory if the dimensions
are those required when working on a large scale?
Technical experiments should not be performed with
apparatus whose principles can not be applied practi-
cally. Results obtained with such apparatus have
little, if any, technical value, simply because they can
not be directly introduced into practice.
In the whole course of the experiments such fre-
quent opportunity is offered to exercise the ingenuity
IOO ELECTROCHEMICAL EXPERIMENTS.
as chemist, physicist, and engineer, that it is unneces-
sary to speak of all the possible conditions.
When the most favorable experimental conditions
have been found and suitable apparatus has been
constructed, advance is made to the next stage of the
experiment by having a small dynamo act for several
weeks or months on a number of baths arranged in
series. A more extended view of the process is thus
gained, and the deficiencies in construction appear
which in the case of smaller apparatus would be over-
looked. When the faults have been largely removed
and the plant works well, a calculation for larger
conditions can be undertaken without fear of stumb-
ling upon great deceptions or mistakes.
E. CALCULATION OF THE NECESSARY
POWER. CHOICE OF DYNAMOS.
To calculate the power required for an electro-
chemical process it is necessary to know the pressure
and current efficiency for each bath.
Suppose that the bath pressure equals s volts, and
the current yield <r per cent., then the work of a horse-
power would be
V V amperes r -'
I HP = ; with a pressure, therefore, of s
volts there would consequently result amperes
CALCULATION OF THE NECESSARY POWER. IOI
to the horse-power. If a grams of some definite sub-
stance is produced per ampere-hour (see Table i), then
for each horse-power hour there would result - - .
a grams. Since the current efficiency is never
quantitative, but only equal to <r per cent., the value
above must be further multiplied by and then it
becomes Finally, a correction must be
S . 100
added for the efficiency v of the dynamo. In the
case of small dynamos v = 75 to 80 per cent., whereas
with larger machines and normal loads this value
rises to 93 per cent.
Hence the actual yield for each horse-power hour is
736 a . ff . v
grams; or introducing the limiting values
of v 75 per cent, to 93 per cent. given above, the
production p for a horse-power hour becomes, depend-
ing upon the size of the machine used
552 a 684 a
p = . a . - -to . a . - .
S 100 S 100
Having thus deduced the production for a horse-
power hour, the daily production, having a definite
power at command, can easily be ascertained, or the
reverse, viz., the power requisite to manufacture daily
ICO kg. of any product.
In projecting a plant, the result above indicated
will not be sufficient. Regard must also be had
IO2 ELECTROCHEMICAL EXPERIMENTS.
to the mixing and transportation of the liquor,
to the working of the adjunct machinery, etc.
If a steam boiler is necessary, attention must be paid
to the consumption of steam in heating the liquor,
and at least to the heating of the working-rooms by
steam during the winter.
As regards the choice of dynamos, it may be said
that, so far as its efficiency goes, it is immaterial
whether medium pressure and powerful current or
high pressure and low current be employed. How-
ever, the following must be observed : If the nature
of the process in any manner permits, i. e., if the baths
are alike and regularly served, they should be ar-
ranged in series. The number thus introduced into
o
the circuit, and the pressure that the machine will
consequently have, will depend upon whether fre-
quent disturbances occur, due to cleansing of the
baths or electrodes, fresh charges, etc. In electro-
lytic copper refining, 40 to 50 baths are arranged in
series, but with a less active plant 15 to 20 constitute a
very respectable number. If each bath requires a
pressure of s volts and if ;/ baths of this kind are to
be used in series, a pressure of V = n . s volts will be
required of the dynamo. As a rule, the pressure of
the machine should be higher than that calculated,
because by constant work it falls in consequence
of the heating of the armature, and also because of
imperfect contacts here and there. The latter can
scarcely be avoided, so that an increased pressure
CHOICE OF DYNAMOS. 1 03
is necessary to maintain an undiminished current
strength. When the entire electromotive force of
the dynamo is not needed, the excess is cut off by
being sent through a shunt regulator.
A reserve dynamo should always be on hand, so
that in case of repairs to the running machine the
action of the current need not be interrupted for days
or weeks. It is recommended to use the one ma-
chine for day work, and the other for work at night.
In this way both machines are better protected than
if they be run until some part or parts are completely
used up. By an arrangement such as the preceding,
the periods required for cleaning can also be minim-
ized, and the person in charge, finally, can have a
better oversight of the dynamos. Each workman
can then supervise his own machine, and can not con-
cgal any mistakes which may have occurred in its
operation. Again, it will be easy in this way to
arouse the sense of honor of each attendant so that
he will strive to keep his machine in the best con-
dition.
In addition to the expense incurred in obtaining
the power it is necessary, when calculating the cost of
the manufacture, to include* interest on the purchase
sums for the entire plant (buildings, machinery, instru-
ments, baths, conductors), the cost of chemicals con-
* See also Berg- u. Hiittenmann. Zeitung, 1893, 5 2 -
IO4 ELECTROCHEMICAL EXPERIMENTS.
sumed in the process, and wages, as well as loss in
interest due to the storage of the necessary supplies
of crude material.
F. PRACTICAL PART.
i. CONSTRUCTION AND CALIBRATION OF A TAN-
GENT GALVANOMETER.
A very simple contrivance shown in the accom-
panying Fig. 19, and answering every purpose, may
FIG. 19.
be made by any experimenter. It requires no special
description. Notches are made in the little table
at the points where the wire passes through it. It
must, however, have margin enough to support a
glass cover, intended to protect .the needle from air-
currents. The needle is a short magnetic steel rod
(a knitting needle). That the angle of deflection on
CALIBRATION OF GALVANOMETER. IO$
the largely divided circle may be conveniently read,
attach to the needle by means of shellac or wax a thin
hollow glass thread filled with ink. A fiber of the
finest sewing silk will answer for the suspension fila-
ment. This also is placed upon the sealing wax, and by
momentary contact with a hot wire is attached to the
needle. A drop of wax or paraffin brought upon
the glass tube will serve to balance the needle. This
device, it is true, is very primitive, but it will suffice
for many purposes, especially when it is only in-
tended to obtain an approximate idea of the current
strength, or whether it changes during the experiment.
Tangent galvanometers are calibrated by bringing
them into circuit with a voltameter (arrangement of
experiment : battery galvanometer voltameter
battery) and observing the deflections a and a. 2 , with
varying current strengths, I and I 2 . According to
I = c . tang, a,
consequently
tang, a
The values for c, deduced from the two experi-
ments, must agree fairly well. Determine their mean
and calculate a table for the different deflections.
EXAMPLE. The length of the needle is 28 mm., and
the diameter of the circle is 167 mm.
Experiment i. In six minutes 0.0508 gram of me-
tallic copper was obtained in the copper voltameter;
H
IO6 ELECTROCHEMICAL EXPERIMENTS.
the deflection in the galvanometer averaged 12.
The precipitation of copper per hour would, there-
fore, be 0.508 gram, and as 1.181 grams copper, per
hour, correspond to I ampere, the calculated cur-
0.508
rent strength would be = 0.430 ampere. The
constants of the galvanometer may, therefore, be cal-
culated from the equation
0.430
c - - = 2.023.
tang. 12
Experiment 2. In six minutes 0.0963 gram of
copper was precipitated. The average deflection was
22. Therefore the current strength was
0.963
I = = 0.815 ampere,
1.181
hence
0.815
c = - - 2.017.
tang. 22
hence
The mean of the two experiments is c = 2.02.
This is, at the same time, the current strength for the
deflection-angle 45 (tang. 45 = i). A table for the
galvanometer can now be constructed :
1 deflection : I = 2.02 tang. 1 = 0.035 Amp.,
2 " I = 2.02 tang. 2 = 0.070 "
3 " I = 2.02 tang. 3 = 0.106 "
etc.
CALIBRATION OF GALVANOMETER. IO/
2. CALIBRATION OF A GALVANOMETER BY
MEANS OF A SHUNT.
If it is desired to enlarge the capacity of this very
sensitive instrument, recourse may be had to a shunt.
This must be considered and arranged as a part of
the main circuit. As all sensitive galvanometers have
a great number of turns, and as these are usually
wound flat, the constants can not be established by
one or two experiments, but must be determined em-
pirically. This is done by making a number of read-
ings with different current strengths, the results being
then transferred to millimeter paper on which the
current strengths appear as the abscissas and the cor-
responding deflections as the ordinates. The points
fixed in this manner are then united into a continuous
curve, from which, by inverting the order, current
strengths corresponding to all deflections can be ob-
tained.
The lower the resistance of the shunt, the stronger
may the currents be for which the instrument is avail-
able. It should be remembered that in consequence
of the sensitiveness of the apparatus the needle may
be appreciably influenced by the main current if the
galvanometer be set up in too close proximity to the
latter. Whenever possible, place the galvanometer
from ^ to I m. from the main current. Should the
galvanometer always occupy the same position, the
disturbance may be disregarded, because it has
io8
ELECTROCHEMICAL EXPERIMENTS.
been fully allowed for in the curve previously con-
structed. If, however, the galvanometer and its shunt
are to be used in different localities, it is quite neces-
sary to remove them from the main current to the
distance mentioned.
When a delicate galvanometer can not be had, a very
simple form can be constructed after the style of the
tangent instrument described in the preceding chap-
FlG. 20.
ter. Instead of the single copper ring, use a coil
of covered copper wire having numerous turns.
The needle may be large in proportion to the diam-
eter of the ring, as no use will be made of the tangent
law.
Example. A delicate instrument provided with a
bell magnet and copper damping was used as the gal-
vanometer (Hartmann and Braun, Frankfort-on-the-
CALIBRATION OF GALVANOMETER.
ICQ
Main) ; its resistance was 3.35 Q. The shunt con-
sisted of I m. of copper wire 2 mm. in diameter, at
each end of which a plate of metal with two clamps
was attached by solder. This shunt N, a copper
voltameter K, and a resistance W, serving to vary the
current strength, were arranged in series in the same
circuit. The galvanometer G, was connected with the
shunt N by means of two copper wires, I m. in length
(Fig. 20). (These two connecting wires form a
part of the branch resistance, hence, in subsequent
measurements, can not be exchanged for any other
two wires.)
Produce with W currents of different strengths ;
note the deflection, and weigh the copper deposited in
a unit of time (6 minutes, preferably, as they equal
^ of an hour).
DURATION OF
EXPERIMENT.
DEFLECTION.
PRECIPITATED
COPPER
CURRENT
STRENGTH.
MINUTES.
DEC.
IN GRAMS.
AMPERES.
6 . .
. 5-6. ..
. . 0.0114 . .
. 0.096
6 ...
. . 7-5 .-.
. . 0.0154 . .
. 0.130
6 ...
. . 10.8 .
. . 0.0222 . .
. 0.187
6. . .
. . 14.3 . . .
. . 0.0307 . .
- 0.259
8 ...
. . 20.0 . . .
. . 0.0599
-379
6 ...
. . 22.1 . . .
. . 0.0502 . .
. 0.423
7 .
. . 24.5 . . .
. . 0.0661 . .
. 0.478
6
28 o
. o 0668
o. ^6^
6 . . .
. . 32.0 . . .
. . 0.0802 . .
. 0.676
6 ...
37.O .
. . 0.0961 . .
. 0.810
6 . . .
. . 43.0 . . .
. . O.I2OO . .
. 1. 012
6 . . .
. . 49-0 . .
. . o. 1488 . .
1-255
110
ELECTROCHEMICAL EXPERIMENTS.
In Fig. 21 the preceding current strengths appear
as abscissas, the corresponding deflections as ordi-
nates, and the resulting points of intersection indicated
by crosses form the connecting curve :
50r
O s
FIG. 21.
The following table can be read from this curve as
a result of the calibration :
O.I
o. 2
0-3
0.4
0.5
0.6
0.7
0.8
0.9
I.O
I.I
1.2
1-25
DEFLECTIONS.
16.3
21.
25-
2 9 .
33-
36.5
39-5
43-o
46.
48.
49-
INSTRUMENT TO MEASURE PRESSURE. I I I
3. CONSTRUCTION OF AN INSTRUMENT TO MEAS-
URE PRESSURE.
If several storage cells are at the disposal of the
operator, the galvanometer used in the preceding
experiment may be calibrated so as to measure pres-
sure. When a single accumulator has lost a small
part of its charge, its pressure may be accepted as
fairly accurate if taken as 2.0 V. If the cell be
discharged by a German silver wire, the resistance of
which has been so chosen that the wire never becomes
too hot and the current allowed for the cell is never
exceeded, there will exist between the terminals
of the German silver wire a pressure of two volts ; at
the middle it will have one volt, and between the one
terminal and the first quarter 0.5 volt, etc. If a wire
I m. in length has been selected, each cm. of it will
correspond to T i 7 of the pressure prevailing at the
terminals. Hence it will only be necessary to suc-
cessively connect the galvanometer by its wires to the
points of the German silver wire representing o.i, 0.2,
0.3, etc.,V. pressure, and note the deflections in order to
use the instrument as a voltmeter graduated in tenths.
In order to obtain suitable deflections of the galvan-
ometer it must be provided with a series resistance
the size of which will depend upon the pressure and
the delicacy of the instrument. As mentioned on p.
54, the use of a high series resistance increases or en-
112 ELECTROCHEMICAL EXPERIMENTS.
larges the applicability of the instrument. Hence,
having the choice between a less delicate galvanom-
eter with lower series resistance, and one that is very
delicate, and for the introduction of which a high
resistance will be necessary, it will be wise to choose
the latter.
Example. A piece of fine German silver wire, 2 m.
in length, is stretched over a narrow board, which at its
terminals is soldered to binding screws, connected by
stout copper wires to two accumulators, in series, hav-
ing a maximum discharge capacity of ten amperes.
As the resistance of the wire equals 6.67 Q, there
exists in it, during the'experiment, a current strength of
- = 0.6 ampere. To obtain a proper angle of de-
6.67
flection for the pressure of 4.0 V. prevailing at the ter-
minals use a series resistance of about 550 Q. Under
the stretched wire is placed a scale 2 meters in length
and divided into 40 equal parts. The division lines are
five cm. apart and the pressures are written at these
points. The calibration from T x to -fa V. is made in
such a manner that the one binding screw of the gal-
vanometer is connected by a fine wire to the first
binding screw of the German silver wire, while the
other binding screw with the inserted resistance W is
connected with a long, flexible wire. This is carried
along the German silver wire and at each division the
corresponding deflection of the galvanometer noted.
Fig. 22 represents the arrangement as projected.
INSTRUMENT TO MEASURE PRESSURE.
+
-\
, 1 . , , 1 , , , , 1 1 ,
< 1 1 1 1 | 1 1 1 1 1 4-4-
1.0
< * 1 ' 1 | 1 1 1 | 1 1 1 1 1 1 1 1 1 1 i 1 1 1 *
2.0 V-, 3' 4o
FIG. 22.
The values obtained when W = about 550 U
are:
PRESSURES.
o.i V.
0.2
o-3
0.4
0-5
0.6
0.7
DEFLECTIONS.
2.1
4-5
6.6
8-7
10.8
12.8
14.6
3-6
3-7
3-8
3-9
4.0
51.6
52.3
53-o
53-7
54-4
114 ELECTROCHEMICAL EXPERIMENTS.
When a second series resistance of only 100 Q was
used to obtain a greater accuracy for a lower ca-
pacity, the following deflections were observed :
PRESSURES. DEFLECTIONS.
o.io V. 12
0.15 17.2
O.2O 22.5
0.25 27.4
0.30 31.7
0.80 56.5
0.*5 58.1
O.QO 59-4
0.95 60.7
1. 00 61.8
The pressure corresponding to each degree of the
galvanometer may be ascertained by carefully con-
structing a curve, as shown on p. no, from the
observations made and then taking from it the num-
bers not directly observed.
Series resistances may be constructed by winding
covered nickelin or rheotan wires,* of 0.2 mm.
diameter, upon wooden spools.
Each wire has an approximate resistance of 15 Q
per meter. Stout copper wires should be soldered to
the terminals and the whole apparatus be then placed
* May be obtained from F. A. Lange, Berlin C., Seydelstrasse.
,
OF THE
UNIVERSITY
REGULATING RESISTANCE.
in a box or under a glass cover, the terminals of the
copper wire alone projecting beyond the latter.
4. THE CALCULATION FOR AND CONSTRUCTION
OF A REGULATING RESISTANCE.
A battery of storage cells is at the disposal of the
chemist. Each cell is allowed a maximum discharge
rate of ten amperes. The resistance in circuit may be
so regulated that only a portion of the four cells need
be connected for pressure. This, however, would be
a crude arrangement, and, furthermore, the cells would
be discharged unequally, so that after the lapse of a
certain period some of the cells would be exhausted,
while others would still be in a workable condition.
It would be best, if possible, to utilize all the cells in
every experiment. They would then sustain equal
discharge, and could subsequently be equally charged.
A wire is therefore run from the battery to the experi-
ment table. Somewhere in the line introduce a reeu-
o
lator, which will enable the operator to reduce the
current in every possible way and without making any
great jumps. When several experiments are to be
conducted simultaneously by means of the same
battery, throw in such a resistance between each
individual experiment and the main circuit. The
resistance wires are of such a size that the passing
currents are not sufficient to ignite them. The wires
should not be too long and not too different in kind.
Il6 ELECTROCHEMICAL EXPERIMENTS.
The ordinary arrangement of the battery is such
that the four cells are placed in series. Their yield
would then be 8 V. and 10 amperes. The group
arrangement of 2 X 2 cells with a yield of 20
amperes and 4 V. rarely occurs. The maximum
current strength, due to the resistance in question
is, therefore, 20 amperes. Disregarding altogether
the resistance in the experiment under consideration,
the 20 amperes appear with a resistance of - - = 0.2
Q and 10 amperes flow in a circuit where the re-
g
sistance is = 0.8 &. The wire for the resistance
10
subdivisions under I Q should be so selected that
it may safely carry 20 amperes. From the table
on p. 63 nickelin wire 1.5 mm. in diameter would
answer. If it is desired to regulate down to 0.2
o
ampere, a total resistance of =40 Q would be
requisite. Indeed, with 40 @ it is possible to
get even below this point, because the resistance of
the conducting wires in the particular experiment
must always be added, and its decomposition-pres-
sure must be deducted from the 8 or 4 V. of the
battery.
Selecting the subdivisions 0.25, 0.25,0.5, i, 2, 2, 5,
10, 20 Q, the corresponding current strengths in the
separate subdivisions, and the diameter of the nickel-
in wire, suitable for the same, may be found as
follows :
REGULATING RESISTANCE. I \J
SUBDIVISION. CURRENT STRENGTH. DIAMETER OF WIRE.
Up to I G 20 10 amperes 1.5 mm .
2 5 " 41.6 " 0.7 "
10 40 " 0.8 0.2 " 0.3 "
The entire regulating resistance then arranges it-
self as follows :
SUBDIVISION. LENGTH AND SIZE OF NICKELIN WIRE.
0.25 Q 1.09 m.
0.25 " 1.09 " V 1.5 mm.
0.5 "
1. "
2. " 1.92 " }- 0.7
2.
5- "
10. " 1.79 " f 0.3
20. "
The first of these wires is arranged in a long loop,
provided with a sliding contact, so that its resis-
tance can be changed from o. to 0.25 Q by mere altera-
tion in position. Two thin copper plates, bound
together, answer for the sliding contact. They are
pushed over the wire loop as indicated in Fig. 23, a.
The remaining wires are wound in spirals and at-
tached to a frame, each of their ends being soldered *
* Every person engaging in electrolytic experiments should possess
some skill in soldering. Contacts which are to act well for long
periods are best prepared by soldering. The bright, clean terminals of
the two wires are wrapped about each other, moistened with a solution
of zinc chloride, heated in the flame of a Bunsen or alcohol lamp, and the
heated point touched with a thin hammered piece of tin, until the
latter melts and flows smoothly over the soldered spot. If the solder
does not spread, the wires are again moistened with " solder-water,"
115 ELECTROCHEMICAL EXPERIMENTS.
to a piece of stout copper wire which dips into a cup
of mercury.
To throw out a resistance subdivision, unite the
corresponding mercury cups with a copper wire hav-
ing this form
FIG. 23.
5. WORKING A COPPER LIQUOR CONTAINING
ARSENIC.
Problem. A factory produces daily 5 cb. m. of
liquor, which contains 40 grams of copper as sulphate
and the manipulation repeated. With practice the end may soon be
attained with ease. Each solder-point must be washed to remove any
adherent acid, which would gradually destroy the metal.
COPPER LIQUOR. I IQ
and 10 grams of arsenic as arsenic acid, per liter.
Other heavy metals are present only in small quan-
tity, while chlorine and nitric acid are absent. The
attempt is to be made to purify the copper as far as
possible in the electrolytic way, and to separate it in a
marketable form. Make an estimate of the probable
cost of the plant with the necessary power, keeping in
view its future expansion.
It is a fact well known to every chemist that copper
can not be separated quantitatively, as metal, in the
electrolytic way from solutions containing arsenic
without some of the latter being carried down with it.
At the beginning of the electrolysis pure copper,
bright red in color, separates ; but it gradually
grows paler, and in a comparatively short time it is
steel gray, or even black, from the co-precipitated
arsenic. The bright blue color of the solution, at
this point, is evidence that the liquid is not yet free
from copper.
In the electrolytic separation of two bodies, such as
are presented in this case, two considerations must be
observed : the proper current density nx\A the agitation
of the liquor. When the current density is high, the
velocity with which chemical reactions must occur at
every point of the kathode will leave very little time
for the current to select from among the separate
bodies in the solution, so that two or even more of
them will be simultaneously precipitated. With low
current density, however, the possibility of such a
I2O ELECTROCHEMICAL EXPERIMENTS.
selection will occur. Hence a very definite relation
between the copper and arsenic content of a solution
exists for every current density beyond which point
the copper precipitated will contain arsenic. These
limits should be determined.
The liquid at the kathode, from which copper has
been deposited, becomes specifically lighter and rises
to the surface. This upward movement of liquid at the
kathode can be readily shown by carrying out the ex-
periment in a glass vessel and examining the solution
by transmitted light. The liquid rising in the vicinity
of the kathode becomes, in consequence of the dimin-
ished copper content, relatively richer in arsenic, as
compared with the major portion of the solution.
Unfavorable mixture conditions, therefore, exist about
the kathode unless the difference is destroyed by
constant stirring. Thorough agitation of the liquor
becomes necessary in proportion as the kathode
plates are longer.
With the preceding observations before us, the
course of the investigation is clearly indicated. To
obtain as much pure copper as possible, the liquor must
be constantly agitated. It must also be determined,
with varying current densities, at what copper-content
of the liquor the experiment should be interrupted to
prevent the deposition of arsenic. The current effi-
ciency and the bath pressure should be ascertained by
an inserted voltameter. The last two factors form a
basis for the calculation of the requisite power, while
COPPER LIQUOR. 121
the most favorable current density determines the
necessary size of plant.
Execution of Experiments. Two paraffined cigar
boxes will answer for containing vessels. They should
be 1 1 cm. in depth and in length and be 6 cm. wide.
The one contains the solution to experiment upon,
while the other does service as a copper voltameter
(see p. 40). The electrodes of the experimental cell
consist of two plates of lead as anodes, between which
is suspended a thin plate of copper, serving as kathode.
Each metal plate is 10 X 10 cm. The electrodes of
the voltameter should be a thick and a thin plate of
copper of equal size. Agitate the solution by a stirrer
which constantly moves two glass rods back and forth
between the electrodes (30 revolutions per minute).
A horizontal, not vertical, movement is preferable,
because in actual practice the former is cheaper. With
an up-and-down movement the weight of the agitator
must be raised with each lifting ; this means a con-
siderable consumption of energy. With a to-and-fro
movement of the agitator, however, a rake can be
attached to a bar passing over firmly attached carry-
ing-rolls. This does away with the weight of the
agitator, and the work required for stirring, occa-
sioned by the resistance of the liquid and the friction
of the carrying rolls, is reduced to a minimum.
The current from two storage cells is made to flow
through a regulating resistance, and afterward passes,
in regular succession, the shunt of a galvanometer,
i
122 ELECTROCHEMICAL EXPERIMENTS.
the experimental cell, the copper voltameter, and then
back to the battery. The liquor contained 39.29
grams of copper and 9.9849 grams of arsenic per liter.
The experimental cell contained 530 c.cm., corre-
sponding to 20.822 grams of copper and 5.291 grams
of arsenic.
The first experiment was conducted with a current
I = 1.257 amperes (calculated from the voltameter),
the kathode surface immersed in the liquor was 10 X
8.4 cm., hence the surface used was 168 sq. cm., and
the current density was D = ~ = o .748 ampere
for each sq. dm. In trade, whole numbers are pre-
ferred ; hence, in calculating the current density, the
square meter, one hundred times as great, is taken
as the unit. The current density in the preceding case
would then be " 75 amperes per sq. m." The pres-
sure remained nearly constant at 1.92 V.
The experiment was interrupted at the close of
eight and one-half hours. The precipitated copper
had a perfectly bright color, and weighed 12.109
grams. In the voltameter 12.613 grams of copper
were deposited, consequently the current efficiency
was
12.109
12.613
= 96 per cent.
After two additional hours the plate of metal in
the voltameter had increased 2.8925 grams in weight,
while the sheet in the experimental bath had in-
COPPER LIQUOR. 123
creased 2.820 grams. Hence the carrent efficiency
was 97.49 per cent., the mean current strength 1.224
amperes, and the current density 73 amperes per sq.
m. The precipitate was perfectly satisfactory in
every respect.
The following morning the experiment was con-
tinued for two hours more. The pressure equaled
1.95 V. In the voltameter 2.8195 grams of copper
had been deposited, and in the experimental bath the
quantity of copper equaled 2.7828 grams. The cur-
rent strength, therefore, was 1.194 amperes, the cur-
rent density 71 amperes, and the current efficiency
98.7 per cent.
As the copper now began to take on an earthy
color, it was assumed that the limit for the current
density, so frequently mentioned, had been attained.
The copper precipitated in all was
12.109 grams.
2.820 "
2.783
Total, 17.712 "
hence, there still remained in solution 20.822
17.712 = 3.11 grams of copper and 5.29 grams of
arsenic, or, as these quantities were dissolved in 530
c.c. of liquor, the latter contained, per liter, 5.87
grams of copper plus 9.98 grams of arsenic i. e.,
in round numbers, 6 grams of copper for 10 grams of
arsenic.
124 ELECTROCHEMICAL EXPERIMENTS.
Again, a new plate of copper was suspended in the
solution. The experiment was then continued with
a more feeble current, consequently with a lower cur-
rent density. At the expiration of two hours there
was an evident gray-colored deposit of arsenic. The
observations were as follows :
The copper deposited in the voltameter was 1.4485
grams.
The copper deposited in the experimental cell was
I -3974 grams.
Pressure equaled 1.85 V.
The current strength, deduced from these data,
equaled 0.613 ampere; the current density equaled
36.5 amperes per sq. m., and the current efficiency was
96.47 per cent. The liquor still contained 3.11 I 40
1.71 grams of copper, together with 5.29 grams of
arsenic, or in every liter there remained 3.23 grams
of copper plus 10 grams of arsenic.
The gray-coated metallic plate was now replaced by
another, and the current was still further diminished
to 0.25 ampere for a kathode surface of 144 sq. cm.
The current density, calculated from this, equaled 18
amperes per sq. m. The pressure was 1.69 V. After
three hours the copper precipitate was still a beau-
tiful red in color. The deposited quantities were
0.9089 gram and 0.8354 gram, from which the cur-
rent efficiency of 92 per cent, was calculated.
At the end of another hour and seven minutes the
copper became coated with a gray deposit of arsenic,
COPPER LIQUOR.
125
so that the experiment was now definitely interrupted.
The solution contained 2 grams of copper and 10
grams of arsenic per liter.
These results can be best understood from the
following tabular statement :
CURRENT
DENSITY PER
SQ. METER.
71 Amp.
?6 "
18 "
PRESSURE.
'95
1.85
1.69
V.
CURRENT
EFFICIENCY.
97 %
THE FINAL LIQUOR.
6 g. Cu -)- 10 g. As per L.
3-2 g. " " " " " "
2 rr " *
It may be again mentioned that these numbers
relate to liquors which were well agitated during the
electrolysis.
Assuming that the preceding results have been
confirmed by several controls, steps can then be
taken for the calculation of the plant. The lower
the current density selected, the more completely can
the liquor be freed of copper. The current density
of 1 8 amperes, tested last, is certainly not to be recom-
mended. It gave only 4 grams more of copper than
the current density 71, but to do this it required an
electrode surface four times as large, hence a larger
plant, and the efficiency also was about 5 per cent,
less. The middle current density, that of 36 am-
peres, is more favorable. This, or in round numbers
40 amperes, may be made the basis of the following
calculation, taking the pressure as 1.9 V. and the final
liquor as containing 4 grams of copper per liter.
According to these conditions, the liquor at first con-
126 ELECTROCHEMICAL EXPERIMENTS.
tains 40 grams of copper per liter, and is to be
worked down to 4 grams of copper per liter, hence
for each liter or each cubic meter there should be
obtained 36 grams or 36 kg. of copper respectively.
The working of the daily production of 5 cb. m.
should then yield 180 kg. of copper per day. Since
the current efficiency is 96 per cent., there would be
deposited for each ampere hour, not the theoretical
i.i 8 1 grams, but only I.i8l X ~ ~ = I-I33 grams of
copper. Consequently, the precipitation of 180 kg.
of copper would require a quantity of electricity
180,000
equal to = 158870 ampere hours. As the
electrolysis demands a pressure of 1.9 V., therefore
the work would be 158,870 X 1.9 = = 301,853 volt-
ampere hours. This calculated into horse-power
hours is L = 410.1 H. P. hours. If twenty-four
hours are to be worked per day, the machine must
have - = 17.1 H. P. The efficiency of a 17
horse-power dynamo can be considered equal to
about 90 per cent., so that eventually the power re-
quired for the work will be found equal to
0.90
19.0 H. P.
In calculating the size of dynamo necessary for the
above purpose, it may be assumed that there is
a daily consumption of 301,853 volt-ampere hours.
COPPER LIQUOR. I2/
This would require in 24 hours a steady output of
- = 12,577 V. amperes. As I bath requires 1.9
V., the work projected could be done with a machine
of 1.9 V. and 6620 amperes (as 1.9 X 6620 = 12578).
Two baths, in series, would require 2 X 1.9 = 3.8 V.,
and - = 3310 amperes. Four baths, with similar
3- 8
arrangement, would demand: pressure = 4.19 =
7.6 V., and current strength: = 1655 amperes,
etc.
The greater the number of baths arranged in series,
the greater must the pressure of the machine be, but
the current strength can be correspondingly low.
The current strengths mentioned above are too high
for a machine. It is better, as already mentioned on
p. 72, not to exceed 500 amperes. In using a current
of 300 amperes the corresponding pressure would be
= 41 .9 V., and with this pressure it would be pos-
4! .Q
sible to arrange - = 22 baths in series. With this
relatively simple working of baths twenty-two will not
be too many, and the current strength will not be too
high, therefore a machine of the model 42 V. AND 300
AMP. can be selected for the work. It is always well
to allow a certain latitude in such matters ; therefore
it is best to consider propositions for a machine
which, with the lowest velocity possible, will yield a
128 ELECTROCHEMICAL EXPERIMENTS.
current of 300 amperes with a pressure of 42-45 V.,
but it must still be possible to increase it, without
danger, to 350 amperes.
After the energy consumption for the process has
been calculated as 19 H. P. and the dimensions of the
machine are of such proportions as to give 42 V. and
300 amperes, the size of plant can be next considered
i. e., the number and size of baths and the number
and size of electrodes. The number of baths to be
42
arranged in series is = 22. As the current density
must equal 40 amperes per sq. m. there should be
= 7.C sq. m. of electrode surface in each bath. A
40
suitable and convenient size for the electrodes is 50
X 50 cm. As both sides of the electrodes are used,
the surface becomes 0.5 sq. m. Therefore, to get 7.5
sq. m. surface for each bath it will be necessary to use
- = 15 such plates. The lead plates, serving as
anodes, are of equal size; in number they are 16, or I
more than the kathode strips, this is because an anode
should be suspended opposite the reverse side of the
last kathode plate. The lead plates should be about 2
mm. thick and the copper plates 0.3 mm. in thickness.
It will be necessary to purchase the copper plates
only in the initial experiment, because as the work
progresses these will be produced in the process.
If the kathode surfaces be covered with an exceed-
COPPER LIQUOR.
I2 9
ingly thin layer of fat, a little thicker on the edges,
they soon become coated with layers of copper,
which can readily be removed in sheet form.
FIG. 24.
The electrodes are hung parallel in the bath, with
alternating anodes and kathodes, the copper plates
being connected with one another, and the plates of
I3O ELECTROCHEMICAL EXPERIMENTS.
lead in like manner. The current is carried to the
adjacent bath by the device a or b, Fig. 24.
Directions as to the manner of fastening the elec-
trodes, etc., will not be here given. They can be
found in the various volumes of Berg- u. Huttenman-
nischen Zeitung, as well as in Balling's Grundriss der
Electrometalhirgie^ Stuttgart, 1888.
Dimensions of the Bath. In the preliminary experi-
ments the electrodes were removed 3 cm. from each
other. This separation is somewhat too small. It
would be better to make the distance between them
from 4 to 5 cm., so that by the buckling of the elec-
trodes beneath the liquid surface short circuits may
not arise. It is especially at the beginning of the ex-
periment, when the copper sheets are still thin, that
this danger exists. Later, when they have become
thicker and firmer, it is less to be feared. When an
agitator is used, it may diminish short circuiting, but
in spite of this the separation of the electrodes 3 cm.
from each other can still be made the basis of cal-
culation. Thirty-one electrodes give 30 intermediate
places 90 cm., and the distance of each terminal
electrode from the side of the trough being 4 cm.,
there would in all be a full length of 98 cm. The full
width of the trough, granting equal separation of
electrodes, would be 4 -f- 50 -f- 4= 58 cm. To pre-
vent the lead peroxide, which slowly disintegrates,
from becoming incorporated with the copper plate, the
latter should not be allowed to reach entirely to the
COPPER LIQUOR. 13!
bottom of the vessel. Its distance from the bottom
should be 4 cm. Further, the trough must never be
filled to overflowing. A spare of 7 cm. should exist
between the edge and the liquid surface. The depth,
consequently, of the trough would then be 4 -f 50
-f- 7 = 61 cm., and the depth of liquor in it would be
54 cm. The full size of trough should, therefore, be
in length, breadth, and depth 98 X 56 X 61 cm. The
volume of liquor would then be 9.8 X 5.8 X 5.4 =
307 liters. The volume of the electrodes would
cause a reduction of 10 L., leaving in all 297 liters.
Should the dimensions of the bath, using the pre-
ceding calculation as basis, prove too large, it can
be divided into two, or several, smaller baths.
These should be arranged parallel, and the group
that results can then be introduced, in series, as one
single, large bath.
An interesting question is, In what time will the con-
tents of the bath be exhausted ? As 300 amperes are
active in the bath and 1.133 grams of copper are pre-
cipitated with the.observed current efficiency of 96 per
cent, per ampere hour, there would result an hourly
yield, from the bath, of 300 X 1.133 34 grams of
copper. A liter of liquor would yield 40 4 = 36
grams of copper, while the contents of the trough, con-
taining 297 L., would give, consequently, 36 X 297 =
10,692 grams of copper. The time requisite for this
would be - - = 31^ hours. At the end of this
340
132 ELECTROCHEMICAL EXPERIMENTS.
period the entire 22 troughs would have to be re-
plenished. This would require 22 X 297 = 6534 liters
of liquor. During this period there have also been pro-
duced 5 X 6.563 cb.m. of liquor. The calcu-
24
lation, therefore, agrees. The working of the liquor
keeps pace with its production during the operation,
but care should be had for a receptacle, in which can
be collected the liquors obtained from two days'
operation.
The example may be amplified by calculating the
daily production of the plant on the basis that copper
is precipitated in 22 baths arranged in series, and
that the dynamo yields 300 amperes with 42 V.
Such are the statements which generally reach the
public, while the actual process is held as secret.
Let us see how, from such statements, the daily
production, at least, can be calculated. The pressure
need not be taken into account in this calculation ; it
is sufficient for the energy consumption. The current
strength and the number of baths arranged in series,
are, however, important. With 22 baths thus ar-
ranged and a current strength of 300 amperes, 300
amperes would be active in each of these baths, or a
total of 22 X 300 = 6600. It is quite immaterial
whether the 22 baths are simple, or whether 22
groups of 2, 3, or 4 baths each, in series, are worked.
The only difference being that with 22 simple baths
300 amperes fall upon a single bath, whereas with 22
COPPER LIQUOR. 133
quadruple baths 300 amperes will fall to one quadruple
bath, a group, so that a single bath will take
= 75 amperes. But in the entire circuit there will be
a total of 22 X 300 = 6600 amperes doing actual work.
For each ampere hour there is a theoretical produc-
tion of 1.181 grams of copper, while the entire plant
would yield, consequently, 22 X 300 X 1.181 per
hour, and 22 X 300 X 1.181 X 24 daily, which
equals 187,070 grams of copper. This number must
yet be multiplied by the current efficiency. In the
preceding example it was known to be 96 per cent.,
so that the daily production would be 179,587 =
Instead of attempting to free the liquor from cop-
per in a single operation, it may be worked up in
two stages, at first with greater current density,
reducing this toward the end, or the attempt may be
made to arrange the process for continuous work.
This can be effected by letting the liquor slowly
enter the first bath, then conducting it into the next,
and so on until it flows from the last bath fully
deprived of its copper. The first baths, containing
rich liquors, may have, under certain conditions,
greater current densities, consequently less electrode
surface than the end baths. These statements merely
aim to show the possibility of reaching the desired
goal by various methods.
The few grams of copper per liter, remaining in
134 ELECTROCHEMICAL EXPERIMENTS.
the final liquors, are removed by hydrogen sulphide,
by cementation, or by any other good method.
The evolution of oxygen during the electrolysis
causes a disagreeable spattering of the liquor. This
may be much diminished by swimming a sheet of
oiled paper on the bath. Agitation of the liquor is
then out of the question, but it can be mixed by cir-
culation or by similar means.
6. ARRANGEMENT FOR ELECTROCHEMICAL
ANALYSIS.
As a rule, the pressure needed in analyses by elec-
trolysis does not exceed 4 V., therefore, we may use,
as sources of electric energy, either 4 large Daniell
cells, arranged in series, or a thermopile (largest
model with 4 V.), or two secondary batteries arranged
in series. The decided advantage of electrochemical
analysis, its extreme accuracy, as well as the cir-
cumstance that the work in the main is self-acting,
leaving the chemist free to perform other tasks, are
recognized on all sides. As a consequence, electroly-
sis is pushing its way more and more into technical
establishments. In these places many analyses are
conducted simultaneously, hence the source of the
energy must yield a rather powerful current and still
have low internal resistance. The three arrange-
ments previously described render this possible.
Meidinger cells are not well adapted for the work, as
ELECTROCHEMICAL ANALYSIS. . 135
they have great internal resistance. The writer,
using a battery consisting of 4 large Daniell cells (22
cm. high), conducted simultaneously 5 copper deter-
minations, or two nickel estimations, and with two
secondary batteries made simultaneously 12 elec-
trolytic determinations of the most varied character.
To render each electrolysis independent of the rest, as
far as regards current strength, it is only necessary to
insert between each experiment and the main current
a suitable regulating resistance. This is most simply
made as follows (Fig. 25): Two conducting strips,
in the form of flat wire, are run from the battery, on
the wall, along the experiment table. One of the two
electrode stands is connected directly with one of the
metallic conducting strips.while between the other con-
ducting strip and the second stand-support a resistance
wire is inserted (see p. 117 and Fig. 23). The brass
clip, serving as a movable contact, is held firmly by
pushing a short piece of rubber over it. The author
uses for this purpose 1.5 m. of rheotan wire, 0.4 mm.
in thickness, which represents 5.5 U. If, occasion-
ally, greater resistance is necessary, connect the
stands by the conducting metal strips, not with cop-
per wire, but by means of a spiral of German silver.
By pushing the movable contact back and forth the
current strength can be sufficiently regulated. It
may be determined by inserting, during the experi-
ment, a measuring instrument between one of the
stands and the main current. The amperemeter of
136
ELECTROCHEMICAL EXPERIMENTS.
Kohlrausch * (Fig. n) answers admirably for this
purpose.
The customary electrodes are of platinum. A few
FIG. 25.
words may be mentioned in regard to their form.
The electrodes, originally proposed by Luckow and
* Compare Classen, Quantitative Analyse durch Electrolyse. 3te
Auflage, S. 52 ; also Smith, Electrochemical Analysis, second edition.
ELECTROCHEMICAL ANALYSIS. 137
later introduced into the Mansfield works, consisted
of a closed cylinder or cone. This was used as
kathode, while the anode was a platinum wire. Alex.
Classen, who has rendered marked service in the
extension of electrolysis, particularly recommends
(loc. cit.) platinum dishes.
The writer does not favor this idea, because the
dish allows of no other interruption of the analysis
than to siphon out the liquid contained in it, water
being poured in at the same time to replace that
which was removed. If the liquid remaining from
the electrolysis is to be still used in the determination
of other metals, nothing further remains than to
again concentrate the diluted liquid to a suitable
volume. In scientific work this is not a very disturb-
ing factor, but in practical operations, where the
question of " time " represents an expensive factor,
the plan can not be adopted. By using cylindrical or
cone-shaped electrodes the siphoning can easily be
avoided. The electrodes should have side slits and
should not be suspended from below in the stands,
but from above (Fig. 26).
When the analysis is finished, loosen the screw
with the left hand, pressing the electrode with the
right to the stand. Raise it vertically, taking care
not to touch the anode spiral. The moment the
edge of the beaker glass is passed, the little liquid
adhering to the edge is removed and the electrode
quickly immersed in a glass containing water. The
J
133
ELECTROCHEMICAL EXPERIMENTS.
anode is then immediately removed. It is washed with
water from a wash bottle. The electrodes after washing
with water are dipped into alcohol and dried directly
over a flame. This procedure will perhaps cause a
slight loss in liquid, but it is so slight that in techni-
cal operations it need not be at all considered. The
loss equals about 0.5-0.8 c.cm. ; consequently, with a
residual liquid of 150 c.cm. it would be 0.3-0.5 per
FIG. 26.
cent, of the remaining metals. For example, in the
case of a sample of German silver containing 20 per
20 0.3
cent. Ni, the error would be
0.06 per
100 100
cent. Ni. In contrast with this slight inaccuracy,
falling within the limit of error of other methods, we
have the saving of time, which is very appreciable
and of importance.
The platinum cylinders and cones should have a
number of perforations, so that the lines of force
ELECTROCHEMICAL ANALYSIS. 139
arising at the central anode can exert themselves on
the outer surface of the kathode. If this is neglected,
the major portion of the precipitate will be found on
the inner kathode surface, especially when there is
not much liquid between the electrodes and the glass
vessel. Since much depends, in the separation of
some metals, on the current density, perforation of the
kathode surface is to be recommended, so that little
difference will exist between the current density upon
the inner and outer surface. A wire is usually made
to serve as the anode. It is bent in the most various
forms. Any one in practice who would bend the
anode into fantastic shapes would justly arouse
astonishment. In scientific laboratories this is ap-
parently a matter of no moment, because we find it to
be almost universally the case that the bending of the
anode rarely accords with the kathode. The two
electrodes should be equidistant at all points, there-
fore a cylindrical anode is by far the most satisfactory
when using a cylinder. When a cone is used, the
bent wire should have the form of a cone-shaped
screw (Fig. 26). The anode wires should always
stand or rest on the bottom of the beaker glass, be-
cause the gas evolved about them will then effect the
mixing of the solution; consequently, the lower liquid
layer would stagnate did not the anode extend to the
bottom of the beaker. The forms of electrodes just
described and the manner of work just mentioned,
have been used and followed by the writer for years,
I4O ELECTROCHEMICAL EXPERIMENTS.
and in several thousand electrolytic determinations
have answered excellently. They fulfill all practical
demands.
In communicating an electrolytic method for the
determination or separation of metals, the nature of
the solution, the approximate concentration, but espe-
cially the current density should be mentioned. Cylin-
drical or conical electrodes must be considered with
both surfaces. If there be references with the reverse
statement in literature, then introduce an ampere-
meter between the electrolytic stand and the main
current, and by means of the resistance-strip, or with
a rheostat, produce the current strength which has
been calculated from the given current density and
the size of the electrodes in use. In this way alone
is it possible to use the observations of others, with-
out necessitating a minute copying of the details of
the first author. Having once ascertained for a new
process how much resistance must be thrown in with
the arrangements at hand, in order to arrive at the
prescribed current density, it will not be absolutely
necessary, in later trials, to insert the amperemeter in
the circuit. It will suffice to use the resistance of
the amperemeter as found.
TABLES.
141
G. TABLES.
I.
ELEMENT.
SYMBOL
AND
VALENCE.
ATOMIC
MASS.
QUANTITY DE-
POSITED PER
AMPERE HOUR.
Aluminium,
Al'"
Sb"'
As"'
Ba"
Bi'"
Br'
<M"
Ca"
Cl'
Cr'"
Co"
Cu"
Cu'
FT
Au'"
H'
If
Fe"
Pb"
Li'
Mg"
Mn"
Hg"
Hg>
Ni"
N'"
O"
p t iv
K'
Ag'
Na'
Sr"
S"
Sn"
Zn"
27.04
119.6
74-9
136.86
207.5
79.76
111.70
39-91
35-37
52-45
58.6
63.18
19.06
196.2
i
126.54
55-8
206.39
7.01
23-94
54-8
199.8
58.6
14.01
I5-9 6
194-34
3903
107.66
23.00
87.30
31-98
JI7-35
64.88
0-337 gr-
1.491
0-934
2-559
2-587
2-983
2.088
0.746
1-323
0.654
1.096
1.181
2.363
0.713
2.446
0.0374
4-732
1.045
3-859
0.262
0.448
1.025
3.736
7.472
1.096
o.i75
0.298
1.817
1-459
4.026
0.860
1.632
0.598
2.194
1.213
im
Antimony,
Arsenic,
Barium,
Bismuth, .
Bromine, .
Cadmium,
Calcium,
Chlorine, ....
Chromium, . .
Cobalt,
Copper,
Fluorine, .
Gold, .
Hydrogen, .
Iodine,
Iron, ... . ....
Lead,
Lithium,
Magnesium,
Manganese,
Mercury, .
Nickel,
Nitrogen,
Oxygen,
Platinum,
Potassium,
Silver, ...
Sodium,
Strontium,
Sulphur,
Tin,
Zinc.
142
TABLES.
II. THERMOCHEMICAL, DATA.
(from JVaumann's Thermochemie.}
HYDROGEN.
ARSENIC.
(H 2 ,0)
68360
(As 2 , 0.)
154590
CHLORINE.
(As 2 ,O 3 , aq)
(As 2 , 6 )
147040
219400
(Cl, H)
22OOO
(As 2 , 5 , aq)
225400
(Cl, H, aq)
39320
(C1 2 , 5 ,aq)
- 20480
POTASSIUM.
SULPHUR.
(S, O 2 )
(S, 2 ,aq)
(S0 2 , 0)
(S0 2 , 0, aq)
(SO 2 aq, O)
71070
78770
32160
71330
63630
(K,0, H)
(K,0, H,aq)
(K, S, H, aq)
(K 3 ,0,aq)
IK, ci)
(K, Cl, aq)
(K 2 , O, SO 3 aq)
I 04000
116460
65100
164560
105610
101170
195850
( O. O )
103230
(S0 3 , aq)
39170
SODIUM.
(S,0 4 ,H 2 )
(S, 4 ,H 2 ,aq)
192910
210760
(Na, O, H)
(Na, O, H, aq)
102030
111810
(S, H 2 )
4510
(Na, S, H, aq)
60450
(S, H 2 ,aq)
9260
(Na 2 , 0, aq)
155260
IODINE.
(H, I)
(H,I,aq)
6040
(Na 2 , O, SO 3 aq)
(Na, O, Cl, aq)
(Na, Cl)
(Na, Cl, aq)
186640
83310
97690
96510
BROMINE.
(Br, H)
(Br, H, aq)
8440
28 3 80
CALCIUM.
(Ca, 0)
(Ca, 0, aq)
131360
149460
NITROGEN.
(Ca, C1 2 )
170230
(N,H 3 )
11890
(Ca, C1 2 , aq)
187640
(N, H 3 , aq)
(N 2 ,0)
(N,0)
(N 2 ,0 3 ,aq)
(N,0 2 )
(N 2 ,0 5 ,aq)
20330
- 18320
21575
6820
2OO5
29820
STRONTIUM.
(Sr, 0)
(Sr, O, aq)
(Sr, C1 2 )
(Sr, C1 2 , aq)
i 30980
157780
18455
195690
(N,0 3 ,H)
4I5IO
BARIUM.
(N, 3 ,H,aq)
49090
(Ba, O)
130380
(Ba,0, aq)
158260
PHOSPHORUS
(Ba, C1 2 )
194250
(P,0 4 ,H 3 ,aq)
305290
(Ba, C1 2 , aq)
196320
TABLES.
MAGNESIUM.
LEAD.
(Mg, 0)
145860 (Pb, O)
50300
(Mg, 0, H 2 0)
148960
(Pb, C1 2 )
82770
(Mg,0 2 ,H 2 )
217320
(Pb, Cl 2 ,aq)
75970
(Mg, C1 2 )
151010
(Pb, 0, S0 3 aq)
73800
(Mg, Cl 2 ,aq)
186930
(Pb, 0, N 2 5 aq)
68070
(Mg, 0, S0 3 aq)
180180
(Pb, S)
20400
ALUMINIUM.
COPPER.
(A1 2 , C1 6 )
321870
(Cu 2 , 0)
40810
(A1 2 , C1 6 , aq)
(A1 2 , 3 , 3S0 3 aq).
47556o
45 '770
(Cu, 0)
(Cu 2 ,Cl 2 )
37160
65750
MANGANESE.
(Cu, C1 2 )
(Cu, C1 2 , aq)
51630
62710
(Mn, C1 2 )
(Mn, C1 2 , aq)
(Mn, O, H 2 0)
(Mn, 2 , H 2 0)
(Mn, 0, S0 3 aq)
111990
128000
94770
116280
121250
(Cu, O, SO 3 aq)
(Cu, O, N 2 O 5 aq)
(Cu 2 ,S) '
CADMIUM.
(Cd, C1 2 )
5596o
52410
20240
93240
ZINC.
(Cd,Cl 2 ,aq)
96250
(Zn, O)
85430
(Cd, O, SO 3 aq)
89500
(Zn, O, H 2 O)
82680
SILVER
(Zn, C1 2 )
(Zn, C1 2 , aq)
(Zn, O, SO 3 aq)
(Zn, S)
N^ICKKI
97210
112840
106090
41989
(Ag 2 , 0)
(Ag, Cl)
(Ag, Br)
(Ag, I)
5900
29380
22700
13800
(Ni,0, H 2 0)
(Ni, C1 2 )
(Ni,Cl 2 , aq)
60840
74530
93700
(Ag 2 , 0, N 2 5 aq)
(Ag 2 , 0, S0 3 aq)
(Ag 2 , S)
16780
20390
53io
(Ni, 0, S0 3 aq)
86950
MKRCURY.
OoBALX
(Hg 2 , 0)
42200
(Co,0, H 2 0)
(Co,Cl 2 )
(Co, Cl,, aq)
(Co, 0, S0 3 aq)
6^400
76480
94820
88070
(Hg, 0)
(Hg 2 ,Cl 2 )
(Hg, C1 2 )
(Hg, C1 2 , aq)
30660
82550
63160
59860
IRON.
(Fe,Cl 2 )
(Fe,Cl 2 , aq)
(Fe 2 ,Cl 6 )
(Fe 2 ,Cl 6 ,aq)
82050
9995
192060
255420
TIN.
(Sn, C1 2 )
(Sn, C1 2 , aq)
(Sn, C1 4 )
(Sn, C1 4 , aq)
80790
81140
127240
157160
(2Fe, C1 2 aq, C1 2 )
55520
GOLD.
(Fe, 0, S0 3 aq)
(Fe 2 , 3 , 3 S0 3 aq)
93200
224880
(Au, C1 3 )
(Au, C1 3 , aq)
22820
27270
(Fe, S)
3554
(Au, C1 3 , HC1 aq^
31800
III. WIRE RESISTANCES.
DIAMETER.
CROSS-
SECTION.
RESISTANCE PER i M. OF WIRE.
Nickelin.
li
Rheotan.
O
Copper.
O.IO
0.15
O.2O
0.25
0.008
0.018
0.031
0.049
51
22
13
8
60
26
15
9-5
2.2 3
0.99
0.56
0.36
0.30
0-35
O.4O
0-45
0.50
0.071
0.096
0.126
0.159
0.196
5-6
4-1
3-2
2-5
2 O
6-7
4-9
3-7
2.9
2.4
0.247
0.182
0.139
O.I 10
0.089
0-55
o 60
0.65
0.70
0.75
0.238
0.283
0.332
0-385
0.442
1.68
1.41
i. 20
1.04
0.90
1.99
1.67
1.42
1-23
1.07
0.074
0.062
0.053
0.045
0.040
0.80
0.85
0.90
o-95
I.O
0-503
0.568
0.636
0.709
0.785
0-79
0.70
0.63
0.56
0.51
0.94
0.83
0.74
0.66
0.60
0.035
0.031
0.028
0.025
O.O22
.2
3
4
5
0.950
I.I31
1.328
1-539
1.767
0.42
0-35
0.30
0.26
0.23
0.50
0.42
0-35
0.31
0.27
0.018
0.016
0.013
O.OII
O.OIO
.6
7
.8
9
2.O
2.009
2.270
2-545
2-835
3-!4i
0.199
0.176
0-157
0.141
0.127
0.235
0.208
o.i 86
o. 167
o. 150
0.009
0.008
0.007
0.0062
0.0056
2.1
2.2
2-3
2-4
2 -5
3-464
3.801
4.155
4-524
4.909
o. 1 15
0.105
0.096
0.088
0081
0.137
0.124
0.114
0.105
0.096
0.0051
0.0046
0.0043
0.0039
0.0036
2.6
2.7
2.8
2.9
3-o
5-309
5-725
6.158
6.605
7.069
0.075
0.070
0.065
0061
^7-
0.089
0.082
0.077
0.072
_ 0^)67
0.0033
0.0031
0.0028
0.0026
0.0025
I
UNIVERSITY OF CALIFORNIA LIBRARY,
. BERKELEY
THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW
R^VC tint returned on time are subject to a fine of
ration of loan period.
20m-l,'22
/67C2
