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OSMANIA TTMIVERSITY LIBRARY
C all No. 'Accession No.
A uthor
Title
This book should be returned on or before the date
last marked below.
ASSOCIATION THEORY OF SOLUTION
AND
INADEQUACY OF DISSOCIATION THEORY
BY
JITENDRA NATH RAKSHIT, RAI SHAHEB, F i.e., P.C.S.,
ELLIOTT ?RTzEMA~^6roZd Medalist),
Opium Chemist, Government of India.
Calcutta
S. C. AUDDY& CO M BOOKSELLERS AND PUBLISHERS
58 & 12, WELLINGTON STREET
1930
Printed and published by J. Banerji for Messrs. S. C. Auddyfc Coc
At the Wellington Printing Works
10, Haladhar Bardhan Lane and 6 & 7, Bentinck Street, Calcutta,.
PREFACE
The mechanism in the phenomenon of solution has
l been a subject of experiment and study for more than
one generation. In these few pages interpretations of
representative experimental observations on the subject
have been* done in ways not exactly recorded before.
Muoji; needful and relevant experimental verification with
protracted discussion by many scientists will test their
usefulness in time. It is my keenest ambition to take
part in the discussions that may arise on account of this
publication. The media through which a scientist may
give publicity to his views are so vast that there are
reasonable chances of escaping notice of some, even if I
remain on the look out for all. I therefore draw atten-
tion of all scientists, who may be pleased to enlighten
their fellow scientists by the publication of their own
opinion, may kindly inform me also about the same. ,
Pointing out mistakes of any kind will be grate-
fully appreciated. Private criticisms and correspondences
are also cordially invited.
Many figures have been taken and derived from
Landolt Bronstein, Tabalen 5 Auflage 1923 published
by Julius Springer, and a few quotations have also been
made from 'Solubility 1 by Hilderbrand published by the
Chemical Catalog Company, Thyeico-Chemical Tables'
2nd edition 1920 by Castell-Evans published by Charles
Griffin & Co. Ltd., 'Theoritieal Chemistry* by Nernst
and 'Physical Chemistry* by Walker published by
Macmillan & Co. Ltd., 'Solutions' by Osfcwald and 'Che-
mical Constitution* by Smiles published by Longmans
Green & Co. Ltd., 'Chemists* Year book 1 by Atack pub-
lished by Sherratt Hughes, 'Theories of solutions' by
Arrhenius published by the Yale University Press, and'
Systematisk gennernforte termokemiske Undersogelsers
numeriske og teoretiske Resultater, 1905, by Thomson
published by Det Kongelige Danske, Videnskaberne&
Selskab for which indebtedness is expressed to the authors
and to the publishers for their kind permission.
It seemed suitable to write this book in English.
It would have been happy if I had more control over
this language, Readers may have an unavoidable addi-
tional inconvenience on account of the book being written
in a language foreign to me. I shall always remain sorry
for this.
I have great pleasure in expressing my best thanks to
my friends who helped in many ways in this connection.
1930.
CONTENTS
PAGE.
Introduction ... ... ... ... 1
CHAPTER I.
Solubility ... ~ ... ... 10
CHAPTER II.
Specific Gravities of Solutions ... ... 18
CHAPTER III.
Contractions in Solutions
CHAPTER IV.
Surface Tensions of Solutions... ... ... 80
CHAPTER V.
Viscosities of Solutions ... ... **. 88
^.. CHAPTER VI.
Osmotic Pressures of Solutions .., ... 105
CHAPTER VII.
Thermal Effects of Solutions ... ... ... 123
CHAPTER VIII.
Optical Properties of Solutions ... ... 203
CHAPTER IX.
Electrical Effects of Solutions ... ... 247
Index of Authors ... ... . 287
Index of Subjects ... ... ... 291
INTRODUCTION.
Respect for the gifted past and for the aged is religion,,
politeness to seniors and to fellow-scholars is good nature,
but devotion to one's own subject and to truth is duty
and therefore obligatory. Once one assumes a scholastic
life duty towards his own subject must stand supreme.
This sense of obligation and call of duty would give en-
ough rsason to record the truth that he may discern
inspite of many other influences, however powerful and
dissuading they may be. Thus the assumption of a
scientific life, some times has to be signalised by the pro-
duction of what is practically a work done to order. It is
true the choice of the subject of such a work is left to
the aspirer, but on<r*Yesult of such work of compulsory
interest is unavoidable. A book written from such sense
and pressure is bound to lack the inspiration which may
be expected when an author writes because the inner
spirit compels him to do so. If, however, my readers will
bear with patience the theme before them in the absence
of such inner stimulus, may not tell too severely on them.
With all respect for the mighty workers on the subject an
honest and sincere attempt will be made to put forward
my humble views with an expectation that the enterprise
would not be wasteful.
I desire to consider what steps and what attitude
should be taken with regard to the numerous publications
2 ASSOCIATION THEORY OF SOLUTION
on the chemistry of solution that are presented before the
scientists and I wish to make a few observations on what
students and investigators should do for the preservation
and promotion of the best interest of that great branch of
science. I intend to proceed deliberately and cautiously
on such lines as I find <piite suitable and appropriate after
mature consultation and discussion with many of my
worthy friends who choose to have sympathy for my
honest efforts. I will take the liberty of just indicating
to my fellow scientists the broad and general lines on
which it behoves them to follow, if they are solicitious of
maintaining the status and rights, and the preivileges that
a scientific mind and spirit should enjoy.
Scientists have always been on the side of free think-
ing and the orderly progress of their dear subject on
rational lines, and any biased movement should never
have their countenance or support. It is only right and
proper that a student of science should put forth all his
efforts to maintain the full openness of mind and to adopt
all measures that are conductive to the good and bene-
ficent advance of the subject. In this connection I
call upon all students of science who have an interest in
the theory of solution to join together and make a common
cause to make honest attempts to discover the truth in
the way in which I desire to deal with experimental
results of many illustrious past investigators. For the
votaries of science the paramount consideration is the
investigation of the secrets of nature. In the work of
unlocking the secrets of nature, men are taught by
tradition and training to bring to bear on the problems
INTRODUCTION 3
before a mind free from bias, prejudice and preconceived
ideas.
Probably many of my readers have noticed that in
recent times there has grown a dangerous tendency among
the young investigators and this is that of generalisation
with quite insufficient data, and the worst of all this is
that the unfortunate tendency is overlooked by many
learned societies in publishing such communications.
Studies in the electrical properties of dilute aqueous
solutions of salts, acids and bases gave results of con-
ductivity measurements which afforded formation of
several mathematical formulae. Some of these experi-
ments were done by very eminent scientists by a batch
of great physical chemists of the time, such as Van't
Iloff, Kohlrausch, Arrhenius, Ostwald and Nernst. These
investigators being the leaders of the contemporary scienti-
fic opinion forced their views upon the dissociation of
salts, acids and base's in aqueous solution, which unfor-
tunately, however, had a further support by an agreement
of some such electrical results with those of PfefBer's
Osmotic pressure determinations, as also those of the
determinations of the lowering of freezing point and of
elevation of boiling point of solutions.
Although electrolytes form only a fraction of the
very vast general subject of solution yet basing on results
obtained from them considerable calculations, and generali-
sations have been published which mostly are more
contradictory than discordant amongst themselves, Mathe-
matical treatment of a few sets of results have been so
often used for the purpose of generalisation that any
4 ASSOCIATION THEORY OF SOLUTION
open minded study on the subject is rendered far more
complicated than what would have been the case had these
results been published without any such misleading
generalisations so full of exceptions, and with mere state-
ment of broad experimental results.
In these few pages attempts are made to explain many
prominent facts observed during different investigations
with the assumption that the solvent and the solute
always form compounds in solution and the phenomena
of combination are attended by changes very much similar
to those of chemical change if not often indentical. This
association theory of solution has already been somewhat
conceived by Berthelot and MendelecfF but unfortunately
during their life- time the complete establishment of theory
could not be achieved perhaps for want of sufficient ex-
perimental data.
In order to establish the complete association theory
of solution in this book all prominent phenomena are
dealt with and explained with simplicity, as far as possible,
in light of the present views.
Experiment of solutions with semi-permeable parti-
tion have been considerably developed in attempting to
explain many or rather all phenomena of solutions but
success so far achieved appears to be so insufficient that
other modes of explaining them became necessary. So
many varieties of experiments are done to tackle the
theory of solution that it would be beyond the scope of
this book to include many details of them ; besides it is
not necessary to deal with papers which could not produce
any results of sufficient importance. All important
INTRODUCTION 5
Experimental results on solutions have been explained on the
assumption of the association theory of solution and thus
ensuring its complete establishment.
In conclusion I take the opportunity of drawing the
attention of the readers that science has gradually
achieved the freedom of human mind from the domain of
superstition and unquestioning belief. The history of
science reveals the class between the impersonal attitude
of the scientific mind and the pre-conceived notions of
the age, to which belonged privileged votaries of science,
who devoted themselves to win the victory for truth and
reason. Facts were observed, investigated, catalogued,
correlated and classified by me, and formed a bias upon
which I raised a conception of association theory of
solution and humbly place before such votaries of science.
CHAPTER I
SOLUBILITY
The term 'Sol ability' indicates the quantity of a
substance soluble with another substance at any particular
condition. There is scarcely any distinction between the
terms solvent and solute but, for the purpose of con-
venience the constituent present in larger quantity in the
resultant mixture is called solvent. Sometimes the subs-
tance with lower melting point is called the solvent. The
distinction between the terms solubility and miscibility
is also not ordinarily made. Perhaps it would have been
better to have distinguishing terms for two kinds of
solutions or mixtures ; the mechanical and the non-
mechanical attended with changes of energy and property.
Instances of the first type of solution yet remain to be
properly established and those of the second type are all
noticeable ordinarily.
Two or more gases mix in all proportions and their
mixtures do not develop any appreciable simultaneous
change in energy and property. But such phenomena
may hardly be said to be instances of solutions.
When gases are suitably brought in contact with
liquids, mixtures are formed with the occurrence of
changes of energy and property, and such mixtures are
easily called solutions. The formation of the law that the
solubility of a gas at any given temperature in a definite
volume of liquid is directly proportional to the pressure
SOLUBILITY 7
was first done by Henry 1 . Several investigators 2 sub-
sequently verified the law and established its general
validity. Deviations, however, have also been observed
when the gases are highly soluble.
Dalton 3 extended Henry's law by observing that
when a mixture of two different gases is brought in
contact with a liquid each dissolves proportionately to its
partial pressure.
Determination of the solubility of a gas in a liquid
may be carried out by bringing together known volumes
of the gas and of the gas-free liquid, to shake them until
no more diminution of gas is noticeable and then to
determine the volume of the gas, by measuring the
volume of gas expelled on exhausting and boiling out,
by chemical process if any such suitable is applicable, or
by finding out the partial pressure of a gas in its solutions.
The last process* is useful in certain cases to give accurate
results. *"*
In the majority of cases, the solubility of a gas in a
liquid decreases as the temperature rises. Irregularity in
the change of solubility data of a gas in liquids with the
temperature has been noticed in several cases. Existence
of a minimum solubility at about 6(P has been shown for
hydrogen 5 and for rare gases 6 in water. Solubilities 7 of
nitrogen, hydrogen and carbon-monoxide in a number of
organic solvents are greater at 25 than those at 20.
Attempts have been made to show a relationship
between solubility of gases and other properties of solvent
and solution. The phenomenon of the diminution of the
solubility of a gas in a liquid with the rise of temperature
8 ASSOCIATION THEORY OF SOLUTION
has been connected with its viscosity 8 , the absorption
coefficient for any temperature interval is approximately
proportional to the corresponding diminution in the
viscosity coefficient of the solvent. The maximum solu-
bility of gases in mixed organic solvents has been com-
pared with the occurrences 9 of minimum surface tension.
The introduction of one or more substances in a
solution reduces the solubility of a ga;> in a liquid. A
considerable number of measurements have been made to
study the influence of salts and other substances on the
solution of gases in water. It has also been proposed by
numerous investigators that the diminished solvent power
of a salt solution as compared with pure water is mainly
determined, not merely by the specific nature of the
dissolved gas, but by some factor involved in the relation-
ship of the solvent and the solute. It has been suggested
that the influence exerted by salts is a function of internal
pressure 10 or the compressibility of the solution, that the
interaction between the molecules or ions' 1 of the dissolved
substances cause the salts to lower the solubility, and that
the lower solvent power of a salt solution as compared
with water is connected with the hydration 13 of the
salt.
The phenomenon of taking up of gases by solids can
hardly be said to be an instance of solution, since it is
influenced by the surface of the solid body, and changed
by variation of temperature and pressure. Considerable
studies on this subject are needed 13 from the technical
stand-point, as the results on the absorption of % gases
by molten metals could be applied to regulate blow
SOLUBILITY 9
holes ill a casting and its subsequent other physical
properties.
The mutual solubility of a pair of liquids may be
divided into two classes : (1) Soluble in all proportions
and (2) soluble in limited proportions. Determination
of solubility of one liquid in another when not soluble
in all proportions', may be done by bringing together a
large known volume of one with a small known volume
of the other, shaking until saturation has been obtained
and then finding the volume of the undissolved portion
of the smaller liquid 14 . Several chemists 15 determined
the solubility by shaking suitable quantities of two liquids
together until they are mutually saturated and then
analysing portions from each one of the- layers. When
mutual solubility is considerably influenced by variation
of temperature it is often determined by the following
method 16 . Weighed quantities of two liquids are put in
a tube, which is--then sealed and suspended in a bath of
water or other suitable liquid The temperature of the
bath is altered until the contents of the tube become
uniform and determinations of this point are then re-
peatedly alternated with determinations of temperatures
at which, as indicated by appearance of turbidity, the
homogeneous contents of the tube begins to seperate into
two layers.
There are some liquids the mutual solubilities of which
vary with temperature, and which become soluble in all
proportions at certain temperatures called "Critical solution
temperatures". Fig. 1 represents the solubility rising with
the temperature up to critical solution temperature of
ASSOCIATION THEORY OF SOLUTION
phenol and water, benzoic acid and water, carbon disulphide
and methyl alchol, and methylethyl ketone and water.
The mutual solubility increasing with the fall of tempera-
ture up to lower critical solution temperature is represented
by Fig. 2. for triethylamine and water, p-collidine and
water, 1 methyl piperidine and water. The solubility
curve becomes a closed ring when both upper and lower
critical solution temperatures occur, nicotine and water 17 ,
2 methyl piperidine and water 18 and guaieol 19 ; and such
cases are represented by Fig. 3
High pressures 20 influence the mutual solubility of
liquids. Introduction of a third substance generally
interferes with the mutual solubility of two liquids ;
and this phenomenon is utilised in everyday practice for
''salting out" many organic compounds from their
aqueous solutions by the introduction of a suitable quan-
tity of salt.
Composition. Com/iosftion.
FIG. i.
FIG. 2.
FIG. 3.
It is rather difficult to know since when the solubility
of solids in liquids has been studied but it is from the
time of Davy 21 that the solubility of very common things
in water, ordinarily appearing like in soluble, .y., glass,.
clay, etc., has been established.
SOLUBILITY 1 1
Ail accurate method of the determination of solubility
has been described by Farrow 32 which may be conveniently
used for special purposes. But ordinarily solubility of
solids in liquids are determined in various other ways of
which the following are the principal ones : (I) preparation
pf a saturated solution at a fixed temperature and then
determination of the amount of solute present in the
known volume or weight of the solution, (2) finding out a
temperature 28 at which a minute quantity of the solid in
contact with its solution of known strength neither in-
creases nor diminishes in amount, (3/ in the case of
sparingly soluble salts* 4 , comparison of electrical con-
ductivity of the saturated solution of the substance with
that of pure water, and (4) finding out potential differ-
ence 25 between the saturated solution of sparingly soluble
salts, and a suitable electrode.
In the majority of cases, the solubility of solids in
liquids increases -at the temperature rises. Solubilities of
a few substances* 6 like sodium sulphate, calcium sulphate,
calcium hydroxide, etc., do not vary in the same direction
uniformly with the variation of temperature. Solubility
of some salts in water reaches maximum or minimum with
variation of temperature and these phenomena are very
characteristic in the cases of calcium and barium salts of
fatty acids.
Pressure 37 also slightly influences the solubility of
solids in liquids. The solubility of a salt is increased by
pressure if, during solution, a contraction occurs $ and con-
versely the solubility of a salt is decreased by pressure if
an expansion occurs during solution. This subject need
12 ASSOCIATION THEORY OF SOLUTION
more exhaustive investigation to disentangle the nature
of solution,
The solubility of a solid in a liquid is considerably
influenced by the introduction of a third substance in the
solution. The increase or decrease of solubility in such
cases entirely depends on the nature of the solute, the
solvent and the third substance. The increase of solubility
of iodine in potassium iodide and that of silver chloride in
potassium cyanide is due to the formations of definite
compounds. The solubility of salts in water is often
diminished by the introduction of another salt containing
one of the two radicales, negative or positive, common to
the original solute j and it is also often increased when
none of the radicals of the third substance is common to
any of the first salt. The solubility of a non-conducting
solute in water is raised or lowered according to the res-
pective individual nature of the solvent and two solutes.
Considerable* 8 studies have been on this subject but
conclusions forthcoming from them are, unfortunately,
unable to lead to any satisfactory generalisation.
Solubility phenomenon 29 has been stated to be en-
countered with substances which are closely allied
chemically. Toe solubility of salts in water depends on
the ability of the molecules or ions to surround themselves
with a water mantle and thus to approximate in character
to the solvent. When the molecule is capable of taking
up the solvent, e.g., in the form of subsidiary balance
compounds, it is then soluble even though undissociated.
Thus compounds containing H 2 of crystallisation are
-usually soluble in water. If a connection exists between
SOLUBILITY 13
^the capacity of the solute to take up solvent and the
solubility, the latter must be related to the structure of
the 'molecule. Several applications of these considerations
are dealt with briefly by Fritz Ephram, who also considered
that the formation of precipitate is probably due to changes
in constitution.
All the representative phenomena described in con-
nection with the solubility of gases, liquids and solids in
different liquid solvents could be explained by the
assumption that in solution the solutes form compounds
with solvents in proportion equal to the dilution. The
nature of the solubility curve depends entirely on the
stability and on other properties of such compounds under
the conditions of the experiment. Assuming that all the
solute molecules form compounds or associate with solvent
molecules each oilier in proportion as their dilution con-
siderable complication in explaining all phenomena con-
nected with solubility could be obviated. It is advantageous
to ignore the separate existence of any solvent molecule
in a solution containing solute in any of its states of
existence, gas, liquid or solid. When a third substance is
introduced in a, binary solution a readjustment of formation
of compounds takes place depending on the comparative
affinity to form such associations under the conditions of
the experiment. Maximum solubility indicates the limit
of the ratio, for the formation of compound, of the
solute and solvent molecules under the conditions of the
experiment. The relationship 80 of some salts in dilute
aqueous solutions has been determined by their influence
on the critical solution temperature of the system phenol-
14 ASSOCIATION THEORY OF SOLUTION
water and the results obtained could be only explained by
admitting the association theory of solution and not by
the dissociation theory. Combination of two solutes
simultaneously with the solvent may be nicely illustrated
from Van't HoiFs* 1 results. He found that if a solution
containing sodium sulphate and magnesium chloride in
equivalent proportions is placed in a dilatometer, no
anomalous expansion is observed at 5, but if two
molecules of sodium sulpuate are present for each molecule
of the magnesium salt, there is a very considerable expan-
sion if the liquid is heated above 5, and a considerable
contraction if it is cooled below this temperature. Although
this experiment was not properly done yet the results show
the difference of properties of the different compounds
formed with solvent.
In studying the solubility of substances like sodium
sulphate in water transition temperatures are noticed
which have been erroneously explained by stating that a
turning point in the solubility curve shows (hat the solid
phase in the saturated solution is changing. The decrease 32
in the solubility of certain salts at higher temperatures
may be explained by the assumption that the associations
of solute and solvent in certain proportions are unstable
beyond those conditions at which the curves cut or change
directions, but some others having different molecular
proportions are stable under the same circumstances,
(1) Henry, Phil. Trans., 1803, 93, 2 ( J, 274.
(2) Bunsen, Annalen, 1885, 93, 1 ; Kharikof and
SOLUBILITY 15
Longuinine, Ann. Chim. Phys,, 1869, 11, 412 ; Woukul-
off, Compt. rend., 1889, 108, 674 >, 109, 61.
03) Dalton, Mem. Lit. Phil. Soc. Manchester, 1805.
1. 273.
(4; Gaiis, Zeit. anorg. Chem., 1900, 25, 23(3 ; Abe^-g
and Riesenfeld, Zeifc. Phys. Chem., 1902, 40, 84 ; Jones,
Trans. Chem. Soe., 1911, 99, 392; Dobson and Masson,
ibid , 1924, 125, 668 ; Dunn and Eiddal, ibid., 67(3.
(5, Bohr and Bock, Ann. Physik., 1891, 44, 318.
(6) Estreicher, Zoit. Phys. Chem., 1899, 31, 176 ; von
Antropoff, Proc. Hoy. Soc., A. 1910, 83, 474.
(?) Just, Zeit. phys. Chem., 1901,37, 342.
(8) Winkler, Zeit. phys. Chem., 1892, 9, 171 ; Thorpe
and Rodger, Jour. Chem. Soc., 1894, 65, 782.
(9) SUiiTOw, Zeit. phys. Chern., 1902, X, 41, 139 j
Christoff, ibid., 1906 > 55, 622.
(10) Euler, Zeit. phys. Chem., 1899. 31,368 ; Geffeken,
ibid., 1904, 49, 23^5 Ritzcl, ibid., 1907, 60, 319.
(11) Levin, Zeit. phys. Chem., 1906, 55, 503; Ruth-
mond, ibid., 1909, 69, 523.
(12i Rothmond. Zeit. phys. Chem., 1900, 33, 413-
Baur, Ahren's Samrnlung, 1903, 3, 466 ; Lovvry, Trans.
Ear. Soc., 1905, 1, 197 ; Philip, Jour. Chem. Soc., 1907.
91, 711; Hudson, Zeit. Elektrochem., 190S, 14, 821.
(13) Iladlield, Trans. Ear. Soc, 1919, 14, 173.
(14; Schuncke, Zeit. phys. Chem. 1894, 14, 331 .
Bodtker, ibid., 1897, 22, 511 ; Herz, Ber., 1898, 31,
2669 ; Rex, Zeit. phys. Chem., 1906, 55, 355.
(15) Chancell and Parmetier, Compt. rend., 1884 99
892 ; 1885, 100, 773 ; Walker, Zeit. Phys. Chem., 189o!
1 6 ASSOCIATION THEORY OF SOLUTION
5, 196 ; Klobbie, ibid., 1897, 24, 616 ; Euler, ibid., 1899,,
31, -364 ; Osaka, Mem. Coll. Sci. Eng. Kyoto. 1909-1910,
2. 21.
(16) Gullirie, Phil. Mag., 1884 18, 22, 495. Alexeeff.
Ann. physik., 1886, 28, 305 j Rothmond, Zeit. phys.
Chem., 1896, 26, 483. Bingham, Amer. Chem. Jour.,
1907, 37, 54y ; 38, 91 ; Flaschner and Mac Ewen, Jour.
Chem. Soe., 1908,93. 1000.
(17) Hudson, Zeit. phys. Chem., 1904. 47, 113.
(18) Flaschner and Mac Ewen, Jour. Chem. Soc.,
1908, 93 1000.
(19) Mae Ewen, Jour. Chem. Soc., 1923, 123, 2286.
(20) Kohustamn and Timmermans, Proe. K. Akad
Wetenseh. Amsterdam. 1913, 15, 1021 ; Arch NeerlaniL
19:12, 6, 147.
(21) Watt's Dictionary of Chemistry Vol. IV. 1920.
175.
(22) Farrow, Jour. Chern. Soc., 1926, 51.
(23) Aiexeeff, Ann, Physk. 1886, 28, 305; Schroeder,
Zeit, Phys. Chem., 1893, 11, 453.
(24) Hollernan, Zeit, phys. chem., LSU3. 12. 125 :
Kohlrausch and coworkers, ibid., 1893, 12, 234; 1901,
50, 355 ; Sit/ ungsber. K. Akad. Wiss. Berlin. 1901.
1018; Bottger, Zeit. phys. Chem., 1903, 46,521, 1906,
56, 83 ; Weigel ibid., 1907, 58, 293.
(25) Goodwin, Zeit. phys. Chem., 1894. 13. 641;
Morgan, ibid., 1895, 17, 533 ; Thiel, Zeit. Anorg. Chem.,
1900. 2149 ; Immerwahr, Zeit. Elektrochem., 1901, 7, 477.
(26) Tilden and Shenstone, Phil. Trans., 1884, 175, '23
Etard, Compt. rend., 1383, 106, 206, 740; Lieben and
SOLUBILITY 17
others, Montash., 1894, 15, 404 ; Lumsden, Jour, Chem.
Soc., 1902. 31, 350 ; Roozeboom, Zeit. phys. Chem.,
1893, 10, 477.
(27) Cohen and Coworners, Zeit. phys, Chem., 1910,
75,257; 1909, 67, 432; Piezocheme Kondensiertcr
system, Leipzig, 1919; Sill, Jour. Amer. Chem. Soc.,
1910, 38, 2032 ; Sorley, Proc. Hoy. Soo., 1863, 12, 538 ;
Phil. Mag.. 1854, (4). 27, 145 ; Brann, Wied. Ann., 1887,
30, 250, Zeit. phys. Chem., 1KS7, 1, 259; Johnston,
Losungen, Hamburg, 1907.
(28) Rothmond's Loslichkeit and Loslichkeits-beein-
flussung. Vol. VII of Bredig's Ilanclbeuch der angewand-
ten Physikalische Chemie ; British Association Report
1910,425; 1912,795; Hildebrand, Jour. Amer. Chem.
Soc., 191G, 38, 1452 ; Bronstead, Jour. Chem. Soc., 1921,
574; Jour. Amer. Chem. Soc., 1922, 44, 933.
(29) Fritz Ephram, Ber., 1921, 54. B. 379.
(30) Duckett~and Patterson, J. Physical. Chem., 1925.
29. 295 ; Carrington, Hickson and Patterson, Jour. Chem.
Soc., 1925, 2544.
(31) Van't Hoff. Rec. Trav-Chem., 6. 30-42 j 91-94 ;
137-139, and Abst. Chem. Soc., 1888. 404.
(32) Rakshit, Zeit. Elektrochem., 1927, 581.
CHAPTER II
SPECIFIC GRAVITY OF SOLUTIONS
The terms specific gravity and density are often used
in connection with solution practically conveying the same
idea, although the density of a substance is defined as
its mass per unit of volume and the specific gravity of a
substance is defined as a ratio of the mass of a given
volume to the mass of the same volume of water under the
same conditions. For the purpose of this book the use
of the term specific gravity would be suitable because most
of the investigators on solution expressed their results in
such terms. These terms are, however, interchangeable
without committing much mistake on account of the fact
that the unit volume of water at a standard temperature
is taken as the standard unit of mass and the same volume
of water at the same condition is taken as standard for
the purpose of specific gravity determination.
Scientists at many laboratories have determined specific
gravities of solutions by various methods and have
expressed their results which widely differ in their modes
of expression, the chief item being the temperature
difference. Specific gravities were determined by knowing
the weights of a fixed volume of solution at various
temperatures and comparing these with the weight of the
same volume of water at the same or different tem-
peratures, These differences, in recording the results, are
SPECIFIC GRAVITY OF SOLUTION 19
so inconvenient that the figures of one can hardly be
compared with those of the other. Although there may
be 'reasons for working under conditions which varied so
widely, yet it is only desirable to have such determinations
done under uniform conditions, which could be fixed by
international arrangement. There does not seem much
difficulty in expressing specific gravity of solutions at any
temperature compared with water at that very temperature,
and such figures would be more helpful for theoretical
considerations.
Determinations of specific gravity could be done by
hydrometers, specific gravity bottles, pyknomcters and
plummets. Some of the observations on the subject lii-e
those recorded by Young and Forty' and Washburn and
Mac In lies* should be studied by anv beginner in such
investigations.
Specific gravity determinations are not only made for
the purpose of living theoretical problems but for many
important practical purposes, in connection with the
manufacture of caustic soda, caustic potash, sugar,
sulphuric acid, alcohol, etc. It is often necessary to know
for commercial purposes the strength of the solute in
solution ; and it is considered (juite convenient to do so
by the determination of specific gravities, if these figures
could be converted into the actual amount of substance
present in the solution. Accordingly many authors plotted
their results into curves and by interpolation prepared
tables by which, from the specific gravities of solutions of
pure substance, strengths of these could be obtained.
Such curves are, however, neither very regular nor could
20 ASSOCIATION THEORY OF SOLUTION
be represented by simple equations on account of the fact
that the molecular volumes of solute, solvent and their
associations at different ratios are related by chemically
allied phenomena which differ from one another.
Association theory of solution assumes that when a
solute dissolves in a solvent a molecular combination take^
place simultaneously as the formation of solution, in
proportion same as their dilution, and when this pro-
portion is disturbed or changed an immediate correspond-
ing association of solvent and solute takes place uniformly.
When this reaction takes place in solution along with the
manifestation of disturbance of other properties, a change
in volume is also accompanied, and this factor alone guides
the alteration of specific gravity of solutions. The specific
gravity curve of solutions is an expression of the force with
which the molecules of solute and solvent are associated
and ignorance of this led to the development of many
equations to disentangle the theoretical nature of
phenomenon of solution. It would be quite worth while
to describe a few of them here, to show how the importance
of the subject was felt by the earlier scientists.
Ostwald presumed that there are cases of concentrated
aqueous solutions where no changes of volume on mixing
occur and the speciiic gravity could be expressed by a
complicated formula, which he said to be "somewhat
obscure expression", 3
o -c -4. (m-r-mo)SSo
Specinc gravity =- - -
J
= Specific gravity of the concentrated solution.
SPECIFIC GRAVITY OF SOLUTIONS 21
So = Specific gravity of pure solvent,
m = weight of the concentrated solution.
m = weight of the pure solvent.
Such equation, however, is useless as there is no solution
known which does not undergo change in volume during
dissolution.
Valson 4 determined a series of specific gravities of salt
solutions at various dilutions and proposed a generalisa-
tion which is commonly known as " Valson 's law of
moduli" This paper attracted considerable attention of
the scientists because quite a number of salt solutions gave
agreeable results. When various salt solutions, each
containing one gram equivalent per litre, are compared,
it is found that the difference between the specific gravities
of solutions which contain two specified metals in combina-
tion with same j^eid is equal, whatever be the acid, and
similarly, that the difference between the specific gravities
of the solutions of two salts of the same acid is independent
of the nature of the metal of the salts. The specific
gravity of the salt solution is obtained by adding, to a
normal value, two numbers, one of which is determined by
the metal and the other by the acid. These numbers are
called moduli. "Valson selected a low specific gravity
solution of ammonium chloride, sp. gr. = 1.015, as the
standard solution. Practically nothing has been said why
water has not been chosen as standard, it appears however
that water might have been a better standard, and then it
would have been only necessary to alter the series of
moduli by 15. The following Valson's figures are
22 ASSOCIATION THEORY OF SOLUTION
multiplied by LOGO, i.e. before using these should be*
divided by 1000.
Ammonium .. Barium
Potassium . 30 Maganese
Sodium ... 25 /inc 1$
Calcium .. 26 Copper ... 42
Magnesium ... 20 Cadmium ... 61
Strontium ... 55 Lead ... 103
Chlorine ... Silver ... 105
Bromine 34 Nitrate ... 15
Iodine 64 Carbonate ... 14
Sulphate ... 20 Bicarbonate ... 16
The specific gravity of a normal solution of sodium
carbonate may be thus obtained :
Standard number 1'015
Sodium 0'()25
Carbonate 0'014
Sp. gr. of N.Na 2 CO 3 Solution T054
This figure very approximately agrees with that obtained
by actual experiment.
Valson's numbers are referred to equivalent quantities
(H-=l) and the rule holds only for dilute solutions.
This rule was extended to solution of any concentration by
Bender 5 who divided the differences of specific gravities
by the number of equivalents in a litre solution, the
quotients agreed with Valson's moduli. The extended rule
is illustrated by the following table, where M number
SPECIFIC GRAVITY OF SOLUTIONS 23
pf equivalents and A is the difference of specific
gravities :
M
NH 4 C1
Sp. gr.
KC1
Sp. gr.
KC1
A
M
Nad
A
M
Lid
A
M
^BaC
A
M
1
1-0157
1 -0444
237
244
78
738
2
1-0308
1-0887
239
240
78
786
3
1-0451
1-1:317
239
233
732
4
1-0587
234
79
5
1-0728
232
76
Figures under Kd~ , Nad-" Lid
M' 2
are
the differences of specific gravities divided by the number
of equivalents contained in a litre of solution. Moduli
of Valson are obtained by ^ , and the specific gravity
of any solution containing M equivalents in a litre is
obtained by adding Moduli of the salt multiplied by M, to
the specific gravity of solution of sal-ammoniac which
contains the same number of equivalents. Bender obtained
the following moduli at 18C in -- - . units :
NH 4
i/n
... 410
K
... 296
^Cd
... 606
Na
... 235
iCu
... 413
Li
... 720
Ag
... 1069
iBa
... 739
Cl
4Sr-
... 522
Br
... 370
24 ASSOCIATION THEORY OF SOLUTION
iCa ... 282 I ... 733
iMg ... 221 NO 3 ... 160
iSO 4 ... 200
Thus the specific gravity of sodium nitrate containing
3 equivalents in a litre of solution may be calculated :
Sp. gr. of NH 4 C1 Solution (3N) T0451
Na ... 00235
NO 3 ... 0-0160
0-0395x3 ... =0-1185
Sp. gr. of SNNaNOs ... =11036
Groshams investigated on the subject on a more
rational method by regarding the phenomena on a
molecular basis. He found that the difference between
the molecular volume of a salt-solution decreases as the
quantity of water increases in a decreasing rate. He
proposed a formula,
where d~ specific gravity
vvr:= quantity of water referred to unit quantity
of the salt,
a and (3 = Constants.
All these however may be considered useless in solving the
theory of solution as it has not been set forth clearly how
a,nd on which stoichiometrical relationship they are based.
Figures are also compared inspite of their obtaining under
SPECIFIC GRAVITY OF SOLUTIONS 25
dissimilar conditions j gram equivalents of salts are taken
and made up to one litre with varying quantities of water.
The following table is prepared from their figures for
normal solutions :
c , , . water per
substance. Sp.gr. grams or salts lit-,
per litre. ------- '~7~r
wt. Mol.
KC1 T0444 75-5 924'5 51-3
NH 4 C1 1-0157 53-5 946-5 53-6
It is thus very clear how absurd it is to compare data
obtained by mixing one gram-molecule of ammonium
chloride with 52'6 gram molecules of water with those
obtained by mixing one gram molecule of potassium
chloride with 5 1*3 gram molecules of water. The apparent
agreement brought forward is entirely due to the taking
of figures in unreasonably round numbers.
Walker 7 states "there is an undoubted regularity in
the density of aqueous salt solutions. If we consider,
for example, the density of normal solutions of a number
of salts, we find that the difference in density between a
chloride and a corresponding bromide is constant ; that
the difference between a chloride and the corresponding
sulphate is constant ; in short, that the difference between
corresponding salts of two acids is approximately constant,
no matter what the base is with which the acids are
combined On (he other hand we find that the difference
in the densities of equivalent solutions corresponding
salts of two bases are always the same and independent
of the acid with which they are united. Examples are
26
ASSOCIATION THEORY OF SOLUTION
given in the following table, where the densities are tho^e
of normal s
olutions :
01
Br
I
iS0 4
N0 3
K
1-0444
1-0800
1-1135
1-0662
1-0591
NH 4
1-0157
1-0520
1-0847
1-0378
1-0307
Difference
00287
0-0280
0-0288
0-0284
0-0284
K
Nrf
NH 4
iSr
iBa
NO 3
1-0591
1-0540
1-0307
1-0811
1-1028
Cl
1-0444
1-0306
1-0157
1-0607
1-0887
Difference 0'0147 0*0144 0'0150 0'0144 0-0141
From a consideration of this table, it is evident that
we can obtain the density of normal solution of any salt
by adding to the density of a salt chosen as standard two
numbers, or moduli, one of which is characteristic of the
base and the other characteristic of the acidic portion of the
salt. This regularity is known as ' Valson* s Law of Moduli"
Text book writers attach full importance to Yalson
without considering the rationality of his statement.
Using the above specific gravity figures and specific gravi-
ties of salts given in Kaye and Laby's tables, 1921, let
us calculate and consider (1) solution, (2) volumes of salts
used for the purpose, and (3) contraction occured during
the process of solution, on the following principle :
wt. of 1000 cc. of ^ KC1 solution = 1044*40
KCL used for the purpose = 7456
Gram or c.c. of water* used 969'84 c.c.
* One gram of water taken to be one c.c. under the conditions of the
experiment.
SPECIFIC GRAVITY OF SOLUTIONS 2j
Gjam or c.c. of water used =-969'84 c.c,
Volume of KC1 used * = 37-4,7 cc.
-
Sum of the volumes of solvent and solute... = 1007'31 c.c.
Actual volume of solution , = 1000-00
Change in volume = 7 31 c c.
contraction.
(1, Difference in quantities or volumes of water present.
Cl. Br. I $S0 4 N0 3
K 969-84 960-98 947-5 979-07 95799
NH 4 962-20 954*04 939-7 971-73 950-65
Difference
7-64
6-94 7-8
7-34 7'34
K
Na
NH 4
Cl
96$34
98114
9( 2-20
NO 3
957-99
96899
950-65
Difference
11-35
1215
11-55
(2) Difference in the volumes of salts used.
Cl Br I JSO, NO*
K 37-47 43-11 54'61 32-80 48-14
NH 4 35-21 42-05 58*01 37'34 4655
Difference 2'26 1'06 -3'40 -450 1'59
K Na NH 4
NO 8 48-14 37-45 46'55
Cl 3747 26-94 35-21
Difference 10'67 10-51 1134
28 ASSOCIATION THEORY OF SOLUTION
(3) Difference in contractions in solutions.
Cl Br. I 1S0 4 NO,
K +7-31 -1-4-09 +2-11 +11*67 + 6'13
-200 -3'91 -2-29 - 9'07 -2'60
Difference
9-91 8-00
K
Cl
7-31
N0 3
(M3
Difference
1-18
4-40
Na
8-08 -4-60
6-44 -2-80
1-64 -0-20
These figures do not produce any similarity that may
induce or support Val son's Law, on the contrary, they
prove that for such calculations, disagreement is easily
detectable if fourth place of decimals are considered. There
does not seem to exist much justification in considering
the figures in round numbers up to 3rd place of decimals
where differences definitely perceptible on consideration
of figures up to tth place of decimals lead to some
valuable conclusions.
Some of these densities of salt solutions have
been wrongly applied the in light of the electrolytic
dissocation of them in water by Nernst. He has
introduced an irrational formula to represent the change
of volume :
M+JW_ W
v _ __
SPECIFIC GRAVITY OF SOLUTIONS 29
Where V = Change in volume in solution.
S = Density of solution,
W = Weight of water.
M = Molecular weight of the salt in grams.
So = Density of the pure water at the same
temperature.
This equation is erroneous in the sense that it dees not
make any mention about the effects of dilution, since
contraction varies quite widely when their solutions are
equally diluted and on account of excluding the volumes
of the salts used, which are neither the same for all salts,
nor it is reasonable to presume that they occupy no
volume in state of solution. The equation should be
modified by the introduction of the volume of the salt
used, as follows :
IVJ + W W M^
v -*g So S((i
where, S m = the specific gravity of the salt. The changes
in volume as recorded by Nernst would be then,
I Diff. II Diff. Diff. I-II
KC1-7-31 . Nad 8-08 < -077
KBrr=4'09 NaBr = 6'49 -2-4.0
KI =211 198 Nal -5-92 ' 57 -3*81
Conclusions drawn from density figures to harmonise with
dissociation theory of solution are thus exploded. These
figures, however, fully support the association theory of
solution, all substances coming in solution combines with
the solvent in ratio entirely dependent on dilution, forming
30 ASSOCIATION THEORY OF SOLUTION
molecules, which differ in property from any one of those
of the original solute or solvent, and from any of those
associated ones that may be formed at any other dilutions.
The properties of the resultant associated molecules will
differ from those of the average of the components.
(1) Young and Forty, -lour. Chem. Soc., 1902, 730.
(2) Washburn and Mac Innes, Jour. Amer. Chem.
Soc., 33, 1686.
('I) Ostwald, "Solutions" Translation. By Muir,
1891, 249.
(4) Valson, Corapt. Rend., 1874, 73, 44].
(5) Bender, Wied. Ann., 1883, 20, 560.
((3) Groshams, Wied. Ann., 1883, 20, 492,
(7) Walker, Introduction to physical chemistry, 10th
Edition. 1927. Page 183.
(8) Nernst, Theoretical chemistry. Translation from
eighth-tenth German Edition, By Cod. 1923, 453 :
Traube, Zeit anorg. Chem., 1893, 3. 1.
CHAPTER III
CONTRACTIONS IN SOLUTION
The subject whether any change in volume occurs when
a substance dissolves in a solvent has been under inves-
tigation from very early days. P. Gassend, A. Nollet,
and M. Euller observed that salts dissolve in water
without change in volume, but R. Watson seems to be
the first to notice that a change in volume also takes
place in some such cases 1 . Investigation on the subject
was taken up by Dalton* with considerable seriousness.
He found that contraction does occur in solution and
confirmed the results of some of his earlier investigators,
that the solid matters like the carbonates, the sulphates,
the nitrates, the^chlorides, the phosphates, the arsenates,
the oxalates, the citrates, the acetates,, etc., etc., add to
the weight and the water adds to the bulk.
In those early days accurate methods of determination
of volume changes in solution were neither known nor
need for them much felt. When this matter was again
taken up by a few other later investigators L it was
found by Holker, Playfair, Joule, and Marignac, 1 that a
certain amount of contraction does take place when
substances pass into solution, and that the degree of
contraction varies with different salts, and in some cases
such as calcium hydrate, the volume of the solution is
less than that of the water present in the solution.
32 ASSOCIATION THEORY OF SOLUTION
Some study on the effect of dilution on the con-
traction which occurs during the process of solution
was made by Michel and Krafft and Kremers. They
noticed that contraction in solution of different substances
is differently influenced by change of dilution. Their
results, however, were not accurate and did not give
conclusions which could stand the test of modern accurate
methods. It would be interesting to compare the fol-
lowing remarks with the results that will be tabulated
hereafter. Wanklyn, Johnston and Cooper, 8 noted "the
venerable Dal ton made the great discovery, about the
year 1840, that contraction occurs when salts dissolve
in water. This is strictly exemplified by lime water,
which occupies less space than the water which it
contains. On the other hand there are cases where the
volume of the solution of a mineral salt is almost as
great as the sum of the volume of the salt plus the
volume of the water in the solution. (The solution of
nitrate of silver is a case in point). There are even
instances where expansion takes place. This is exemplified
by some ammoniacal salts, where the volume of the
solution has been found to exceed the sum of the volumes
of water and dry salt." These three joint authors, 4 relying
on their inaccurate data remarked "Dalton also experimen-
ted upon sugar, but failed to notice that in that case there
is no contraction ; and it has been reserved for ourselves,
after a lapse of half a century, to record that there are
cases such as sugar where there is rigid maintenance of
volume."
Lumsden 5 found that organic compounds suffer change
CONTRACTIONS IN SOLUTION 33
ii\ volume in passing into solution into hydrocarbons.
But his assumption, that the change in volume which
occurs on solution to depend only on the solute, is not
reasonable. He found that the molecular solution volumes
of solutes did not show any sudden change on passing
through the melting point or boiling point of the solute
which fact obviously leads to the conclusion that combina-
tion of solvent and solute taking place the original pro-
perties of both get altered. He has also found that change
of volume in solution depends not only on the solute, but
also on the solvent, and to a certain extent also on the
concentration ; this subject, however, has been more
thoroughly investigated afterwards. 6
Contractions in solutions of halides of alkaline metals
at certain conditions have been determined by Baxter and
Wallace, 7 who tried to explain the phenomena by Richard's
hypothesis of eojppressible atoms and by the assumption
of hydration in solution. Their following figures are
interesting because they have determined them with
considerable accuracy, where C = change in volume during
solution at 25 in c.c. per gram-molecule,
c
C
C
LiCl = - 2-03
LiBr
= +0-16
Lil =+3'40
NaCl - - 8-48
NaBr
= -6'94
Nal= -4-50
KC1 =-8-71
KBr
= -7'72
KI =-6-31
RbCI=-9'19
RbBr
- -8'70
Rbl = -7'86
CsCl = -1-09
CsBr
= - O'OO
Csl = + 177
These investigators, however, did not properly consider
the effects of dilution and variation of temperature.
3
34 ASSOCIATION THEORY OF SOLUTION
Besides they considered that during solution changes in the
molecular volumes of the alkali halides take place, and did
not much consider what happens to the molecules of the
solvent in the state of solution. There are also a few
other 8 investigators who determined changes in volume in
solution, but their results are not quite free from experi-
mental error and the range of variation is not wide enough
to draw much attention. Favre and Valson thought that
the change in volume during solution is the combined
result of two opposing influences, (1) the contraction of
the solvent under the influence of the solute, and (ii) an
increase in the volume of the salt owing to its dissociation
into its components.
The majority of the scientists, however, have attributed
contraction in solution to be due to hydration of either of
the salt molecules or of their ions. Tammann 9 argued
that since the solutions behave in approximately the same
manner when subjected to temperature and pressure, as
the same bulk of water, at a higher pressure, there must
be a compression of the water by the solute owing to an
increase of internal pressure which he calls the Binnendntck.
Attempts have been made by some scientists to use
this phenomenon of change of volume in generalising and
correlating with other properties of solution. In the
previous chapter it has already been mentioned about
Valson's 10 remark that the differences between the specific
gravities of solutions containing one gram-equivalent of
various salts per litre of two specific metals with same
acid radical are nearly identical and therefore independent
of the nature of the acid, and conversely the differences
CONTRACTIONS IN SOLUTION 35
l)e.tween the specific gravities of solutions of various salts
of two specified acids with same metal are equal and
therefore independent of the nature of the metal. He
found that this law is true for dilute aqueous solutions
of salts and he explained all irregularities by the
assumption of volume changes during the formation of
salts in solution. Bender 11 tried to extend this law to
concentrated solutions. Observations of these investi-
gators are true only empirically, and there is no reason
why these results should be so. Some ordinary text-book
writers 1 * paid considerable attention to this Yalson's law,
which, however, does not appear to be rational. This sub-
ject has been critically dealt with in the previous chapter.
Traube 13 found that when a gram-molecule of a non-
associated liquid is dissolved in water to form a dilute
solution, a contraction amounting always to 12' 2 c.c. occurs
on account of an jjbtractive force exerted by the solute on
the solvent. Those substances which when prepared into
a dilute solution in water are accompanied by a contrac-
tion in volume less than that of the above 12*2 cc. indicate
to be associated substances according to Ramsay and
Shield's method. Traube proposed that the diminution
in contraction is proportional to the degree of association
of the substance ; and on this assumption it is possible to
ascertain the degree of association by observing its
molecular contraction in aqueous solution. Thus,
Degree of Associations! r L
& molecular contraction in dilute aqueous solution.
Traube worked out a large number of figures by this
method and also by another one by using Kopp's method
36 ASSOCIATION THEORY OF SOLUTION
of finding out molecular volume of a liquid by the sum
of atomic volumes of the elements and radicals composing
the molecules. These results are, however, not important
in disentangling the nature of the relationship between
solvent and solute in the state of solution, because molecu-
lar contractions, as will be seen hereafter, are not constant
for the same solute and solvent at all dilutions and tempe-
ratures. In a few particular cases, under particular
conditions, Traube found that when some substances pass
into solution in water molecular contractions are approxi-
mately the same and calculations made on these limited
figures are hardly useful because the molecular contraction
is not the same if dilution or temperature is altered.
Before proceeding further, it would be better to-
discuss the conception of volume in this connection. The
volume of a substance, solid, liquid or gas, is really the
space occupied by it under a specific condition at which it
exists. An idea is to be formed if this volume indicates
any measure, or bears any relationship with the volume of
the molecules composing the mass. The volume of a
solid body may be imagined as a heap of molecules like
cubes or spheres having spaces or gaps between them. It
may have been noticed by many that if a piece of brick
which appeared to be a solid compact mass originally,
when placed under the wheels of a running railway train
is smashed and a small part is left behind on the railway
line with a more compact and denser composition than-
it had before. A similar phenomenon is also observed if
any such piece of solid mass is hammered on the anviL
Thus even a solid mass may assume a lesser volume when
CONTRACTIONS IN SOLUTION 37
subjected to a suitable pressure. It is also well known in
the cases of liquids and gases that the volume is dependent
on the pressure. Consequent!}' it remains to be considered
in this line of argument to what extent it is reasonable to
think that the volume of a mass is an index of or
proportional to the volume of its composing molecules. If
the Kinetic theory is admitted the volume of a body is
really a measure of the force with which the molecules
composing the body is capable of bombarding the sides of
the vessel in which it is kept.
Molecular volume or molar volume, as it is called now,
is the volume occupied by a gram-molecule of a substance,
And the figures for molar volumes are often compared as
if they have something to do with the volumes of the
actual molecules composing the body. Such measure-
ments, however, do not seem to have much relationship
with the actual volumes of the molecules. Molecular
volume represents the pressure or the force with which
the walls of the vessel containing the body is bombarded
by its molecules present in a gram-molecule. Consequently,
when a change in molecular volume is observed as a
result of solution of a solute in a solvent it does not
necessarily mean that the resultant associated or disso-
ciated molecules, whatever they may be in solution, are
correspondingly suffered in volume. It seems probable
that the average space between the molecules are affected
as a result of solution. This change in volume by solution
may only mean that the bombarding capacity is changed
without any reference to the volume of the particles
themselves in their new state of solution. It is not
38 ASSOCIATION THEORY OF SOLUTION
unreasonable that the volumes of the individual associated
or dissociated molecules and the average space between them*
are simultaneously changed with the formation of solution
but the changed volume of a body, as will be shown below,
will probably have very little to do with volumes of
the molecules. The change in volume when a substance
is dissolved in a solvent would mean that the mass and
the Kinetic movements of resultant molecules are different
from those of the averages of the pure components.
Contraction in solution has been a subject of experi-
mental study by the author 14 basing on the determination
of specific gravities of solute, solvent and solution. Thfr
volume relationship may be expressed by the following
formula
where,
C Contraction occured during solution of a gram-
molecule; when this figure is negative it
means expansion.
Mv Molecular volume of the solute
Molecular weight.
Specific gravity
Sv Volume of the solvent present in the solution-
containing one gram-molecule of solute.
S'v=s Volume of the solution containing one gram-
molecule of the solute.
By knowing the specific gravities of solute, solvent
and solution of known strength these figures may be
easily obtained for the purpose of determination of
molecular contractions in solution.
CONTRACTIONS IN SOLUTION 39
The following figures are interesting :
(1) Molecular contraction increases with concentration.
Calcium oxide in water.
CaO. Sp. gr, = 308
% w/v sp. gr. Contraction per
at 20/20C gram- molecule
in c.e.
0-0133 1-00003 -249
0-0899 1-00025 - 2-7
0-0665 1-00072 + 22-8
01064 1-00142 +36-9
0-1330 1-00200 +46-4
Calcium hydroxide in water.
Ca(OH; 2 Sp. gr. = 2-078
0017G A 1-00003 -25*8
0-0528 ' ' 1-00025 - 3*3
00880 1-00072 +22*2
0-1408 1-00140 -4-35-2
0-1760 . 1-00200 +45-7
Carbon disulphide in benzene.
0-87250
1 087500 -9-3
5 088920 -35
10 0-90715 -27
50 105780 -10
80 T17610 -03D
100 1-25770
40 ASSOCIATION THEORY OF SOLUTION
Acetone in benzene.
0-87250
1
0-87102
-5-7
5
0-86730
-2-0
10
0-86350
-0-8
50
0*83220
-0-08
80
0-80855
-0-03
100
0-79290
Methyl alcohol in benzene.
0-87250
1 0-87100 -3*1
5 0-86762 -0-75
10 0-86372 -0-32
50 0*83210 -0'04
80 0-80785 -003
100 079105
Ethyl alcohol in benzene.
0-87250
1 0-87075 -6-5
5 0-86682 -2-0
10 0'86336 -0'66
50 0-83542 +0'48
80 0-81455 + 0-54
100 0-79100
CONTRACTIONS IN SOLUTION 41
Iso-propyl alcohol in benzene.
0-87250
1 0-87010 -13-6
5 0-86661 - 29
10 0-86145 - 2'3
50 0-82776 - 1-7
80 0-80495 - 0-08
1OO 0-78900
Acetic acid in benzene.
0-87250
1 0-87230 -13-1
5 0-87752 - 5-1
10 0-88448 - 3'8
50 ^ 0-94960 - 1-6
80 1-00810 - 0-06
100 1-05090
Acetic anlydride in benzene.
0-87250
1 0-87288 -7-3
5 0-88056 -17
10 089010 -1-1
50 0-96816 -1-0
80 103070 -0-13
100 1-07318
42 ASSOCIATION THEORY OF SOLUTION
Chloroform in benzene.
0-87250 -
5 0-90200 -0-6
10 0-93178 -0-4
50 1-17044 -0-08
80 134922 -0-09
100 1-46935
Nitrobenzene in benzene.
087250
1 087358 -25-4
5 0-88675 - 4'6
10 090350 - 1-6
50 103520 + 0'18
100 1-19654
Narcotine in benzene.
087250
1 087350 -113-2
2 0-87660 - 79-0
3 0-88048 - 56-5
4 088340 - 54-3
5 0-88762 - 44*0
100 1386fi
CONTRACTIONS IN SOLUTION 43
(2) Contraction decreases with concentration.
Calcium chloride in water.
CaCl,. sp. gr. = 2-26.
1 1-00805 +27-5
5 1-04000 4-26*9
10 1-07905 4-25-8
30 1-22540 4-21-5
Sodium iodate in water.
NaIO 3 . sp. gr.=s3-56.
1
1-00825
4 20-86
5
1-03990
4-1554
10
1-08970
4- 3-50
Silver nitrate in water.
AgNO 3 . sp. gr.4-35
1 1-00850 4-13-5
5 1-04140 4- 9-7
10 1-08255 4- 9-3
20 1-16308 4- 8*0
44 ASSOCIATION THEORY OF SOLUTION
Toluene in benzene.
%V/v
087250
1
0-87280
+ 50
5
0-87270
+ 1'9
10
087230
+ 1-2
50
0-86690
+ 1-2
80
0-86320
+ 005
100
0-86090
+ 0-003
Ethyl ether in Benzene.
0-87250
1 0-87150 +6-6
5 086635 +6-2
10 0-85890 +2-3
50 0-79930 +T1
80 0-75115 048
100 0-71670
V 3) Contraction increases, reaches maximum and then
decreases with increasing concentration.
Barium oxide in water.
BaO. sp. gr.5-10.
% w/v
0-3875
100428
+45'9
1-1625
101325
+ 51-4
1-9375
1*02165
+ 47-9
31000
1-03445
+ 470
CONTRACTIONS IN SOLUTION 45
Potassium Bromate in water.
KBrO 3 . sp. gr. 3-271.
1 1-00725 +5'1
2 1-01465 +6-5
3 1-02200 +6-5
4 1-02925 +6-2
5 1-0364-0 +5-6
(4) Contraction decreases, reaches minimum and then
increases with increased concentration.
Ethyl acetate in benzene.
% v/v
0-87250
1 0-87244 -258
5 087200 -29-2
10 0-87255 -17-1
50 ^ 0-87882 -13-4.
80 ""' 0-88438 - 0'6-
100 0-88820 -
(5) Contraction in solution varies with solvent.
Water in Methyl alcohol.
% w/w sp. gr. at 0/4C
(By Ditmar and Fawsitt)
0-8102
5 0-8240 +6-4
10 0-8375 +1-8
50 0-9287 4-l'0
80 0-9723 +0'28
90 0'984:J
ASSOCIATION THEORY OF SOLUTION
Water in Ethyl alcohol.
sp. gr. at 15-5 /15-5C.
(By Hehner)
0-7938
5 08089 4-24
10 0-8228 4-1-9
50 0-9182 4-1-0
80 0-9716 4-0-36
90 0-9841 4-1-1
Water in acetone.
% w/w sp. gr. at 15/15C.
(by Squibb).
0-7966
5 0-8113
10 0-8260
50 0-9247
80 0-9755
-4-2-0
4-20
4-1-2
4-0-46'
Water in Glycerine,
sp. gr. at 12-14/12-14*C.
(by Lenz).
1-2691
10 1-2425
50 1-1320
80 1-0498
90 1-0245
4-0-044
-0-072
-0-071
0-038
CONTRACTIONS IN SOLUTION 47
Water in acetic acid.
sp. gr. at 15/4 C-
(By Oudemans)
10558
10
1-0713
+ 3-8
50
1*0615
+ 1-2
80
1-0284
+ 0-37
90
1-0142
+ 0*16
Water in nitric acid.
sp. gr. at 15/4 C C.
(By Lunge and Hay).
1-5204
39'63
1-3754
4-1-82
68-32
T1953
+ 0-53
8184
1-1065
4-0-17
90-15
1-0554
+ 0-25
Water in sulphuric acid.
sp. gr. at 15%C.
By Lunge and Ray.
1-8357 *
10 1-8198 46-6
50 1*3990 -037
80 1-1424 -0-30
90 1-0681 -017
48 ASSOCIATION THEORY OF SOLUTION
(6) Contraction in solution varies with temperature.
In these determinations densities of water at different
temperatures were taken as follows :
Temper-
ature. 10C. 20C. 30C. 40C. 50'C. 60'C.
Density
of
water. TOOlcSO I'OOOOO 0'99784 0'99489 G'99118 0'98650,
Potassium Chlorate.
KClOs. Sp. Gr. = 2-307
1
2
3
4
5
20720C.
Specific Gravity.
1-00730
1 01374
102010
1-02B70
103310
+ 19-4
+ 1-V6
4-H'O
+ 103
4- 91
1
2
3
4
5
3<T/20 e C.
1-00450
1-01110
1-01742
102394
1*03032
4-14-1
+ 11-3
4-10-1
+ 9'4
CONTRACTIONS IN SOLUTION
49
1
2
3
4
5
1-00152
1-00782
1-01412
1-02046
1-02682
4-10-3
4- 8-1
4 80
4 6-9
4 6'3
2
3
4
5
50'/20 3 C.
1-00416
4 7-9
1-00992
4 5-8
1-01650
4-63
1-02260
4 5-0
% w/v.
1
5
10
Sodium Carbonate.
Na 3 CO a . Sp. gr. = 2-500.
10720C.
1-01245
1-05308
1-10500
443-8
4*5-4
1
5
10
30
1-01030
1-05038
1-10100
1-81800
444-9
439*6
436-8
50 ASSOCIATION THEORY OF SOLUTION
30/20C.
I V00820 +42-7
5 1-04718 4-37-0
10 1-09718 H-35'6
30 1-31325 +27-6
40720C.
1
1-00455
+ 39-4
5
1-04320
+ 36-4
10
1-09274
+ 34-2
30
1-30708
+ 27*0
50V20*C,
1
1-00062
+ 332
5
1-03880
+ 34-7
10
l-0877cS
+ 33-1
30
1-30125
4-26-4
60720'C.
5
1-03500
4-371
10
1-08350
4-334
30
1-29632
4-26-6
CONTRACTIONS IN SOLUTION
Sodium Sulphate.
Na 2 SO 4 . Sp. gr. = 2:670.
% w/
1
5
10
I
5
10
5
10
30
1
5
10
30
1
5
10
30
1-01140
4-52-0
1-04880
+ 40-(>
1-09840
4-39-9
20'/20*C.
1-0090
+ 50*8
1-04672
+ 3^-8
1-09350
+ 35'6
30/20C.
1-04350
+ 37-2
1-09018
+ 33*2
T29920
+ 27'0
40/20C.
1-00372
4-33-5
1-03940
+ 33-8
1-08578
4-32-2
1-29340
426-0
50'/20 9 0.
09980
+ 33*2
1-03523
431-7
1-08080
430-8
1-28780
425*3
ASSOCIATION THEORY OF SOLUTION
1
0-99525
4-30-7
5
1-03058
+ 33-8
10
1-07550
+ 30-4
30
1-28220
4- 25'3
Calcium Sulphate (hydrated).
CaSO 4 .2H 2 O. Sp. gr. 2'306.
10/20C.
w/vv
0*1
0-2
1-00288
1-00330
4299-0
4153-5
20/20C.
o-i
0-2
1-00125
1'00210
4 44-2
4 7-8
30'/20C.
01
0-2
0-99860
0'99920
4 78
+ 3-3
o-i
0-2
0-99530
0-99600
- 4-9
- o-i
50'/20'C.
0-1
0-99160
- 4-9
/o w/w
20-31
CONTRACTIONS IN SOLUTION
Manganese Sulphate.
MnSO 4 , Sp. gr.- 3-100.
60/20C.
1-20136
4- 32-3'
Manganese Sulphate. (Hydrated).
MnS0 4> 4H 2 O.=* 2-388.
10720'C.
1
1-00855
+ 16-0
5
1-03480
4-13-7
10
1-06955
4- 7-3
30
1-21970
4- 3-2
20/20C.
1
1-00700
4-25-4
5
1-03300
+ 12-9
10
1-06738
4-11-4
30
121700
4- 30
30720'C,
1
1-00474
4-26-8
5
1-03050
4-11-9
10
1-06470
4-107
30
1-21378
4- 2'7
54 ASSOCIATION THEORY OF SOLUTION
40/20C.
i i-ooiso -firo
5 1-02716 +10-4
10 1-06085 + 8-5
30 1-20990 -f 1-8
50/20'C.
1 0-99740 + 6 2
5 1-02250 + 6-1
10 1-05690 + 8-1
30 1-20560 + 1'G
60V20*C.
^0 1-20136 4- 2-2
Ferrous Sulphate (hydrated).
*, 7H 2 O. Sp. gr. = 1-899.
10/20C.
o/o w/w
1
1-00780
+ 343
5
1-02980
+ 19'8
10
1-05900
+ 20-4
20
1-11880
+ 16-3
20/20C.
1
1-00600
+ 35-1
5
1-02772
+ 19-6
10
105620
+ 18-0
20
1-11540
+ 14-8
CONTRACTIONS IN SOLUTION
55
1
5
10
20
1
5
10
20
5
10
20
5
10
20
o w/vv
1
10
30
50
30'/20C.
1-00330
4-19-7
1-02375
+ 11-7
1-05200
-f-13'3
1 11110
+ 11-7
40'/20C.
0-99980
4- 5-3
1-02015
4- 4-5
1-04828
+ 10-2
1-10720
-f 10-1
1-01620
4 3-8
1-04400
4- 8-9
1-10300
4- 9-7
60V20HX
1-01160
4- 4-3
1-03936
4- 8-9
1-09820
4- 9-4
Cadmium Sulphate (bydrated)
CdS0 4 , 8/3H 2 0. Sp. gr. =
10*/20 3 C.
1-01005
1-03600
1-29120
^ 1-56300
-4-37-4
4-25-7
4-18-4
4-12-0
56 ASSOCIATION THEORY OF SOLUTION
1 1*00830 +37-8
10 1-08354 +24-3
30 1-28770 4-17-6
50 1-3G444 4-11-7
30/20C.
1 1-00550 H-24'4
10 1-08088 4-243
30 1-28458 4-17-5
40/20C.
1-00282
-4-24-3
1-07718
4-21-9
1-28000
H-18'l
1*55588
+ 116
1
10
30
50
50/20C.
1 0-99912 4-24-1
10 1-07300 +21-0
30 1-27588 -4-16-3
50 1-55110 4-111
60Y20C.
1 0*99430 4-29-0'
10 1-06830 4-21-9
30 1-27086 4-16-3*
50 1-54628 +11-4
CONTRACTIONS IN SOLUTION 57
Ethyl Alcohol
C 2 H 6 O. Sp. gr. at 20/20 C = 079350
20/20C.
w/\v
1 0-99845 -f4\S
10 0-98460 +4-8
25 096540 +5-3
50 0-92130 +4'1
70 0-87850 +2'9
90 0-83140 +17
30/20C.
99586
+ 3-0
0-98172
+ 34
0-96034
+ 4-8
0-91414
+ 3'4
0-87070
+ 2-2
0-82480
+ 1-1
1
10
25
50
70
90
40/20~C.
1 099248 -0-5
10 0-97800 +2-9
25 0-95494 +3'9
50 0-90660 +27
70 086185 +1-5
90 0-81592
5 8 ASSOCIATION THEORY OF SOLUTION
50'/2(TC.
1 0-98866 -0-6
10 0-97372 +3-1
25 094852 +32
50 0-89900 +2-0
70 0-85418 -f-0'9
90 0-80882 -0-1
60V20C.
1 0-98440 - 0-4
10 0-96752 +2'1
25 0-94255 4-26
50 089066 +1-3
70 084500 +0'2
90 0-79780 -0-9
Some of the associations of solvent and solute permit
isolation in a free solid state and amongst them hydrates
of salts are very common. The following contractions,
resulted by the combination of water with salt to form
solid hydrates and concentrated solutions, are instructive
to show that the molecular contractions in the solid
hydrated crystals increase with hydration with diminishing
rate of increase, which, however, are not always quite
continuous up to solution. Or, in other words molecular
contraction of a particular compound of solute and solvent
is dependent on the particular ratio of the constituents
and is not so much related to or bear necessarily any
CONTRACTIONS IN SOLUTION
59
similarity with any such compound of similar composition $
this is strikingly illustrated in the cases with manganese
sulphate and sodium acetate, which, in contradistinction
with others, produce larger contractions in solid hydrates
than those in concentrated solutions.
Substance.
Sp. gr.
Mol. Vol.
Mol.
Contraction.
CusO 4
3-606
44-4
, H 2
3-2289
54-3
8'1
, 2H 2 O
2-953
67-0
13'4
, 3H 2
2-663
80-0
18-4
, 4H 2 O
2-645
83-9
23-5
, 5H 2 O
2-284
109-1
25-3
,40H 2 O
' 1-20809
35-6
,48H 2
1-1855
40-1
MgSO 4
2-709
448
, H 2
2-445
55-6
7-2
, 2H 2
2-373
67-0
13-8
, 5H 2 O
1-869
112-4
22-4
, 6H 2 O
1-751
130-8
22-0
, 7H 2
1-676
146-6
24*2
,48H 2 O
1-13026
34-2
,64H 2 O
1-0017
43'3
60 ASSOCIATION THEORY
OF SOLUTION
ZnSO 4
3-624
45-6
, H 2 O
3-280
54-7
8'9
, 2H 2 O
3'958
66-6
15-0
, 5H 2 O
2-208
113-7
21-9
, 6H 2
2-072
130-2
23-4
, 7H 2 O
1-965
146-8
23-8
,17-7H 2 O
1-4740
31-3
,20H 2 O
1-40175
32-6
NiSO 4
3-418
44-6
, H 2 O
65-5
6-1
, 6H 2 O
2-931
129-9
23-6
, 7H 2 O
1-949
1446
26-0
, GOH 2 O
1-14068
42-3
,100H 2 O
1-08603
449
CoSO 4
3-444
44-7
, H 2 O
3-125
55-2
7-5
, 2H 2 O
2-712
70-9
9-8
, 3H 2 O
2-327
97-4
19-a
, 5H 2 O
2-134
114-6
20-1
, 6H 2 O
2019
130-1
22-6
, 7H 2 O
1-918
146-0
24'T
, 60H 2 O
1-13989
41-4
,100H 2 O
1-08554
47-5
CONTRACTIONS IN SOLUTION
6l
PeSO 4
3-346 44*5
, H 2
3-047 56'2
6-3
, 2H 2 O
2773 67-7
12*8
, 4H 2 O
2-227 100-5
160
, 7H 2 O
1-900 145-7
23-8
, 80H 2 O
1-10597
49-8
,100H 2 O
1-10937
60'8
MnSO,
3-282 45-0
, H 2
2-845 55'7
7'3
, 2H 2 O
2-526 73-6
7-4
, SHoO
2-356 86 6
12-4
, 4H 2
2*261 98-0
18-8
, 5H 2 O
2103 114-4
20-6
, 32H 2 O
T16917
0-8
, 42H 2 O
^1-13615
-0-07
NaO 2 H 3 C 2
1-3970 58-69
, 3H 2 O
1-4442 94-32
18-4
, 6-4H 2 O
(Supersaturated solution)
14-6
, 7-OH 2
(Supersaturated solution)
16'3
, 10-2H 2 O
1-16822
17-6
Ostwald 15 noted that the break in the solubility curve
of sodium sulphate at 32-33C is exclusively due to the
change of the solid phase in solution at this temperature.
But he also experimented with saturated solution of this
salt in a dilatometer and observed no break at this tempera-
ture. Water of hydrated sodium sulphate is differently
62 ASSOCIATION THEORY OF SOLUTION
bound vvifch the salt in the solid state and in the state o
aqueous solution. In the solid state the force of union is
overbalanced at 32-33C and it has been shown that in
solution it does not do so even at a higher temperature
than this. Ordinary text book writers often erroneously
considered that at the temperature of the break in the
solubility curves of salts, whose solid hydrates lose water
of hydration at transition temperatures (below 100C)
when heated in the solid state would part with the same
amount of water at higher temperature even in solution.
The break in the solubility curve takes place on account
of the fact that the associations of solvent and solute at
that ratio is unstable at that temperature. The relationship
between water and salt gets altered as soon as the hydrated
solid substance is dissolved in water.
The relationship of water with sodium sulphate, sodium
carbonate and sodium acetate in salid hydrated crystals
and in solutions have been very conclusively established
by dilatometric experiments. -The changes in volume,
when they were heated above their respective transition
temperatures, in saturated solution, in ordinary solution
or in dilute solution, are nominal and negligible. If,
however the hydrates of these salts behave likewise in
solution or in supersaturated solution as they do in the
solid state there would have been corresponding changes
in volume owing to the separation of water molecules
from the hydrated salts.
Molecular contractions in aqueous solutions of varying
dilutions of sodium sulphate and sodium carbonate are
CONTRACTIONS IN SOLUTION 63
given below to show that the state of combination is quite
stable up to temperatures much above those of their
transition temperatures of the solid hydrated salts. These
determinations were made by observing the expansion of
fixed weights of solutions by means of a dilatometer
composed of a 100 cc. flask fitted with a ground hollow
glass stopper with a graduated glass tube of accurately
measured bore. The flask was placed in the waterbath
fitted with electric stirrer. The temperature of the bath
was raised at the rate of 1-C per minute, approximately,
to the boiling point of water, and when the solution in
the dilatometer reached constant volume it was eooled
down at the same rate. The volumes given in the following
tables are the means of two readings at the same tempera-
ture of rising and falling :
^Na 2 S0 4 , 50H 2 0.
Solution = 107-5500 gms.
Temp. Total Volume Mol. Contraction
C c.c. c,c.
15 95-26 30-3
30 9570 29*1
40 96-11 28'5
50 96-56 280
60 97-05 27'8
70 97-65 27-2
80 98-23 27-4
90 98*77 28-5
64 ASSOCIATION THEORY OF SOLUTION
Na 2 SO 4 , 100H 2 O.
Solution = 102-3130, gms.
15 95-76 32-7
30 96-18 31
40 96-58 30'1
50 97-01 29-4
60 97'49 29-5
70 98-11 28-1
80 98-58 30'3
90 9918 31'1
Na 2 SO 4 , 150H 2 O.
Solution = 99-6840 gms.
15 95-24 35*9
30 95-60 34-8
40 95'99 33-4
50 96-40 33-2
60 96-88 33'0
70 97-37 31-5
80 98-01 32-4
90 98-62 34-3
Na 2 SO 4 , 200BUO.
Solution = 98-8155 gms,
15 95-31 43-91
30 95-66 42-8
40 96-00 42 8
CONTRACTIONS IN SOLUTION 6$
Temp. Total Volume Mol. Contraction
c.c. c.c.
50 96-49 39 6
60 96-91 37-8
70 97-38 44-0
80 9798 44-0
90 98-56 47'0
Na 2 C0 3 , 50H 2 0.
Solution -105-9430 gms.
15
30
40
50
60
70
90
95-15
39-2
9575
36-5
96-20
345
96-62
35-3
97-01
36-1
97-51
36-4
98-61
37-9
Solution = 100-8242 gms.
15 9550 42-4
30 95-90 39-9
40 96-05 39'0
50 96-49 38-a
60 97-04 36-9
70 97-30 42-3.
90 98-42 44-6
66 ASSOCIATION THEORY OF SOLUTION
NasCOg, 150H 2 O.
Solution - 99-2494 gms.
15
95-50
42'4
30
95-90
399
40
96-28
38-8
50
9672
37-8
()()
97-18
38-1
70
9773
37-8
SO
98'32
37-9
DO
!)8'82
42-4
Na 2 CO 3 , 200H 2 O.
Solution = 98'0740 gms.
15 9518 45-0
30 95-53 42'8
40 95-89 43-1
50 96-32 42-1
60 96-81 41-5
70 1)7-33 42*1
80 97-92 42-4
90 98-60 46-1
It is seen in these figures that although the molecular
contractions are not widely changed with increase of
temperature still, each solution has a minimum. If mole-
cular contraction is an index of the force with which the
associated molecules bombard the sides of the wall of its
CONTRACTION IN SOLUTIONS 67
container then it would conclude that its variation in
composition is attended with corresponding some other
intramolecular change.
Large number of experiments have been done by
different investigators with aqueous solutions of substances
to find out to what extent the solutes keep up the solvent
in state of combination, They may be discussed in the
following way :
(1) Armstrong and his colaborators determined hydra-
tion by the precipitation of solutes from solution by the
introduction of another solute, by the change of velocity
of reactions such as inversion of cane sugar, and by the
hydrolysis of methyl acetate by dilute acids with or without
addition of salts. These methods presume that the mole-
cules of the two solutes or their molecular associations
with the solvent do not react with each other in any way
.but only remain thefe as physical mixtures ; the position
as regards how the two solutes remain in solution does not
seem to be quite clear now and more statements on them
are needed. In any case the hydration figures obtained by
these methods indicate more what happend when two
solutes are introduced in a solvent than what is the case
with a solution of any one of them. It has been argued
that on the introduction of calcium chloride in a solution
of sucrose, the hygroscopic property and higher solubility
of salt operate, resulting the dehydration with regard to
the effect of dilution on the velocity of the rate of inversion
by hydrochloric acid j but for the very same reason,
however, it would follow that the hydrations would be
anore or less proportional to the comparative solubilities
68
ASSOCIATION THEORY OF SOLUTION
and hygroscopic properties. The following figures majr
be considered in this connection :
Molecular
Solubility
Calculated
Other pro-
hydration
in lOOgms.
molecules
perties.
by
of water
of water
Armstrong
at 15
required
(from
to dissolve
Comey).
one mole-
cule of
substance.
NH t Cl,10H 2
35-2
8'4
Non-
hygroscopic.
KC1,10H 2 O
33-4
12-5
ff
NaCl,13H 2 O
35 '9
9-0
BaCl 2 ,19H 2 O
34*4
33-6
CaCl 2 ,22H. 2 O
66-0
9-2
Hygroscopic.
It is quite clear from this table, when figures are
properly compared, that the solubility and hygroscopic
property could not allow full support to the following
inference by Armstrong, ''in view of the general character
of the effect produced by salts, bearing in mind that easily
soluble hygroscopic salts, such as calcium chloride, have
far greater influence than sodium chloride, for example, it
appears justifiable to regard the acceleration as concentrated
effect due to the withdrawal by the dissolved substance of
a certain proportion of water molecules, which thus
became removed from the sphere of the action of the acid."
The above table shows that the molecules of water required
to dissolve one molecule of substance do not bear any
proportion with the molecular hydrations found by Arm-
strong when figures for calcium chloride are compared
CONTRACTION IN SOLUTIONS 69
with those of other salts. These investigators seemed to
have made no clear and good case, so far, to find out any
relationship amongst each other of the properties of solu-
bility, association with solvent, deliquescence and efflore-
scence. The formation of high hydrate in solution of
efflorescent hydrates of sodium carbonate and sodium
sulphate goes much against the above presumption.
(2) The method of Jones and his coworkers based on
abnormal lowering of freezing points of solutions, assumes
that the solutes "must take up a part of the water forming
complex compound with it, and thus removing it from the
field of action so far as freezing point lowering is concern-
ed." If molecular contraction indicates the molecular
association of solvent and solute, then such phenomena of
abnormal lowering of freezing points should have been
observed in cases of many non-electrolytes whose solutions
gave contraction in ne same direction. Since contraction
in solution is general to many electrolytes and non-
electrolytes it is reasonable to think that their conception
of association of solvent and solute may not be the only
cause to abnormally affect the freezing point of aqueous
solutions.
(3) The method based on the solubility of gases, liquids
and solids in water, produced by the addition of electroly-
tes and certain non-electrolytes, considers that the solute
molecules get associated with some of those of the solvents,
leaving others free to behave and act as if they have
nothing to do with the associated ones ; but such assump-
tion could be considered hardly justified since when the
free solvent molecules are removed from the field the
70 ASSOCIATION THEORY OF SOLUTION
original ratio of molecular association will be disturbed
and changed. The presence of some surplus of molecules
of solvent is required to retain the determined ratio of
hydration. It is more probable that such experiments
would give the indications for only relative associations of
solvent molecules with those of the two solutes present in
the field than what happens when only one of them is
present, When more than one solute is introduced in a
solvent the molecular association of any one of them may
not represent what happens when the other is absent from
the field. In a solution of mixture of two substances the
molecular association of any one of them with the solvent
is a function of their masses present and of their compara-
tive chemical properties with reference to the solvent.
Thus the results obtained by the above method might not
represent with certainty what the case is when one solute
is present. Herzog and Bergen-Thun 17 found that the
addition of a small quantity of sucrose to a solution of
calcium chloride of about 7 N- concentration causes an
increase in the boiling point which is less than that cal-
culated by Raoult's equation. The boiling point of a more
concentrated solution is, however, decreased, whilst for
certain concentrations there is no effect. A depression of
the boiling point is observed when lactose or mannitol is
added to a 8 '09 N-calcium chloride, or when dextrose is
added to a 8*09 solution ; also by the addition of sucrose
to solutions of lithium chloride or calcium thiocyanate.
Very little departure from the calculated increase in boiling
point is observed when sugar is adeed to 5*3 N-sodium
chloride solution.
CONTRACTION IN SOLUTIONS 71
(4) The merit of the determination of hydration of ions
by the measurement of change of concentration at electrodes
caused by the transport of solvents with the ions during
electrolysis has been questioned 18 $ these results could only
give association of solvents with ions taking part in the
electric current and might not possibly represent what the
case is with others.
(5) Although there is not enough data to arrive at a
definite conclusion about the method of determination of
hydration by the measurement of relative migration veloci-
ties of ions, yet it may be stated that the results obtained
by this method would represent only the condition of the
ions taking active part in the passage of electric current
and might not give any idea about those molecules or ions
who do not take part in the conduction of electricity.
(6) The distribution method or the one by determina-
tion of variation ^of partition coefficient of a neutral
indicating substance between water and an immiscible
solvent with the addition of substances in water hydrations
of which have to be determined, assumed that some of the
water was attached to the solute as water of hydration and
thereby was removed from its role o solvent, and that the
lowering of solubility afforded a direct measure of this
"fixed" water, and it also assumed that the indicating
substance was insoluble in the hydrated salt. Sugden I{>
had a few results which agreed with those determined by
other methods, but there were several others which gave
disagreeing results probably indicating the merit of the
general assumption that the associated molecules of solvent
and solute do not dissolve the indicating substance under
72 ASSOCIATION THEORY OF SOLUTION
the conditions of the experiment. If molecular contraction
is a function of association of solvent and solute, the
proposal of Sugden that the figures obtained by this
distribution process as hydration of some particular salts
may be regarded as fixed values for all dilutions becomes
inconsistent. The molecular contraction varies with dilu-
tion indicating the change of association of solvent and
solute. The abnormal behaviour of chlorates and nitrates
of potassium was explained by stating that the solution of
these salts exert a greater solvent power upon acetic acid
than does pure water. If this explanation is to be accepted
then it may just suit the reverse one for the other salts,
that is, the solutions of other salts exert a lesser solvent
power upon acetic acid, and are not free from action at all.
Jablczynski 20 tried to measure the dimensions of ions
in aqueous solutions from measurements of densities of
solution. It is rather difficult to accept his figures on
account of want of reliable proof regarding the relation-
ship between the volumes of ions and that of unit weight
of solution. Even if ions are present in solution the
volume of an unit of mass of solution will also partially
depend on the mean free space in which each one of them
are moving by dint of kinetic movements and not so
much on the volume of each individual compoment of
solution.
It would be interesting to raise in this connection, how
these results of determination of molecular contraction
of substances in solution, reflect on the studies of other
physical properties of solution which have been so long
presumed to have revealed the real nature in which a
CONTRACTION IN SOLUTIONS 73
solute remains in the state of solution. The real study of
combination of two components as such or in a state of
solution in a third substance- the solvent- has been a very
exhaustive, laborious and searching investigation during a
Course of long period by Kendall and his coworkers. 21 He
started with the determination of viscosity, and preparation
of additive compounds of organic acids and other organic
substances in pairs, and subsequently prepared another set
of additive compounds of aldehydes, ketones, phenols,
esters with organic acids. He measured the viscosity of
solutions of solids in liquids to establish a connection
between the viscosity and the composition of ideal binary
mixtures by experiments with mixtures of benzene and
benzyl-benzonate, of benzene and ethyl benzoate, of
toluene and ethyl benzoate, of toluene and benzyl-
benzoate, of benzene and p-napthalene, of benzene and
diphenyl, of toluetfe" and naphthalene, and of toluene
and diphenyl, Results obtained in these determinations
were used in a formula which seem lacking in rationality.
Freezing point determinations of aqueous solutions
lead to the establishment of identity of a few interesting
additive compounds of organic acids with water
of hydration. Discovery of these additive compounds
in aqueous solutions is quite useful in organic and
inorganic chemistry but has misled the investigator and
his co-workers to think that water can function both as a
weak acid and weak base, and that the extent of hydration
in aqueous solutions should be found to increase the
increasing acidity or basicity of the solute. Among the
(reasons why such hypothesis cannot be accepted :two of
74 ASSOCIATION THEORY OF SOLUTION
them seem to be very clear, the experiments conducted
with rather a limited number of samples should not lead
to any such generalisation and even admitting there are
no mistakes in his experiments adequate proof is wanting
that those compounds which separated at freezing point
are really what were present in the solution. When a
substance is subjected to extreme cold, the kinetic
movements of the particles manifestable in terms of
thermal effects gradually decrease and ultimately come to
a point when the solute molecules, in a state of combination
with solvent molecules in proportion the same as their
dilution, become unstable, and the formation of a
stable compound sets in causing the separation of a
frozen mass. If it is presumed that the substance present
in solution could be separated by freezing, then the
molecular contractions of some of the hydrated substances
given in the foregoing pages would have been different.
If the substances were present in solution in a state of
combination with solvent the same as they could be
separated by freezing, the relationship between these
hydrated substances and the rest of the solvent molecules
present in the solution should have been properly
interpreted. It does not seem rational to think that the
two classes of molecules, combinations of solute and
solvent, and pure solvent are present as a mechanical
mixture.
Proving the identity of additive compounds between
100% sulphuric acid and the normal sulphates of the
alkali metals and the acid metal sulphates by the freezing
point determinations, Kendall postulated that the
CONTRACTION IN SOLUTIONS 75
formation of solvent solute complex is a prerequisite to
ionisation in solution. On this assumption it should
be possible to predict the degree of dissociation in any
given solution from (a) the stability of the complexes with
respect to their components, and (b) the instability of the
complexes with respect to their ionisation products.
Attempts have been made to substantiate this hypothesis
with figures available on influences on freezing points
and osmotic preasures, Since influences on freezing points
and osmotic pressures have been differently explained on
the basis of association theory of solution, the support
sought naturally ultra-vires. Kendall pointed out that
the hypothesis of Milner, Ghosh and others, failed to
explain the anomaly of the ionisation of strong electroly-
tes, because they, like the older hypotheses, ignored the
role of the soljy^nt in ionisation. Argument like this
could not be more useless and irrational, since he accuses
others for not taking into consideration the effect of the
bulk of the solvent present in the solution whereas he
himself, has used in his calculation only a portion of the
molecules of the solvent which could be separated with
the solute by means of ^application of extreme cold. There
may be truth in the assumption of the formation of
Hg (NO 3 ) 2 ,8H 2 O ; Hg (C1 4 ) 2 ,6H 2 O ; and HgF 2 ,2H 2 O ;
but he failed to establish that these are the very com-
pounds that are present in solution in an unchanged
condition at all dilutions and at any other changed con-
dition. Substantial support is needed why salts, acids,
or bases should ionise at all, if solute and solvent asso-
ciate why would it ionise afterwards, and why one should
76 ASSOCIATION THEORY OF SOLUTION
take it for granted that if anything conducts electricity
it is due to the presence of ions. All kinds of electroly-
tic dissociation theories presume that there are some
molecules of solvents present in an electrolyte which do
not take part in the conduction of electricity, or in other
words these are non-conductors. There are salts, acids and
bases, and their hydrates which conduct electricity in
the pure states, and it seems irrational to think that when
they are brought in a solution some molecules of each
substance retain or increase their property of electrical con-
ductivity while the remaining lose it altogether. It may
be argued that there are some ions present in the solid
substance which cause the conduction of electric current
but the assumption of a mixture of ionised and non-ionised
molecules can hardly be conceived without their mutual
influence on each other, since, if one is removed the other
can not exist as such and therefore it is convenient to
consider that there is only one kind of molecule present.
At any rate these points should have been made quite
clear before the putting forward of theories by Kendall.
A great deal of argument has been based on comparative
results of strong acids, weak acids, strong .bases, and
weak bases, without properly defining these terms or
clearly establishing lines of demarcation or gradation in
this connection 22 . While studying the problem of weak
or strong acids and bases subtances like borax 23 might
have formed some part on account of their high molecular
depression of freezing points. It appears, however, that
weak or strong acids and bases are more or less relative
4erms, before applying them in such connection it is
CONTRACTION IN SOLUTIONS 77
essential to establish lines of demarcation with a number
of samples which should be far more representative than
what Kendall and his co-workers have done. A relationship
that would take place between solvent and solute depends
entirely on the chemical nature of the substances, as, (a)
some are perfectly neutral and perfectly stable in solution 24 ,
(b) the other class instantly decompose when it is
attempted to dissolve them in water. As the type of the
first class may be taken the alkaline salts, of the second,
mercuric sulphate, nitrate and stannous chloride offer
examples, as also thallic salts of all three.
Attempts have been made to correlate contractions of
volume and refractive index of liquid mixtures 25 but
results could not be said to be satisfactory for want of
consideration of other changes of property that simulta-
neously attend the phenomena,
All the observed facts may be easily explained by the
following assumptions. When a solute dissolves in a
solvent all the molecules of both combine with each other
in proportion to the dilution and when the number of
solute molecules are increased or decreased corresponding
association of solvent and solute takes place uniformly.
All the molecules of the solvent and the solute being
influenced by identical conditions it is unreasonable to
presume that there should be a mixture of different kinds
of combinations e. g. the ionised or the non-ionised. The
process of association of the solvent and the solute is
reversible at all conditions. The associated molecules of
the solute and the solvent need not bear similarity to
those of either component, in respect of chemical,
78 ASSOCIATION THEORY OF SOLUTION
electrical, optical, osmotic, etc., properties. Dilution,
temperature and pressure influence the properties acquired
by the associations of solvent and solute.
REFERENCES.
(1) Watson, Phil Trans., 59. 1770, 325, 354; Holker,
Phil. Mag., 1845, (3), 27, 207 ; Playfair and Joule, ib.,
1845, (3), 27, 453 j Marignag, ib , 1846, (3 , 28, 527.
(2) Dalton, Acids, bases and salts, Manchester, 1840.
(3) Wanklyn, Johnstone and Cooper, Phil. Mag., 1891
(5) 32, 473.
(4) Wanklyn, Johnstone and Cooper, Chem. News,
1891, Vol. LXIV. p. 27 ; Eakshit. Zeit Elektrochem.,
1925. 97, 320.
(5) Lumsden, Jour. Chem. Soc., 1907. 91, 24.
(6) Rakshit, Zeit. Elektrochem., 1925, 321.
(7) Baxter and Wallace, Jour Amer. Chem. Soc., 1916,
38, 70.
(8) Favre and Valson, Compt. Rend., 1873, 77, 802 ;
Traube, Zeit. anorg. Chern., 1892, 3- 1 j Buchanan, Amer.
Jour. Science, 1916, (4), 21. 25; Baxter, Jour. Amer.
Chem. Soc., 1911, 33, 922
(9) Tammmana, Ueber die Beziehung Zwischen den
innern Kraften Uhd Eigenschaften der Losungen, Leip-
zig. 1907, "8.
(10 s ! Valson, Compt. Rend., 1874, 73, 441.
(11) Bender, Wied. Ann. 1883, 20, 560.
(12) Walker, Introduction to physical chemistry, 1927,
183; Nernst, Theoretical chemistry. Trans, from 8th to
10th German Edition, 1923, 454.
CONTRACTION IN SOLUTIONS 79
(13) Traube, Zeit. anorg. Chem., 1895,8,338; Ber.,
1896, 29, 1023 ; ibid, 30, 265.
(14) Rakshit, Zeit., Elektrochem., 1925, 321 ; 1926,
276 j 1927, 578.
(15) Ostwald, Zeit, phys. Chem., 1902, 42. 503-504.
(16) Marie and Marquis, ib., 1903, 45, 566-570;
Rakshit, Zeit Elektrochem., 1927, 579.
(17) Rakshit, Chem. News, 1927. 289, Herzog and
Bergen-Thun Annalen, 1923, 433, 117.
(18) Rakshit, Zeit. Elektrochem., 1925. 31, 320.
(19) Sugden, Jour. Chem. Soc., 1926, 174.
(20) Jablczynski, Rocz. Chem., 1923, 3, 362.
(21) Kendall and his co-workers, Jour, Amer. Chem.
Soc., 1914, 36 1069, 1222, 1722, 2498 ; 1915, 37, 149 ;
1916, 38, 1309, 1712 ; 1917, 39, 1787, 1802, 2303, 2323;
1918,40, 622 ; 1920, 42, 2131 ; 1921, 43, 979, 1391,
1416, 1426, 1470, 1481, 1545, 1826, 1846 ; 1922, 44,
717 ; 1923, 45, 963 ; Proc. Nat. Acad. Sci., 1921, 7, 56.
(22) Compare, Lowry. Chem, News, 1928, 41.
(23) Ostwaid, Solutions, Trans, by Pattison Muir
1891, 212.
(24) Lea, Phil. Mag., 1893, 88.
(25) Counson, Arch. Sci. Phys. Nat., 1923, 128, 361.
CHAPTER IV.
SURFACE TENSIONS OF SOLUTIONS.
The terms surface tension and capillarity are used in
chemistry apparently indicating the same property of
matter. Surface tension is the property of matter acquired
by virtue of the molecular attractions which tend to draw
the molecules as close together as possible. Capillarity
is that property of matter which causes the rise of liquids
in narrow tubes, the spheriodal shape of falling drops and
soap bubles, the spreading of oil on the surface of water,
etc, The phenomenon of surface tension was under ob-
servation by scientists from the very early days. Quanti-
tative measurements seem to have commenced by Jurin 1 ,
who showed that the difference of the levels of liquids
inside and outside a capillarly tube is inversely propor-
tional to the diameter of the bore. Young 2 , and finally
Ramsay and Shields 3 established the following formula
for ordinary practical determinations :
Yirhd in grams, per centimeter.
where, Y== Surface tension, that is the force which acts
normally along a line of unit length on the
surface of the liquid on account of the
existence of mutual attraction amongst the
component particles
r radius of the tube in centimetres ;
h height in centimetres of the liquid column ;,
d density of 'the liquid.
SURFACE TENSION 8 1
Besides the determination of surface tension by means
of measuring the rise of liquids in the capillary tube the
weight of falling drops from a tube of known bore may
be measured and utilised for the purpose. Several in-
vestigators 4 used the following formula in determining
surface tensions of a large number or samples :
Y---
2?rr
where, Y and r are the same as before ;
W = weight of the drop of the liquid.
Both these methods have been criticised by some
authors and several other methods have been proposed
for the determination of surface tension, but since the
bulk of the data are obtained by these two methods it
does not seem so much necessary to consider the latter
ones here. Moreover the critcisms are not impressive
enough.
The progress of the determination of surface tension
of solutions was followed almost simultaneously with that
of the pure liquids. The chief difficulty seemed to have
been noticed by the investigators is that of a suitable
general formula which would express the relationship
between the surface tension of a mixture and those of
its components. Formulas 5 proposed for the purpose
have been found useless on account of the ignorance of
the fact that the solvent and the solute remain in solution
in a state of combination so that the property of the
resultant product will differ from that of either compo-
nents and from that of the avarage of the components.
Attempts have also been made to improve the formula by
6
$2 ASSOCIATION THEORY OF SOLUTION
introducing a factor to counteract the effect of contrac-
tion or expansion on mixing ; but this also did not afford
any satisfactory result. The following pairs on mixing
give surface tension values which lie between those of
the components :
Benzene and toluene Chloroform and ether
,, acetone acetone
Ethyl iodide and methyl iodide carbontetra
Methyl and isobutyl acetate chloride.
The following pairs give values of surface tension on
mixing which fall below those of either components 7 :
Acetic acid and benzene Fatty acids and water
chloroform Carbondisulphide and
Benzene and chloroform ethelene dichloride
,, carbondisulphide and ether
ethyl iodide chloroform.
,, ether
In the former class of mixtures the compounds formed
in solution acquire a property of surface tension which is
close to the average of those of the components but in the
latter cases this property of the resultant product is
definitely lower than those of their average and those of
any one of the components.
Considerable experiments 8 have been done with
mixtures at different temparatures basing on Eotvos's 9
following formula :
where, Y = Surface tension.
MV = Volume of a gram molecule.
SURFACE TENSION 83
T = Critical temperature or the temperature at
which surface energy becomes zero. (Surface
energy Surface tension x area)
T = the temperature of observation.
K = Constant.
Results obtained by these experiments have been utilis-
ed to establish a relationship between the surface tensions
of mixtures and their molecular complexity but no satis-
factory generalisations were obtained because the com-
pounds formed as a result of solution may differ in all
properties from either of the components and from any
.other compound formed at any other dilution.
Surface tensions of binary mixtures may be classified
under three main groups :
(a) Surface tensions decrease with increasing concentra-
tion :
HC1 in water at 20C. Ethyl alcohol in water at 15'C.
o/o HC1 o /oC oH 6 0.
cm. " cm.
73-03 722
5 72-46 10 51-2
10 72'25 20 40-6
15 7196 30 34-7
20 71-44 40 31-2
25 70-74 50 29'1
60 27-7
70 26-6
80 25'4
90 24'1
100 22-5
ASSOCIATION THEORY OF SOLUTION
HNO 3 in water at 20-C.
NH 4 OH in water at 18C.
o/o HN0 3
/(
j NH 4 OH
7-25
7310
730
9-00
72-70
5
66-5
22-00
71-48
10
63-6
37-00
6810
15
61-3
50-00
65'43
20
59-3
70-00
59-36
25
57-7
(b) Surface tensions
increase
with increase
concen-
tration :
KC1 in water at 18C.
CaCl 2 in
water.
o/o KC1
fe? o
k CaClo
dyn
dyn
cm. '
cm.
cm.
at 10C.
at 3CTCL
o-oo
72-41
O'OO
7412
7112
6-73
74-16
3'56
74-28
72-35
12-65
7511
6-05
7618
73-25
21-06
77-67
12-34
78-63
75-72
22-57
83-83
80-38
25-90
85-74
82-76
31-90
89-60
8644
KNO 3 in
water at 18C.
CuSO 4 in
water
o/o KN0 3
o/o CuSO 4
at 10C.
at 30C.
o-oo
72-59
o-oo
74-12
7112
5-75
73-07
6*43 '
74-83
71-87
12-33
75-66
72-72
1519
74-46
14-69
7610
-
21*46
75-41
25-4
_
74-12
SURFACE TENSION 85
NaOH in water at 20X5.
o/o NaOH Cane sugar in water at 18C.
0-00 72-8 o/ C 12 H 22 O n
5 74-6 O'OO 72-28
10 77-3 8'71 72-76
15 80-8 10-73 73*13
20 85-8 2363 73'47
25 90-6
30 95'1
35 99-7
Tsfa 2 SO 4 in water at 18C.
o/o Na 2 SO 4
O'OO 72'47
4-91 73-79
9-22 74-99
12-73 75-52
(c) Surface tensions increase, reach maximum, and then
decrease with concentration.
H 2 SO 4 in water at 18C.
H 2 SO 4
dyn
o/o H 2 SO 4
dyn
cm,
cm.
o-oo
72-82
80*33
71-20
6-57
7288
84-49
68-53
12-70
73-48
90-0
63-56
35-76
7614
92-7
60'30
4758
76-70
95-4
57-59
62-37
75-42
97'1
55-31
76-56
72-56
98-7
53-66
86 ASSOCIATION THEORY OF SOLUTION
The variation of surface tensions with dilution of solu-
tions of electrolytes and n on -electrolytes suits well with the
association theory of solution but does not do so with the
dissociation theory of solution. The latter, however,
practically proposes no theory regarding the existence of
solvent and solute in non-electrolyte solution, and assumes
in the case of electrolyte that the acidic and basic
radicals composing the molecules get gradually separated
with increasing dilution. The variation of surface tension
with dilution is not synchronous with that of electrical
conductivity. The association theory of solution assumes
formation of different compounds at each dilution in
molecular ratio, whose properties differ from those of the
components and from any such compound formed at any
other dilution. This property of a solution also differs
from that of the average of the components. Increase of
temperature has a decreasing effect on surface tension as
may be seen in the cases of solutions of calcium chloride
and copper sulphate. If surface tension is a function of
attraction between the molecules of a liquid, compounds
formed between the solvent and the solute in solution do
not decompose with rise of temperature, and the molecules
of the solute and the solvent do not undergo any change
of aggregation or depolymorisation with rise of tem-
perature then the associated molecules get further away
with the rise of temperature only on account of increase of
their kinetic movements. The actual increase of the
distance between two particles would bear a function with
the co-efficient of expansion of the substances. The
increase in volume bears a relation to the distance between
SURFACE TENSION 87
the particles, and this distance influences the attraction
between themselves or_, in other words the surface tension.
It seems desirable to find out properly if there is any
relationship between co-effiieients of expansion and co-
efficient of change of surface tension of solutions with
variation of temperature in order to establish the validity
of the above assumptions.
REFERENCES.
(1) Jurin, Phil. Trans., 1738, 30.
(2) Young, Phil. Trans., 1805, 95, 65.
(3) Ramsay and Shields, Zeit. Phys. Chem., 1893,
12, 433.
(4) Tate, Phil. Mag., 1864, [4] 27, 176 ; Quineke,
Pogg. Ann., 1868, 134, 356, 135, 621 ; 1869, 138, 141 ;
1870, 139, 1 ; Duclaux, Ann. Chim. Phys., 1878, [5], 13,
75 ; Rayleigh, Phil. Mag., 1899, [5], 48, 321.
(5) Ramsay and Acton, Proc. Roy. Soc , 1874, 56,
182 ; Whatmough, Zeit. Phys. Chem., 1901, 39, 129.
(6) Smiles, Relation between chemical constitution and
some physical properties, 1910, 41.
(7) Smiles, loc. cit.
(8) Ramsay and Acton, Zeit. Phys. Chem., 1894, 15,
92 ; Pekar, ib, 1902, 39, 446 Monatsheft. f . chem., 1907,
28, 831, 891.
(9) Eotvos, Wied. Ann., 1886, 27, 448.
CHAPTER V
VISCOSITIES OE SOLUTIONS
The existence of the property of internal friction
causing some work done in the relative displacement of
the particles of a solid, liquid or gas is called its
viscosity. It is ordinarily measured by observing the
rate of flow through capillary tubes. The phenomenon
was under observation by a few earlier 1 investigators and
the subject was systematically undertaken by Hagenback. 2
Quantitative determination of this property of matter is
based on the assumption (1) that when a liquid moves
through a tube its particles move parallel to the axis of
the tube and (2) that the layer of the particles next the
wall of the tube remain stationary and those in the centre
move at the maximum speed. The formula worked out
for the purpose is
wr^PT V d
^ *L V "87TT
where, 17 force needed to move a layer of the substance
of unit area through a distance of unit length
past an adjacent layer unit distance away.
This value is called the coefficient of viscosity.
r radios of the tube,
P = pressure under which discharge of liquid takes
place.
T = time.
L = length of the tube.
VISCOSITIES OF SOLUTIONS 89
V=* volume of the liquid discharged,
d*= density of the liquid.
If all these could be measured at any temperature
then the absolute viscosity of the substance is known
under those conditions. Practical determinations, however,
of all of them are not only difficult but are not often
vexy accurate. Consequently it has become useful to know
the result by comparing the time of flow of a given
volume of the liquid with that of some standard one
under the same conditions and the results may be applied
to the following formula :
Where 17 viscosity of the liquid.
^s= standard liquid.
T = time of the flow of the liquid.
Ts = ,, standard liquid.
Ordinarily water is taken as a standard because its
viscosity has been very accurately measured at varying
conditions. Mention may be made in this connection of
the popular apparatus devised by Qstwald for the purpose
of comporative measurements. It is a very simple
instrument but gives quite accurate and considerable
amount of data on this subject have been recorded by
'the use of this ; and such figures are called specih'c
viscosities. Amongst the investigators who took part in
the determinations of viscosity in absolute units the
names of Thorpe and Roger 3 may be mentioned, a reference
to their original paper gives an idea as to their satisfactory
methods of determination including the apparatus used
90 ASSOCIATION THEORY OF SOLUTION
for the purpose which undoubtedly ensures considerable
accuracy.
Smiles 4 has made a very fair, collection of all
representative data how intra-molecular relationship of
atoms or groups of atoms in a molecule could influence
the viscosity. In the case of platinum and gold, Ray 5
found the following variation of valency in different
molecules, bi-, ter-, quadri-, and quinque-valent gold
compounds have been prepared. Potassium dithioethelene
glycol was reacted on platinic chloride under varying
conditions of temperature and dilutions to yield ter-,,
quadri-, quinque-, sexa-, and oeta-valent platinum com-
pounds. Dilution remaining same the higher the tem-
perature of reaction the lower the valency of platinum.
Influence of temperature and other conditions at which a
reaction is allowed to take place on the valency of the
compounds formed has been shown in the case of action of
platinic chloride on ethyl sulphide. The products of such
reactions contained tri-, tetra-, and pentavalent platinum
compounds.
In the case of mixtures or solutions the viscosities do
not represent those of the calculated average of the
compounds. Had there been no reaction between solvent
and solute the viscosity of the solution would have been
an average of the componants or would bear a relationship
with the average : but since they do not do so it is not
unreasonable to presume that whenever a deviation from
average is found a combination between solute and solvent
is suspected, Thus the determination of viscosity may be
employed to establish the probable existence in a solution.
VISCOSITIES OF SOLUTIONS 9 1
of compounds which cannot be isolated by the usual
laboratory processes. The converse, however, may not be
true ; if a solution gives a viscosity which is more or less
equal to the calculated average of the components it need
not be concluded that no reaction has taken place because
the resultant associated molecules of solvent and solute
may assume a property, in such cases, the same as the
sum of those of the pure components.
It is worthwhile discussing what the effect would be
on the viscosity of the resultant product if any reaction
has taken place between the solvent and the solute.
Smiles (loc. cit.) has shown how atoms or groups of atoms
produce varying effects on the viscosity of a substance
depending on the nature how they are linked to the mole
cule, and Ray's researches have given an idea how the
valency of an element can change according to its state of
combination with the remaining part of the molecule.
When a molecule of a solute is suitably brought in contact
with one or more molecules of solvent a combination takes
place ; the rearrangement inside such final compound
depends on the condition at which such reaction takes
place as also on the ratio of the molecules of the solvent
available for the purpose. It is not necessary that each
time a solvent is added the property of the resultant
product will be proportionately increased, because at each
dilution an intramolecular rearrangement takes place.
Whenever a considerable deviation is noticed from the
average of the components of the mixture or from those
formed at other dilutions it is concluded that a change in
the constitution has taken place. But the converse,.
92 ASSOCIATION THEORY OF SOLUTION
however, need not be taken to be true. If a solution does
not show any or much deviation from the average of the
components it may mean that the compounds formed
under such conditions have a property the same as that
of the average of the components.
Viscosities of some mixtures are very interesting to
show how association of solvents and solutes take place at
varying conditions.
(1) Viscosity increases with increased concentration :
Aqueous solution of sucrose. Aqueous solutions of glucose.
%w/w Specific viscosity %w/w Specific viscosity
at 25C. at 25 C.
20-10 1-917 24-03 2216
1478 1-570 2014 1901
998 1-329 15-70 T619
4-85 1141 10-20 1-316
2-00 1-054 4-63 1131
1-00 1026 211 1-062
1-00 1-027
Solution of Benzene and ethyl alcohol.
alcohol. Temp. Viscosity.
79-3 0-00317
1-30 74-8 000327
4-30 70-6 00334
6 90 69-2 00336
15-20 67-4 0-00341
22-4 66-9 0-00344
VISCOSITIES OF SOLUTIONS
w/w alchohol.
Temp.
37-3
66-9
47-4
67-1
70-3
69-1
88-0
72-7
100-0
77-1
93
Viscosity.
0-00361
0-00377
0-00416
0*00438
0-00442
Aqueous solutions of potassium Chloride.
Gram equivalent per Specific viscosity
litre solution. at 25 e C.
4-174 1-097
3-757 1-067
8-818 1-023
1-879 0-998
(2) Viscosity decreases with increased concentration.
Aqueous solution of caesium nitrate at 25C.
Gram equivalent Viscosity. Gram equivalent Viscosity,
per litre solution. per litre solution.
0-02314 0008899 0-3173 0-008697
0-0511
008883
0-4520
0-008617
0-1076
0-008844
0-5652
0-008557
0-1557
0-008804
0-2475
0-008742
0-7321
0-008480
94 ASSOCIATION THEORY OF SOLUTION
Aqueous solution of Aqueous solution of
potassium chlorate. Rubidium chloride,
Gram equivalent Specific Gram equivalent Specific
per litre solution, viscosity, per litre solution, viscosity.
at 18C. at 18'C.
0-5 09848 2'0 0'9405
0-2 0;9948 TO 0-9645
01 0-9990 0-5 09790
0*05 T0008 0204 0-9915
O'lOl 0-9969
Aqueous solution of Aqueous solution of
ammonium chloride. caesium chloride.
Gram equivalent Specific Gram equivalent Specific
per litre solution, viscosity per litre solution Viscosity
at 18C. at 18C.
4 0-9677 20 0-9230
2 0-9626 10 0-9510
1 0-9766 0-5 09731
05 0-9367 0*2 0'9883
0-2 0-9944 01 0-9940
01 0-9961
(3) Viscosity increases, reaches maximum and then
decreases with increased concentration.
VISCOSITIES OF SOLUTIONS
95
Aqueous solution of nitric acid.
'Gram equivalent per f) ?;
100 gms. of solution. 0C. 10C.
53-90 0-02945 002324
58-10 003295 0'02470
61-56 0'03459 0-02604
64-30 0-03560 0-02676
66-60 0-03475 0*02584
67-82 0-03422 0-02579
71-24 0-03288 0'02465
7285 0-03276 0*02456
Aqueous solution of Acetic Acid.
Grams of acid
per 100 gms.
of solution.
2-1
57
10-8
13-0
13-3
17-2
19-6
216
23'3
23'9
24-4
27-7
13C.
0-01906
0-02671
0-03105
0-03187
0-03003
0-03330
03354
0-03360
0-03388
0-03322
0-03355
0'03314
20C.
0-01640
0-02222
02540
0-02601
0-02632
0-02694
0-02726
0-62727
0-02739
0-02701
0-02708
0-02664
30'C.
0-01353
0-01752
0-01981
0-02009
0-02069
0-02070
0-02093
002079
0-02091
0-02052
0-02073
0-02038
' r l
40"C.
0-0112S
0-011421
0-01575
0-01595
0-01626
0-01643
0-01635
0-01640
0-01643
0-01618
0-01628
0-01603
80'C.
0-00967
0-01287
0-01304
0-01327
0-01324
0-01327
0-01327
0-01316
0-01314
0-01287
0-01297
9 6
ASSOCIATION THEORY OF SOLUTION
Aqueous solution of
methyl alcohol.
Aqueous solution of
ethyl alcohol.
%
25'C.
%
25C.
100-0
0-005525
99-20
0-0115
79-64
0-01003
78-09
001804
58-61
0-01399
ol-85
0-02173
37-82
001567
45-57
0-02351
19-74
0-01378
39-65
0-02343
o-oo
0-00891
20-71
0-02343
o-oo
0-00891
Aqueous solution of
Aqueous
solution of
n-propyl
alcohol.
allyl
alcohol.
25C.
25"C.
100-00
0-01936
100-00
0-01232
7313
0-02509
83-20
0-01537
59-38
0-02652
69-56
0-01750
28-62
002118
65-00
0*01790
17-40
0-01697
56-63
0-01891
o-oo
0-00891
48-56
0-01892
47-82
0-01891
47-31
0-01867
46-88
001895
45-21
0-01888
36-53
0-01346
35-53
0-01834
33-70
0-01789
25-98
001632
14'06
0-01349
o-o
0-01891
VISCOSITIES OF SOLUTIONS 97
Solution of benzene in methyl alcohol.
o/o w/w Benzene
Temp.
Viscosity.
O'O
63-7
000326
18-14
59-9
0-00347
31-60
58-2
0-00354
41-60
57-6
0-00359
50-10
57-6
0-00359
63-3
57-2
0-00361
70-4
57-3
0-00360
78-5
57-6
0-00362
90-4
59-0
0-00362
91-9
59-6
0-00357
100-0
79-3
0-00317
Solution of ethyl
alcohol in
carbontetra chloride.
o/o w/w ethyl alcohol,
Temp.
Viscosity.
o-o
75-6
0-00499
4-58
65-1
000518
6'71
64-6
0-00521
9-65
64-0
0-00520
20-95
63-8
0-00530
30-2
64-2
0-00530
36-6
64-8
0-00526
58-8
677
0-00310
73-0
70-5
0-00490
100-0
77-1
0-00442
7
9 8
ASSOCIATION THEORY OF SOLUTION
Solution of aniline in acetic acid.
% aniline.
T25C.
O'O
0-0134
15-5
0-0729
247
0-123
37-9
0'219
409
0-214
44*5
0'203
49-6
0-181
62-3
0-118
lOO'O
0-0362
0-0296
0-0565
0-0558
0-0523
0-0382
0-0201
(4) Viscosity decreases, reaches minimum and then
increases with increased concentration.
Aqueous solution of potassium bromide.
Gram equivalent per litre.
4-032
1-973
0-9333
0-503
0-01976
0-0992
Specific viscosity at 18C.
0-9599
0-9285
0-9533
0-9738
0-9887
0-9924
VISCOSITIES OF SOLUTIONS 99
Aqueous solution of Potassium thiocyanide.
Gram equivalent per litre. Specific viscosity at 18C.
3 95 1-0332
1-975 0-9499
1-005 0-9587
0-5025 0-9768
0-201 0-9915
0-1005 0-9974
Aqueous solution of ammonium nitrate.
/o r /10^C. ^]30C. ?}50 C.
49-83
0-015898
0-011423
0-008824
37-22
0012939
0-009239
0-007002
27-08
0-012091
0-008608
0-006298
12-19
0-012054
0-007994
0-005756
5-975
0-012559
0-007994
0-005702
A comparative examination of the above figures will
show that the viscosity of a solution deviates from that of
the average of its components. This deviation from the
average indicates that the components, in order to lose
their respective property must have lost their separate
existence or identity. The components of the solution
must have combined to acquire a property of the solution
different from that of the average.
The effect of temperature on the viscosity of solution
is just as it is on its other physical properties. It may
Jbe seen from the viscosities of solutions of acetic acid and
IOO ASSOCIATION THEORY OF SOLUTION
ammonium nitrate that maximum and minimum points
occur at different dilutions at different temperatures. This
is probably due to interference of other properties which
may be maintaining a balance with this property. While
viscosity is increased or decreased some property or
properties like those of thermal, optical etc., are corres-
pondingly and sisultaneously changed.
In attempting to find out reasons for the influence of
temperature on the viscosity of solutions it may be
considered that kinetic theory applies to the increased
movement of molecules. Now does this mean that the
increase of motion alone influence this property, or any
other assumption is needed ? It has been primarily
assumed that a layer of molecules immediately in contact
with the surface of the vessel is
motionless and the central portion
moves away causing a friction which
is a function of viscosity. In the
following diagram let the particle
A represent those in contact with the
wall of the tube and B t , B t , Bt',
and B t ', be the positions of moving
particles at temperatures t and t'
respectively $ the latter temperature
being the higher of the two. The
diagram only shows the vertical
component movements of the particle B but it may have
other motions which, however, need not be considered in
connection with viscosity determination as they are not
likely to influence such phenomena.
VISCOSITIES OF SOLUTIONS IOI
On account of movements, the particle B assumes the
positions as represented in the diagram and no work is
done against the attraction on the vertical line between A
and B so long as B is not removed from 'these positions
but if the moving particle is removed from B t or Bt'
positions some work is done. Less work is done in
removing B t ' than that is done by removing the other,
the former being further away. Thus, at the higher
temperature t' less viscosity is found. If, however, the
effect of increase of temperature is to increase only the
vertical movements of the particles then the coefficient
of expansion would have been inversely proportional to
the same viscosity. But actually this is not the case
which means that with the rise of temperature the
molecules may suffer some movements other than that
which influences the viscosity ; and probably there are
some intra-molecular changes leading to the development
of other properties.
The dissociation theory of solution postulates that
molecules like KCI break up in water into K and CI ;
that the quantity of this decomposition is influenced by
dilution, and that these dissociated ions receive a coating
of solvent forming outter shells which preserve them
against the action of other kinds of ions present in the
same field. It has also been said that the molecules of
chemically similar substances undergo similar dissociation
under similar conditions. Some 8 authors declared that
these ions do not move alone but do so being surrounded
by clusters of solvent molecules, the frictional resistance
to their motion with the particles of the solvent being
1O2 ASSOCIATION THEORY OF SOLUTION
thus eliminated. Keeping these views in mind a study
of viscosity figures of compounds of chemically similar
cations with same anions is useful in examining the
validity of the dissociation theory to explain the
plenomena. Compounds of potassium and sodium gave
the following results on the determination of their specific
viscosity at different temperatures :
N N N N
1243
NaOH (25)
KOH (25)
1-2535
1-1294
11087
1-0637
1-0560
1-0313
1-0302
1-0130
Difference.
01061
0-0450
0-0247
0-0172
NaCI (25)
KCI (25)
1-0973
0'9872
1-0471
0-9874
1-0239
0-9903
1-0126
0-9928
Difference.
0-1101
0-0597
0-0236
0-0198
NaNO 3 (25)
KNO 3 (25)
1-0655
0-9733
1-0259
0-9822
1-0122
0-9870
1-0069
0-9921
Difference.
0-0902
0-0437
0-0352
0-0148
Na 8 CO 3 (25)
K a CO,(25 c )
1-2847
1-1667
1-1367
1-0784
1-0610
1-6391
1-0310
1-0192
Difference.
0-1180
0-0583
0-0219
0-0118
Na,SO 4 (25)
K J SO 4 (25)
1-2291
1-1051
1-1058
1-0486
1-0522
1-0206
1-0235
1-0078
Difference.
0-1240
0-0572
00316
0-0157
The dissociation theory also presumes that some of the
molecules are dissociated and the rest undissociated, and
VISCOSITIES OF SOLUTIONS lOJ
that these undissociated molecules interfere with the
property of the components of the dissociated ones.
Reasonably assuming that the influence of the
undecomposed molecules of sodium and potassium
compounds is the same or nearly, so under the same
conditions, the difference of viscosities of solution of the
same normality of hydroxides, chlorides, mitrates,
carbonates and sulphates would have been equal, the
effects due to anions being eliminated by subtraction.
The figures, however, as shown above do not support either
as they are, or, even when they are manipulated by some
factor. Agreement of these differences would have secured
a great support to the dissociation theory but un-
fortunately, the experimental results being untoward, the
inadequacy of the theory gets established here also.
Occurrence of maximum and minimum viscosities at
suitable concentrations of some electrolytes, and of decrease
and increase of viscosity with increased concentration with
those of others have not also been properly explained by
the dissociation theory.
The association theory of solution, however, finds no
difficulty in explaining all such phenomena, because the
compounds formed at different dilutions assume properties
which differ from those of the components and of such
compounds at any other dilution.
REFERENCES.
(1) Dubuat, Principles d'hydraulique, Paris (1779);
Girad, Memoiresde Pacadame des sciences, 1816 j
104 ASSOCIATION THKORY OF SOLUTION
Poiseuille, Ann. Chim. Phys., 1843, (3), 7, 50 ; 1846, 21,
76; Stokes, Trans. Camb. Phil. Soc., 1849, 8, 287.
(2) Hagenbach, Pogg. Ann., 1860, 109, 385.
(3) Thorpe and Rodger, Phil. Trans., 1894, 185, A, 397.
(4) Smiles, The relations between chemical constitution
and some physical properties. 1910, pp. 60-72.
(5) Ray, Jour. Chem. Soc., 1923, 133; Jour. Ind.
Chem. Soc., 1924, 63; 1925, 178; 1926, 155.
(6) Castell Evans, Physico-Cheraical Tables 1920. Vol.
2-648.
(7) Ib., 616,
(8) Smiles, The relation between chemical constitution
and some physical properties. 1910, 90; Zeit. Phys. Chem.,
1906, 55. 707.
CHAPTER VI
OSMOTIC PRESSURES OF SOLUTIONS.
When solid or liquid substances are brought in eon-
tact with a liquid solvent, taking precaution so that the
mixing due to agitation is the minimum the process of
solution will immediately commence and continue till a
homogeneous mixture is obtained This process is called
osmotic phenomenon and the tendency to form such
solution is known as osmotic pressure. Attempts to find
out the existence of such a property of solutions were
first made by Abbe Nollet 1 , who found that if a glass
vessel be filled with alcohol, the opening covered with a
bladder, and the vessel immersed in water, then the
volume of the contents of the vessel gradually increases.
Parrot 2 in 1815 repeated the same experiment and came
to the conclusion that miscible liquids show a tendency to
move on their own accord when they are just brought in
contact with, but not agitated in any way, with another
so as to form a homogeneous mixture ultimately. Butro-
chet 3 and Vierordt 4 performed some quantitative measure-
ments and concluded that, if there be a partition of a
membrane of pig's bladder between water and aqueous
solution of salt the water passes through the membrane
more rapidly than the salt. They also found that the
difference between the rates of osmosis of pure water and
of salt solution depends on the nature of the salt, on the
concentration of the solution and on the nature of the
IO6 ASSOCIATION THEORY OF SOLUTION
membrane or the permeable partition used for the purpose.
The influence of the nature of the partition was studied
subsequently by Thomas Graham 5 and Traube 6 . Graham
used animal membranes for all his researches and Traube
was the first investigator to use chemical membranes-
prepared in his laboratory by precipitation. He prepared
precipitation membranes of non-setting glue and tannic
acid, lead tannate, copper tannate, lead silicate, copper
silicate, tin silicate, copper ferrocyanide and copper
ferric) anide. He fdund that these membranes differed in
their permeability to dissolved crystalloids. Ammonium
sulphate and barium nitrate can permeate through glue-
tannic acid membrane but cannot do so through copper
ferrocyanide membrane. Traube thus proved that the
membranes were selective in such action and the phenome-
non was divided into two kinds.
When a vessel containing a solution is closely covered
with a partition placed in another vessel containing pure
solvent the pressure inside the vessel will depend on the
rate with which the solute and solvent molecules enter and
exit through the membrane. In one case the membrane is
permeable both to solvent and solute and in another case
the membrane is permeable to only one component of a
binary solution. The second one attracted considerable
interest and such membranes for a binany solution were
called semi-permeable membranes by Van't Hoff 7 . By the
use of semi-permeable membranes in a solution many
quantitative properties of solutions have been studied.
Absolute permeability and semi-permeability to many
solutes have been studied by several investigators and it
OSMOTIC PRESSURE OF SOLUTIONS 107
has been asserted that no membiane is absolutely im-
permeable to a solute. But several cases have been found
which would easily allow quantitative measurement, being
practically semipermeable. A very specific instance of the
semipermeability has been strikingly established in the
case of cane-sugar solution. Pfeffer 9 made some simple
experiments with sugar solutions by means of an apparatus
wjiich consisted of a small cylindrical pot of porous ware
in the walls of which a precipitate of copper ferrocyanide
was allowed to form by diffusion of 0'25 per cent solution
of ccp}>er sulphate and 0'21 per cent solution of potassium
ferrocyanide from opposite sides of the cell walls. The
pot was previous!} carefully washed, soaked in water for
sometime, filled with solutions of copper sulphate, and
dipped upto the neck in the solution of potassium ferro-
cyanide. This pot was suitably fitted with a closed
manometer arid a tube for filling the cell with the solution,
the osmotic pressure of which was to be determined.
His a] plications of this simple instrument for determina-
tion of osmotic pressure of sugar solution is considered
as classical experiments. Morse 10 , however, laterly per-
formed a most important research on cause-sugar solution
proving the true semi-permeable character of copper
ferrocyanide membrane towaids such solutions ; his experi-
ment extended over a period of sixty days, at the constant
tempeiatuie of 15 r C, which showed that a pressure of
over 12 atmospheres was kept practically constant all
along.
A ccnndeiable number of measurements of osmotic
press uie were carried out by Pftffer, which received very
Io8 ASSOCIATION THEORY OF SOLUTION
valuable support from Van't Hoff. These experiments,
however, were repeated very liberally by Morse 11 , by
means of an apparatus devised by himself and his co-wor-
kers. Frazer and My rick 12 have considerably modified
this apparatus and performed several experiments. Lord
Berkeley and Hartley 13 used another type of apparatus.
All these various investigators worked very keenly on the
line directed by Pfeffer and obtained results which only
confirmed the first three fundamental laws laid by him.
Solutions are said to obey the following osmotic laws :
(1) Osmotic pressure is directly proportional to
the concentration, provided the solution is not too strong.
(2) Osmotic pressure is directly proportional to
absolute temperature.
(3) Equimolecular nonelectolyte solutions of different
substances have equal osmotic pressure.
(4) Osmotic pressure is independent of the nature 14 of
the solvent, provided the dissolved substance has the same
molecular weight in two solvents. Some solutions, like
acetic acid in benzene and acetic acid in water, indicate
difference of osmotic pressure of solution of same strength
on account of the solute behaving as double molecules in
benzene solution.
Analogy very often helps understanding even in
science, and it is therefore generally applied very con-
veniently but in the case of analogy between gas laws and
laws of osmotic pressure of solutions, it is apt to confuse
the fundamental conception of solution. Gas molecules
remain in a space in a state of combination with nothing
whereas the solute molecules in solution remain in a state
OSMOTIC PRESSURE OF SOLUTIONS 109
of combination with the solvent molecules. According to
association theory of solution the solute molecules remain
in a state of solution in association or in combination with
solvent molecules in proportion to the same as their
dilution. It would seem erroneous to think that the solute
molecules move in solution in the same way as gas
molecules in space. Solute molecules while moving carry
with them all the molecules of the solvent with which
it is combined.
It is useful to mention here that the effect of analogy
in this case has furnished some valuable arguments to
Van't Hof. The celebrated- investigator dealt with the
results of Pfeffer in 1887 and elucidated many important
facts which might not have been discovered had he not
compared Pfeffer's results with those of gases. He
presumed that solute melecules in a sufficiently dilute
solution behave like an "ideal gas" molecules ; in an "ideal
solution" the action of the dissolved molecules upon one
another, as well as their actual volume compared with that
of the space they inhabit, are so small as may be considered
negligible. It was thought that the osmotic pressure of
solution is due either to a kinetic cause or to an attraction
of the solute molecules for the solvent molocules. In both
the cases the osmotic pressure should be proportional to the
number of impacts of solute molecules ; and the attraction
for solvent molecules should be also proportional to th&
number of the solute molecules. According to the dissocia-
tion theory of solution, however, some of the molecules of
the solute in an electrolyte break up into ions and then
behave as two molecules and bombard separately on the
I 10 ASSOCIATION THKORY OF SOLUTION
walls of the membrane thus causing increase of pressure
than that obtainable from a non-electrolyte solution of the
same molecular concentration.
It has not been properly discussed how the hypothesis
of attraction, of solute molecules for solvent molecules as
the cause of osmotic pressure, is affected in the case of
solutions which are electrolytes and which have partly or
completely dissociated molecules of the solute. Like the
other hypothesis it would be also necessary to presume
that each of the ions, split up from the solute molecules,
acquires the same property, so far as the osmotic pressure
is concerned, as an entire undissociated original molecule.
And if osmotic pressure of solution is due to the
attraction of solutes inside the cell for the pure solvent
outside the membrane then this attraction is proportional
to the number of molecules consisting of entire and
broken up individual ions.
Abnormality of electrolytes has been explained by the
assumption of Arrhenius's dissociation theory. Solute
molecules gradually split up into ions each of which
separately acquires properties pertaining to osmotic
phenomena the same or similar to the original molecule.
Thus it has been presumed that the osmotic pressure in an
electrolyte is due to undissociated and dissociated
molecules. If, there are 100 molecules of sodium chloride
in one litre, of which 25 are dissociated, the osmotic
pressure will be due to 75 molecules of NaCl, 25 of Na-or
NaOH and 25 of Cl or HC1, or, in other words the
action will be due to 125 ( = 75 4-25 + 25; molecules in the
place of 100. It has thus been assumed that the osmotic
OSMOTIC PRESSURE OF SOLUTIONS I I I
pressure of an electrolyte will be due to three different
kinds of solutes, NaCl, Na, and Cl, in a salt solution.
These differ widely in chemical properties from each other
and the dilution of the first one does not agree with that
of the other two. In view of the analogy of osmotic
laws with gas laws and particularly in the application of
Dalton's law for pressure of mixed gases it is reasonable
to presume that each of the particles of NaCl, Na, and
Cl will behave with respect to osmotic pressure as if
the other two are absent in the field. Now, if the first
law of osmotic pressure be applied, it would follow that
the osmotic pressure due to NaCl molecules is not the
same as Na or Cl, since their concentrations are not
always the same but would be the same only when the
dissociation is 50 per cent. Had the problem been taken
up in this light and necessary calculations made the
validity of such assumption would have been better or
rather correctly recorded.
Osmotic pressure is assumed to be due the bombard-
ment of solute molecules on the walls of the membrane
and the abnormal osmotic pressure of electrolytes is
explained by the help of Arrhenius's electrolytic dissociation
theory. Each ion formed by splitting up of the solute
acquires osmotic properties the same as the original
undissocisted molecule ; thus, in the case of NaCl the
osmotic effect in an aqueous solution will be not due to
NaCl alone but due to NaCl, Na, and Cl. It is also
necessary to presume in this connection that the osmotic
properties of Na, and Cl are either same or similar as
those of unbroken NaCl.
112 ASSOCIATION THEORY OF SOLUTION
Bates 15 did a number of experiments on osmotic pres-
sure of electrolytes of varying concentration and calculated
the degrees of ionisation. His results have brought about
considerable confusion on the relationship between the
theory of osmotic pressure and that of ionic dissociation
of Arrhenius. Views have been expressed that osmotic
pressure is not regarded as due to bombardment of the
membrane by the molecules of the solute and it is thought
very doubtful whether any very large number of the
solute molecules ever reach the semi- permeable membrane
at all. It has also beee proposed by Schay 16 that the
osmotic pressure is primarily connected with the solvent,-
and only secondarily with the solute. It has been
concluded that the divergence from Oswald's dilution law
exhibited by strong electrolytes may be due to the behavi-
our of either of the ions, of the non -dissociated molecules,
or of the both and that Van't HofPs law 7rV = RT,
(where TT Osmotic pressure) does not hold either for one
or both of these molecular species. The osmotic pressures
of the ions and of the non-dissociated molecules in solu-
tions of electrolytes have been calculated by means of
conductivity data, together with measurements of one of
the collegative properties, such as lowering of the freezing
point and the electromotive force of concentration cells.
The osmotic pressure of the univalent ion is, in ganeral^
a little below that calculated from Van't HofFs law, whilst
that of the non-dissociated molecules of a strong univalent
electrolyte is consider nb!y greater, the deviation being
about 15 per cent, in a concentration of 0*0001 N. The
bivalent ions deviate much more than univalent ions.
OSMOTIC PRESSURE OF SOLUTIONS II $
whereas non-dissociated molecules of bivalent salts obey
the law fairly closely.
Bates's 15 results are extremely interesting and useful
in proving the failure of the electrolytic dissociation theory
as his figures decidedly establish the uselessness of the
hypothesis that the abnormality of osmotic pressure of
electrolytes is explained by the assumption of occurrence
of ionisation, and this fact has unfortunately been formed
to be one of the pillars on which Arrhenius based his
theory. Bates, however, took a round about method of
explaining all these discrepancies, instead of pointing
out the uselessness of the dissociation theory on this basis
he tried to meet the irregularities by the assumption that
the dissociated and undissociated molecules are hydrated
in solution. It is true that the solute molecules are
hydrated in solution as electrolyte or non-electrolyte but
not necessarily in the way in which he has taken them
to be.
Mendeleeff 17 has regarded solutions as strictly definite
chemical combinations which may be formed at tempera-
tures higher than their dissociation temperatures and at
ordinary temperatures, and stated that results of deter-
minations of osmotic pressure, isotonic coefficients, vapour
pressure of weak solutions, molecular depressions and
electrical conductivities could not show the methods of
hydration of the substance dissolved in water. Bates's
results give indirect support to the above. Hydration is
usually considered as a cause of increasing the osmotic
pressure of the freezing point lowering of a solution
largely by lessening the amount of "free" water in the
8
1 14 ASSOCIATION THEORY OF SOLUTION
solution but his figures show that hydration has some
effect besides that due to the removal of "free" water in
a solution ; his results are in harmony with the assumption
that any water or solvent present in the field must be in a
state of combination with solute molecules in a ratio
identical with the dilution.
In a solution the solute and solvent molecules are all
combined and none of them are free. The ratio of their
combination is same as their dilution. If any one
portion of them are removed different compounds are
formed with different properties. No dissociation takes
place in electrolytes as professed by advocates of electrolytic
dissociation theory. Solutes have a tendency to increase
their combination, if possible, with more molecules of
solvent and similarly, solvents have a tendency to combine
with those of the other. The solution in an osmotic
cell consists of uniform compounds of solvent and solute,
and each such associated molecule tries to unite with an
additional molecule of solvent through the membrane ;
the pure solvent in the outside also tries to combine with
solute molecules. If the membrane is semipermeable the
solute molecules will not pass out whereas the solvent
molecules will permeate through and increase the volume
or increase the pressure inside the cell in the case of
restricted volume. Such pressure is called osmotic
pressure.
Osmotic pressure is directly proportional to the
concentration because this phenomenon depends on the
number of associated molecules that come in contact per
unit of area of membrane and attract the solvent
OSMOTIC PRESSURE OF SOLUTION 115
molecules from its outside. The irregularities that are
observed occasionally in concentrated or dilute solutions
are due to the difference in affinity for pure solvent mole-
cules of different associated molecules of solute with
solvent. Affinity for combination with another molecule
of water by CuSO 4 ,10H 2 O and by CuSO 4 ,llH 2 O may
not be the same. Irregularity of the first law of osmotic
pressure is due to this cause and may not be due to any-
thing else.
It has also been found in many cases that the osmotic
pressure is directly proportional to the absolute temperature.
It is commonly known that the rate of chemical reaction
is greatly increased by the rise of temperature and it is
also similarly known that the solvent or solution property
is also considerably increased similarly. Osmotic pressure
being an index of the affinity between solvent and solute
for their combination it is quite rational that this property
would also increase with temperature. The increase of
osmotic pressure consequent on the rise of temperature has
been ascribed by Van't Hoff to the increased kinetic move-
ment of the solute molecules only. This, however, need
not be admitted by the association theory of solution,
because Van't Hoff's law indirectly presumes that the
solute and solvent molocules are not in a state of com-
bination but exist separately somewhat like a mechanical
mixture in solution.
Solutions of equimolecular concentrations have almost
the same osmotic pressure in the case of non-electrolytes,
the affinity of a molecule of cane sugar for a certain
number of water molecules is almost the same as that of
Il6 ASSOCIATION THEORY OF SOLUTION
any other non-electrolyte producing solute for the same-
number of molecules, so long as other conditions of the
solutes are the same. In the case of electrolytes, how-
ever, such corresponding affinities differ from any non-
electrolytes but will agree amongst themselves in many
cases. Irregular osmotic pressure of electrolytes has
been ascribed to the splitting up of the molecules into
ions with dilution thereby changing the number of impacts
on the walls of the vessel. The association theory of solution
ignores such explanations and considers that the variation
of osmotic pressure with dilution in the case of eloctro-
lytes is due to the variation of affinity of the solute-
molecules to combine with various number of solvent
molecules.
The evil effect of analogy of osmotic laws with the
gas laws reached its climax when Nernst 18 stated that the
osmotic pressure is independent of the nature of the
solvent. He mathematically treated the problem and
concluded that by dissolving the "same quantity of iodine
in a litre of water as in a litre of carbon disulphide", the
osmotic pressure obtainable would be the same in two
solutions Unfortunately he did not quote any experimental
figures to support this view. It would have been very
convenient if reliable figures were available to deal with
in this connection. The association theory of solution
does not consider it necessary that a fixed quantity of
solute will have the same osmotic pressure when dissolved
in different solvents to produce solutions of the same-
volume. Osmotic pressure in each case will depend on the
affinity of the associated molecules inside the cell, for the-
OSMOTIC PRESSURE OF SOLUTION
117
pure solvent kept outside, In any case, however, Nernst's
above statement needs modification as the concentration
will be different if a definite weight of the substance be
dissolved in a litre of each of the solvent since the con-
tractions in solution are not the same under the circum-
stances. Besides, if the osmostic pressure becomes
independent of the nature of the solvent it becomes in-
consistent with other properties of a solution , -electrical
conductivity 19 , viscosity, specific rotation etc., are depend-
ent on the nature of the solvent. Walden obtained the
following variation in molecular conductivities of ex-
tremely dilute solutions of tetraethyl ammonium iodide for
23 different solvents at 25C.
Acetone 225
Acetonitrile 200
Acetyl chloride 172
OPropionitrile 165
Ethyl nitrate 138
Epichlorohydrine 66'8
Ethyl alcohol 60
Benzenitrile 56*5
Furfurol , 56
Diethyl sulphate 43
Nitrobenzole 40
Methyl Alcohol 124
Nitromethane 120
Methyl rhodanide 96
Ethyl rhodanide 84'5
Acetyl acetone 82
Acetic acid hydride 76
Benzylcyanide 36
Asymmetric ethyl
sulphate 26'4
Ethyl cyanacetate 28'2
Salicylaldelyde 25
Anisaldehyde 16'5
Water 112*5
Instances of the influence of solvents on the rotation
of optically active compounds have been worked out by
Jl8 ASSOCIATION THEORY OF SOLUTION
Patterson 20 and specific rotations of oil of turpentine and
ethjl tartrate are very interesting in this connection.
Considerable confusion has been introduced by th&
topics of ideal and non-ideal solutions in connection with
osmotic phenomenon of solutions and it seems worth
while discouraging any such analogy with gas laws be-
cause no advantage could be gained now. Bancroft 21 has
shown that even when the solutions are very dilute gas-
laws are not nearly obeyed, if marked heat effects ac
company the admixture. When heat is evolved on mixing,
the osmotic pressure is considerably greater than that
calculated on the basis of the gas laws j when heat is
absorbed, the osmotic pressure is considerably less than
that calculated as before. No one has been able to explain
this statement on the basis of the bombardment view of
osmotic pressure, the bombardment being due to the mole-
cules of the solute.
Several theories have been put forward to explain the
osmotic phenomenon, merits and demerits of which have
been carefully discussed by A. F. Findlay 22 and it is not
necessary to repeat them here. Evidences 23 seem to be
more in favour of the theory which directly, indirectly, or
partially accepts the association of the solvent and the
solute in a solution.
Attempts have been made to establish relationship 24
between osmotic pressure, reduction of the freezing point
and electrical conductivity ; and it has been found that
the amount of dissociation of different salts into their
ions in dilute solutions when calculated from these differ-
ent methods, did not always compare well. The numbers
OSMOTIC PRESSURE OF SOLUTION 119
obtained by these methods are in fair agreement in the
cases of potassium and ammonium chlorides, calcium
nitrate and potassium ferrocyanide. With magnesium
sulphate, and the chlorides of calcium, lithium, strontium,
and magnesium, the agreement is by far no means a satis-
factory one. Thus these results do not help the dissocia-
tion theory in any way and the phenomena seem to have
nothing to do with the theory ; on the contrary, however,
all these facts may be easily explained by the association
theory of solution.
For the purpose of comparison of osmotic properties of
substances, data with solutions in molecular ratios of solvent
and solute are not available. Unfortunately, investiga-
tors determined osmotic pressures of solutions, containing
varying weights of solutes in a litre of solution. Such
figures are not, however, very useful in a comparative
study in establishing relationship between the solvent and
the solute. But, if the osmotic pressures of aqueous
solutions of potassium nitrate and cane sugar (Landolt-
page 1422) be compared, it will be of interest to find that
while the sugar solution increases in osmotic pressure
proportionately with increased concentration, that of
potassium nitrate behave in an entirely different way.
Column 4 of the following table shows the difference of
osmotic pressure of the two substances of same molecular
dilution. Admitting the accuracy of determinations, the
difference would have been highest at the highest dilution,
according to the dissociation theory of breaking up of
KNO 3 ~K4~NO 3 . Thus the figures do not support the
hypothesis very much.
ASSOCIATION THEORY OF SOLUTION
1
2
3
4
Molecules
per litre.
KNO 3
(Pressure in
atmospheres)
Sugar
(Pressure in
atmospheres)
Difference of
cols. 2 and 3.
C'0125
0-466
0-3176
+ 0-1484
0-0250
0-890
06350
+ 0-2550
0-0500
1-560
1-2700
+ 0-2900
o-iooo
2-390
2-5400
-0-1500
0-1330
2-870
33790
-0-5090
0-2000
4-500
5-0820
-0-5820
Association theory of solution would explain the
phenomenon in its own way as being due to the compounds
formed with potassium nitrate at those dilutions, which
have their affinities to form compounds with larger
number of molecules of water and such affinities are
partially expressed in terms of those pressures.
REFERENCES.
(1) Abbe' Nollet, Historic de P Acad. Roy. des
sciences, 1748, 101.
(2; Parrot, Gibl. Ann., 51, 318.
(3) Dutrochet, Annales chin. Phys., Vols. 35, 37, 49,
51.
(4) Vierordt, Pogg. Ann., 1848, 73, 519.
(5) Graham. Phil. Trans., 1854, 144, 117.
(6) Traube, Archio. f. Anat. Und Physiol., 1867, 87.
(7) Van't Hoff, Zeit. Phys. Chem., 1887, 1, 481 ; Phil.
Trans., 1888, 26, 8.
OSMOTIC PRESSURE OF SOLUTION 121
(8) Walden, Zeit. Phys. Chem., 1892, 10, 699 :
Tammann, ibid., 1892,9, 97; 10; 255; Neerburg, 1893,
11, 446 ; Quincke Annalen der Physik., 1902, (4), 7, 681 ;
Kohlenberg, ibid., 1900, (4) 3, 578 ; Jour. Physical. Chem.,
1906, 10, 141 ; Ponsot, Compt. rend., 1898, 125, 867 - 9
1899, 128, 1447.
(9) Pfeffer, Osmoticsche Untersuchungen, 1877.
(10) Morse, Amer. Chem. Jour., 1911, 45, 558.
(11) Morse, Amer. Chem. Jour., 1901, 26, 80 j 1902,
28, 1 ; 1903, 29 137 ; 1904, 32, 93 -, 1905, 34, 1, 39 ;
1907, 37, 324, 425, 558 ; 1907, 38, 175 ; 1908, 39, 667 ;
1908, 40, 1, 194, 266, 325 ; 1909, 41, 1, 92, 557 ; 1911,
45, 91, 237, 283, 517, 554 ; 1912, 48, 29.
(12) Frazer and Myrick, Amer. Chem. Jour., 1916, 38,
1907.
(13) Berkeley and Hartley, Phil. Trans., 1906, A, 266,
486.
(14) Nernst, Theoretical Chemistry 8th 10th Edition.
English translation, 1923, 151, 159,
(15) Bates, Jour. Amer Chem. Soc., 1915, 37, 1421-
1445.
(16) Schay, Zeit. Phys. Chem., 1923, 106, 378.
(17) Mendeleeff, Jour. Chem. Soc., 1887, 778 ; Chem.
Soc. Abst., ii, 1890, 326 ; Kakshit, Zeit. Elektrochem.,
1925, 325.
(18) See 14.
(19) Walden, Zeit, physik. chem., 1906, 55, 207 ; 1910,
73, 257 ; 1902, 39, 525.
(20) Patterson, Trans. Chem. Soc. 1901, 79, 169, 477 ;
1902, 81, 1097, 1134.
122 ASSOCIATION THEORY OF SOLUTION
(21) Bancroft, J. Physical. Chem., 1906, 10, 322.
(22) Findlay, Osmotic Pressure, 1919, 94-106.
(23) M. Traube, Bull. Soc. Chim., 1911, (4) 9, 857 ;
Tammann, Annalen. d. Phys., 1900, (4) 3, 578 ; Bouty,
J. de Physique, 1895, (3) 4, 165 ; Walden, Theorien der
Losungen (Ahreno'oehe sammlung) I. Traube, Ber., 1884,
17, 2294 ; Phil. Mag., 1904, (6) 8, 704; Pfluger's, Archiod
Physiologic, 1904 ; Kahlenberg, J. Physical chem., 1906,
10, 141 ; Jones, Carnagie. Inst. Publications, 1907, No. 60.
(24) Van't Hoff and Eeicber, Zeit. Phys. Chem., 3. 198.
CHAPTER VII
THERMAL EFFECTS OF SOLUTIONS
Studies in the disturbance of thermal equilibrium by
the solution of a substance in a solvent have been thought
to be more carefully applied than that is ordinarily done
in disentangling theories of solutions. Being carried
away by the electrolytic dissociation theory of solution
the most valuable results of Thomson have not been
sufficiently treated by the ordinary text-book writers to
impress on juvenile minds the correct nature of solutions
as could be deduced from the classical researches of the
celebrated Danish Chemist. In a short treatise like this
it would not be possible to discuss his entire results but it
is desired to take up enough data which would be fairly
helpful in explaining the following phenomena in the
light of the association theory of solution :-
(1) Heats of solutions and dilutions or hydrations,
(2) Specific heats of solutions.
(3) Freezing points of solutions.
(4) Vapour tensions and boiling points of solutions.
As early as 1840, Hess 1 declared the law of thermo-
neutrality for all chemical processes by clearly stating
124 ASSOCIATION THEORY OF SOLUTION
that when the same chemical change takes place between
definite amounts of substances under the same conditions,
the same amount of heat is always given out, provided that
the ultimate products are the same; and this law
gradually helped a good deal in the formation of the "Law
of Conservation of Energy." In studying thermal effects
of solutions the law will always be applied and it will be
necessary to remember that the differences of energies
between two identical conditions of the system must be the
same, irrespective of the method by which the system is
transferred from one condition to the other.
In measuring the thermal effects of solution, calories
(cal.) are used as units but sometimes larger units are
used, K = 100 calories and Cal. = 1000 calories. The last
one is now considerably used. The terms used in this
connection may be defined as below :
1,1) Heat of solution of a substance is the thermal
effect produced by dissolving one gram molecule of a
substance in a given number of molecules of solvent
(2) Heat of dilution of a solution is the thermal
effect produced when the quantity of solution containing
one grammolecule of a solute is further diluted by a
given number of molecules of solvent.
(3) Heat of hydration is the thermal effect produced
by the combination of one grammolecule of substance
with a definite number of molecules of water to form a
definite hydrate.
Thomson determined heats of solutions of several
substances from which the following figures 2 are taken,
which are true at about 18C.
THERMAL EFFECTS
125
HEATS OF SOLUTION.
fa) Compounds of non-metals.
1. Gases.
Substance.
Molecular
Molecules
Heats of solution
(Gaseous).
formula.
of water
in colories of one
in the
gram molecule of
solution.
the substances.
Hydrogen chloride
HC1
300
17,315
Hydrogen Bromide HBr
400
19,940
Hydrogen Iodide
HI
500
19,210
Ammonia
Ntf 3
200
8,430
Sulphur Dioxide
SO,
250
7,700
Carbon Dioxide
CO 2
1500
5,880
2. L
iquidp.
(Liquid)
Sulphur Dioxide
SO 2
300
1,500
Sulphuric Acid
H 2 SO 4
1600
17,850
Sulphuric Acid
hydrate
H 2 SO 4 ,H 2 O 1600
11,470
Nitric Acid
HN0 3
300
7,480
Phosphoric Acid
H 3 P0 4
200
5,350
Phosphorous Acid
H 3 P0 3
120
2,940
Hypophosphorous
Acid
H 3 P0 2
200
2,140
Formic Acid
CHoOo
200
150
Acetic Acid
C 2 H 4 2
200
375
126
ASSOCIATION THEORY OF SOLUTION
3. Solids.
Molecular
Molecules
Heats of solution
formula.
of water
in calories of one
in the
gram molecule of
solution.
the substances.
H 3 P0 4
120
+ 2,690
H 3 PO ;J
120
- 130
(Solid)
Phosphoric Acid
Phosphorous Acid
Hypophosphorous
Acid. HgPOo 200 - 170
BoracicAcid B 2 O 3 ,3H 2 O 800 -10,790
Ammonium Chloride NH 4 C1 200 - 3,880
Ammonium Bromide NH 4 Br 200 - 4,380
Ammonium Iodine NH 4 I 200 - 3,550
Ammonium Sulphate (NH 4 ) 2 S() i 400 - 2,370
Ammonium Nitrate NH 4 NO :J 200 - 6,320
Ammonium Hydro-
gen Sulphate (NH 4 )HSO 4 200 - 20
Oxalic Acid C 2 H 2 O 4 300 - 2,260
Oxalic Acid (cryst) C 2 H 2 O 4 ,2HoO 530 - 8,590
Citric Acid C G H 8 O 7 600 - 3,600
C H 8 7 ,H 2 O 400 - 6,430
(b) Compounds of the metals ; bases and salts.
KC1
200
- 4,440
KBr
200
- 5,080
KI
200
- 5,110
KC10 3
400
- 10,040
KBrO 3
200
- 9,760
THERMAL EFFECTS
127
(Solid)
Molecular Molecules
Heats o solution
formula. of water
in calories of one
in the
gram molecule of
solution.
the substances.
KIO 3 500
- 6,780
KNO 3 200
- 8520
KoCO 3 400
+ 6,490
KOH 250
+ 13,290
NaCl 100
- 1,180
NaBr 200
- 190
Nal 200
4- 1,220
NaNO 3 200
- 5,030
Na 2 CO 3 400
+ 5,640
NaOH 200
+ 9,940
LiCl 230
+ 8,440
LiNO 3 100
-f 300
BaCU 400
4- 2,070
BaBr 2 400
-f 4,980
BaI 2 ,7H 2 O 500
- 6,850
Ba(NO 3 ) 2 400
- 9,400
BaO
+ 34,520
Ba'OH) 2
+ 12,260
Ba(OH) 2 ,8H 2 400
-15,210
SrCl 2 400
+ 11,140
SrBr 2 400
+ 16,110
Sr(NO 3 ) 2 400
- 4,620
SrO
+ 29,340
Sr(OH) 2
+ 11,640
Sr(OH) 2 ,4H 2 O ...
-14,640
CaCl 2 300
+ 17,410
128
ASSOCIATION THKORY OF SOLUTION
.Solid)
Molecular
Molecules
Heats of solution
formula.
of water
in calories of one
in the
gram molecule of
solution.
the substances.
CaBr-o
400
+ 24,510
Cal 2
400
+ 27,690
Ca,NO 3 ) 2
400
+ 3,950 '
CaO
2,500
+ 18,330
Ca(OH; 2
2,500
+ 2,790
McrClo
800
+35,920
MgS0 4
400
+ 20,280
AloC),
2,500
+ 153,690
ZnClo
300
15,630
ZnBro
400
15,030
ZnI 2
400
11,310
Zn(NO 3 ) 2 ,6H 2 O 400
-5,840
ZnSO 4
400
+ 18,430
CdClo
400
+ 3,010
CdBr 2
400
+ 440
CdL
400
-960
Cd(N0 3 ) 2 H 2
O 400
+ 4,180
CclSO 4
400
+ 6,050
MnCl 2
350
+ 16,010
Mn(NO 3 ) 2 6H 2 O 400
-6,150
MnSO 4
400
+ 13,790
FeClo
350
+ 17,200
Fe 2 CI 6
2,000
+ 63,360
CoCl 2
400
+ 18,340
NiCl 2
400
+ 19.170
CuCIo
600
+ 11,080
(Solid)
THERMAL
EFFECTS
129
Molecular
Molecules
Heats of solution
formula.
of water
in calories of one
in the
gram molecule of
solution.
the substances.
CuSO 4
400
4-15,800
T1 2 C1 2
9,000
-20,2000
T1 2 (N0 3 ) 2
COO
-19,940
T1 2 S0 4
1,600
-8,280
T1 2 O
570
-3,080
T] 2 (OH),
470
-6,310
PbClo
1,800
- 6,800
PbBr 2
2,500
- 10,040
Pb(N0 3 >>
400
-7,610
SnCl 2
300
300
SnCl 4
300
29,920
HgClo
300
-3,300
Ag 2 (N0 3 \j
400
-10,880
Ag 2 S0 4
1,400
-4,480
AuCI 3
900
4,450
AuBr 3
2,000
-3,760
From such results Thomsen concluded along with
others that ;
(a) The heats of absorption of gaseous substances are
always positive, on account of the gases changing their
states to liquids in addition to any, often occuring
secondary reactions.
(6) The heats of solution of liquids are positive.
(c) The heats of solution of solids very widely depend-
ing on the nature and compsition of each,
130 ASSOCIATION THEORY OF SOLUTION
It has been said 3 that the molecules of the solveut and
the solute form a homogeneous solution where the mole-
cules acquire an uniform motion ; but such an equalisation
of molecular motions, retaining the acquired momentum,
must result in a development of heat.
There seems to be no sufficient reason why heat should
be produced as a result of the equalisation of molecular
velocities since no work is done in any way according to
this assumption. Only two energies are added to form an
average. On the contrary it could be concluded that the
evolution of heat is an indication that the solvent and the
solute react to produce solution, which naturally forms a
support of the association theory of solution.
Thomson 4 showed some regularity in hoats of
solutions of halide salts but that is not very reasonable
since figures of different dilutions have been used for
the purpose of comparison and such salts produce heats
of dilution.
Thomsen also tried to show the dependence of the heat
of solution on the molecular weight of the substance but
no mention has been made about heats of ammonium salts
( AmCl = - 3880 ; AmBr = - 4,380 ; Ami = - 3,550),
which go against any such ganeralisation.
Heats of solution of sparingly soluble or insoluble
substances have been determined by Thomsen 5 by an
ingenious method. He found that the heats of neutrali-
sation of equivalent quantities of aqueous solutions of
bases of alkalies, alkaline earths and other oxides were
constant for the same acid, but there was very much
bigger thermal effect when the salt formed is partly or
THERMAL EFFECTS 131
wholly precipitated from the solution simultaneously. He
presumed that the degree of solubility of the compound
would not influence the true heat of neutralisation and
that the increased evolution of heat is due to the heat of
precipitation of the substance. His results of a number of
specially devised experiments are given below :
Substance, Heats of solution
PbCl 2 - 6,800 c
PbBr 2 -10,040
PbI 2 -15,970
T1C1 -10,100
TIBr -13,750
Til -17,850
AgCl -15,740
AgBr -20,100
Agl - 26,410
The figures for heats of solution have been utilised for
the estimation of heat of hydration of salts. The
difference between the heats of solution of the anhydrous
salt and hydrated salt gave the heat of hydration. The
heat of hydration may be due partly to the affinity of the
salt for water and partly to the latent heat of water, since
water molecules change their state of liquid aggregation
to become the constituents of a solid body. The heat of
hydration varies with the nature of the salt and with
the number of molecules of combined water. The
following figures of heats of hydration of hydrated
crystals are due to Thomsen and refer to a temperature
of about 18C.
ASSOCIATION THEORY OF SOLUTION
Heats of total Hydration.
MsCI 2 ,GH 2
32,970 c
BaBr 2 ,2H 2
9,110 c
SrBr 2 ,6H 2
23,330
BaCl 2 ,2H 2
7,000
SrCl 2 ,6H 2
18,640
CuC! 2 ,2H 2
6,670
CaCI 2 ,6H 2
21,750
AuCl 3 ,2H 2
6,140
CoCL,6H 2
21,190
SnCl 2 ,2H 2
5,720
NiCJ 2 ,6H 2
20,330
NaI,2H 2
5,230
NaiPfcClo^HoO
1 ),170
NaBr.2B 2
4,520
Na,PtBr fi ,0H 2
18,540
CdCl 2 ,2H 2
5,290
KoMgiS0 4 ) 2 6H 2
20,620
Na 2 HPO 4 ,12H 2
28,470
K.2fo(SO 4 ) 2l 6H 2
19,810
Na 4 P,0 7 .10H 2
23,520
K>Cu(SO 4 \>,6H 2
22,970
Na 2 C0. 3 ,10H 2
21,800
MnCI 2 ,4H 2
14,470
Na 2 SO 4 10H 2
19,220
e,CU,4H>()
15,150
MgSO 4 ,7H 2
24,080
CdBr 2 ,4H 2
7,730
ZnSO 4 ,7H 2
22,690<
K>Mn(SO 4 ) 2 ,4H 2
12,820
CuSO 4 ,5H 2
18,550
Ca(N0 3 > 2 ,4H 2 o
11,200
MnSO 4 ,5H 2
13,750
Sr(NO 3 )a,4H 2
7,680
Na 2 S 2 G ,2H 2
6,280
Li 2 SO 4 ,H 2
2,640
The above numbers give the heats of total hydratioa
and Thomsen also measured heats of hydration of partially
hyd rated salts of salts hyd rated with lesser number of
molecules of water than it can form at other conditions.
He placed the finally powdered weighed salt upon a flat
platinum plate continued drying for a considerable period
at a constant temperature, weighed from time to time to
control the progress of dehydration, then removed from
the drying apparatus when the weight showed that the
required number of molecules of water had been driven
THERMAL EFFECTS
133
off, the salt analysed, and then the heat of solution per
gram molecule in 400 gram-molecules of water measured
at the usual temperature of about 18c. Although it is
difficult to suggest what other better method there could
be to perform such elaborate experiments, yet it is quite
clear that the samples of salts hydrated with varying
numbers of molecules of water may contain an admixture
of the same salt hydrated with different numbers of mole-
cules of water, keeping of course the average water content
correct. Heats of solution of salts, whose various
hydrated crystals are definitely known and of a few others
of interest are given below :
Heats of
Solution.
CuSO 4
+ 15,800 C
ZnSO 4
+ 18,4300
,H 2
+ 9,330
,H 2
+ 9,950
,2H 2
+ 6,160
,2H 2
+ 7,670
,3H 2
+ 2,830
,3H 2
+ 5,270
,4H 2
+ 630
,4H 2
+ 3,500
,5H 2
- 2,730
,5H 2
+ 1,300
,6H 2
- 840
,7H 2
- 4,260
MgS0 4
+ 20,280
MnS0 4
+ 13,790
,H 2
+ 13,300
,H 2
+ 7,810
,2H 2
+ 11,050
,2H 2
4- 6,240
,3H 2
+ 7,490
,3H 2
-f- 4,150
,4H 2
+ 4,240
,4H 2
+ 2,240
,5H 2
+ 2,010
,5H 2
+ 40
,6H 2
- 100
,7H 2
- 3,800
134 ASSOCIATION THEORY OF SOLUTION
CaCl 2
+ 17,410 C. 2K 2 CO 3 +12,980
,1-67H 2
+ 10,800 ,H 2 + 8,560
,1'98H 2
+ 10,036 ,3H 2 - 760
,2-75H 2
+ 6,927
,3-49 H 2
+ 3,752
,3-76 H 2
+ 2,971
,6-07 H 2
+ 4,340
MgCJ 2
+ 35,920 C
,3-03 H 2
+ 14,871
,4'51H,0
4- 8,360
,4-61 H 2
+ 7,731
,5-05 H 2
+ 6,181
,611H 2
+ 2,950
Na 2 CO,>,
+ 5,636
" ,H 2
+ 2,254
,2H 2
+ 43
,3H 2
- 2,067
,4H 2
- 4,202
,5H 2
- 6,638
,6H 2
- 8,412
,7H 2
-10,765
,8H 2
-12,623
,9H 2
-14,387
,10H>()
-16,160
Na 4 P 2 O 7
+ 11 ' 850 2470 = 1x2470
'2H0
+ 7*030 2,350 = 1x2,350
'sH.O
,10H 2 O
'' 6,980 = 3x2,327
-11,670 ^20-5x2,344
THERMAL EFFECTS 135
In reviewing the results of determination of heats of
hydration of salts with an even number of molecules of
water Tbomsen 6 considered that the thermal effects corres-
ponding to the addition of the individual molecules of
water are far more uniform on account of molecules of
water symetricall}' arranged around the nucleus of the
salt, and in the case with others containing an uneven
number of molecules of water the addition of the first
molecule of it being attended with a considerable thermal
effect, produces a disturbance in the symmetry of the mole-
cule. It is rather difficult to give full support to this view
because there is nothing to show that the thermal effect
is the only indication how each of the individual molecules
of water is linked with the molecule of the substance. It
is also not possible to say that the thermal effect is a
measure of strength of the linkage between the water and
the substance, s ; nce there are some hydrates or partial
hydrates which on addition of the last molecule produces
a negative thermal effect. This negative effect would then
mean that the final molecule of water would reduce the
force of linkage between the substance and the other water
molecules or the final molecule could not be attracted on
account of this having a negative thermal effect, the force
of linkage would be negative or in other words there
would be repulsion.
According to Thomson's figures it is evident that the
individual molecules of water in the hydrated salts are
bound with unequal strength if thermal effect is a function
of the binding force. Sodium phospate is the only excep-
tion to have all 10 molecules of water bound in the same
136 ASSOCIATION THEORY OF SOLUTION
manner with a strength corresponding to about 2352 e
for each gram-molecule of water. Unfortunately,
however, the process by which the hydrates of
substances were prepared are not beyond doubt and it
is not improbable that the different hydrates with which
he experimented were not free from contamination with
other hydrates.
Hydrate formation does not seem to be quite allied to
the chemical property of a substance. Sodium carbonate
and potassium carbonate, though extremely chemically
alike, do not combine with similar number of water
molecules. Heats of hydration do not indicate the capacity
of the salt for combining with water since potassium
carbonate though liberates more heat (8,560 c) in combin-
ing with the first molecule of water than that does sodium
carbonate (2,254 c) under the same circumstances, the
former could not combine with correspondingly higher
number of molecules of water. The chlorides of calcium
and magnesium have very large heats of hydration, 21,750
c and 32,970 c respectively. But these can only combine
with 6 molecules of water whereas there are many others
which have much lower heats of hydration yet they could
easily retain much larger number of molecules of water in
fiolid crystalline state.
It is thus seen that the similarly in chemical nature
has neither much relationship with the heats of formation
of hydrates nor does it suggest much about the number of
water molecules with which combination would take place :
there is also hardly much justification in drawing generali-
sations about hydrated substances from their heats of
THERMAL EFFECTS 137
hydration. On the contrary, however, it is undeniable that
as the salts gradually get hydrated, many other properties
along with the thermal effects get changed. The force
with which the substance and water are combined is not-
only expressed in terms of thermal effects but also in
terms of several other forms of energy which may manifest
simultaneously. When a substance combines with water
the resultant product differs from the original two in
volume, general, thermal, optical, electrical, etc., properties.
Assuming energy is indestructible it may be concluded
that when heats of formation of hydrate is negative the
energy is getting transformed into some other form to an
equivalent amount. It is unnecessarily thought by several
authors that the magnitude of thermal effect is the sole
index of the force with which the water molecule is bound
with the molecule of the substance,
Although it is not quite within the scope of this book
to discuss the inner structure of the anhydrous or the
hydrated molecule, yet, it may not be out of place to say
'that the different water molecules may be differently placed
with respect to the different atoms of the substance, and
'manifestation of variation of different properties with
variation of hydration is a result of the position of water
with respect to the different atoms composing the molecule.
It may be possible that by gradual addition of different
water molecules to a substance for the purpose of hydration
a readjustment in equilibrium of all kinds of energy takes
place to establish final equilibrium in the last stage of the
compound. Or in other words thermal effects of hydration
oay be positive or negative, according as the sum total of
138 ASSOCIATION THEORY OF SOLUTION
other properties, optical, electrical etc., taking place
simultaneously.
Heat of solution is unavoidably connected with heat
of dilution and it is impracticable to determine heat of
solution alone. But considerable light could be thrown
on the subject also if heats of dilutions were known. As
will be seen, heat of dilution depends (1) on the nature of
the dissolved substance, (2) on the amount of water present
and (3) on the temperature of the experiment. Thomsen 7
considered that when different hydrates are formed in
aqueous solutions the change of thermal effect with the
amount of added water must show certain fixed points
indicating their formation, and otherwise the thermal
effect must vary as a regular and continuous function of
the amount of water. This, however, does not seem to be
a reasonable argument since fixed points are not always
found in cases with even solid definite hydrates which are
well isolated. Absence of fixed points in the heats of
dilution is not, therefore, a proof of the absence of hydrates
in solution because more than one hydrate may occur
in a solution according as water is available for the
purpose.
Variation in heats of dilution supports the idea of
reaction taking place at each dilution. The solvent and
the solute do not form a mechanical mixture ; dilution is
attended with changes in different forms of energies which
would ordinarily take place when a chemical reaction
takes place. The following figures of heats of dilution
determined by Thomsen would give considerable support
to the association theory of solution :
THERMAL EFFECTS 139
4 HN0 3 H 8 PO 4 HC1 HBr HI
c c c c c
1 6,3793,2851,741(5,375) - 172 -152
2 9,418 11,365 (13,860) (12,540) 167 -156
3 11,137 5,710 3,298 13,362 15,910 14,180 -~
4 _ _ _ _ _ _ __ -111 _
5 13,108 6,655 14,959 17,620 17,380
O __ _ _ _ _ _ ___ __ ') _
10 7,318 16,157 19,100 18,580
19 16,256 _^____
20 7,4584,93816,756 19,470 18,990 4-173-3307
49 16,684 _ __ _
50 5,16917,115 19,820 19,140 126 -f-278-3452
99 16,858 - ____._
100 7,4395,26917,235 19,910 19,180 148 +335-3516
199 17,065 - ______
200 5,355 149 +375 -3566
'^00 __ _ _ _ _ __ _ _____ _ _
320 7,493 ______
400 __ ._ _ -3600
500 19,940 19,210
For the determination of the following heats of dilution
of caustic alkalis Thomsen used their solutions in &
molecules of water originally and finally diluted them
up to 200 gram-molecules of water j m = water oE
dilution.
140 ASSOCIATION THEORY OF SOLUTION
Heats of dilution of caustic alkalis.
Gram molecules
of total water Heats of dilution in c,
present in
solution (m + 3)
KOH,(m 4- 3)H 2 O NaOH(m+ 3)H 2 O
5 +1,496 +2,131
7 +2,095 +2,889
9 +2,364 +3,091
20 +2,678 +3,283
25 - +3,286
50 +2,738 +3,113
100 +2,748 +3,000
200 +2,781 +2,940
A concentrated solution of ammonia, NH 3 ,3'2H 2 O
developes respectively +324, +350, and +380 c on dilu-
tion with 15, 25, and 50 gram molecules of water.
Heats of dilution of the following salts of representa-
tive nature were studied by Thomson, which already con-
tained n molecules of water, with the addition of m mole-
cules of water,
Heats of salt solutions,
(a) Heats of solution and dilution are positive.
Ou(NO 8 ) 2 2(NH 4 C 2 H 8 0,) ZnCl, CuCl a
ri12 n = 10 n=4 n=5 n10
10 - +1,088 +1,849 -
15 +262 +744
THERMAL EFFECTS 141
n + m
Mg(NO s ),
Cu(N0 3 ) 2
2(NH i C 8 H 3 (
) f ZnCl,
CuCl,
n-12
n-10
n 4
n5
n = 10
20
+ 412
+ 940
+ 1,800
+ 3,152
+ 1,630
30
-
+ 2,458
50
404
+ 904
+ 2,584
+ 5,317
+ 3,336
100
+ 364
+ 776
+ 2,988
+ 6,809
+ 4,052
200
+ 370
+ 729
+ 3,250
+ 7,632
+ 4 510
400
+ 421
+ 3,432
+ 8,020
(b) Heats of solution and dilution are negative.
n + m 2NaCl 2NaNO : , 2NH 4 NO, (NH 4 ) 2 C 4 H 4
n = 20 n 12 n = 5
10
_.
-1,282
-
15
20
_
-2,518
30
-296
50
-2,262
-648
100
-1,056
-3,288 -
- 4,584
-1,014
200
-1,310
-3,860 -
- 5,018
-1,242
40
- 1,410
-4,192
- 5,288
- 1,358
(c) Heats
of solution and of dilution are of
opposite sign.
n + m
Na 2 SO 4
Na 2 CO :5
K 2 CO B KHS0 4
n50
n = 30
n = 10
n = 20
50
-566
-122
-64
100
-655
-1,190
-406
-30
200
-1,132
-1,601
-598
+ 108
400
- 1,383
-749
+ 382
800
-1,483
_..
+ 766
142 ASSOCIATION THEORY OF SOLUTION
Thomsen tried to apply these figures only in putting
forward suppositions regarding the formation or existence
or non-existence of hydrates in solution. It has already
been pointed out that there is poor justification in making
such suppositions. If hydrates are formed by the associ-
ation of solvent and solute the reaction would naturally be
followed by manifestations of readjustment of all forms
of energies that remained latent in the components before
the reaction. Thermal form of energy is not the only
one that gets disturbed. The algebraic sum of all the
the energies must be the same all along. These figures
however clearly prove that each time a solvent is intro-
duced or withdrawn a redistribution of heat energy takes
place thus indicating a reaction between solvent and
solute.
According to the association theory of solution, the
combination of solvent and solute is dependent on the
molecular ratio in which they are present in solution and
each such reaction is frequently complete simultaneously
as the solution is complete. Manifestations of different
forms of energy that follow each reaction are dependent
on the nature of each particular compound or association of
solute and solvent. It is not necessary that manifestation
of any particular energy measurable after any such parti-
cular reaction should always be followed or preceded by
mathematically proportionate liberation or absorption of
the same energy as a result of a similar reaction. The
distribution of energy consequent on the formation of an
association of solute and solvent depends on the position of
the lastly added solvent molecule with respect to the
THERMAL EFFECTS 143
components of the solute and vice-versa, keeping, of
course finally the sum of energies in all forms present
in the latent form constant before and after the
reaction.
The evolution of heat due to mixing up of one gram-
molecule of sulphuric acid with increasing amounts of
water shows the rate of increase of thermal effect much
higher up to 19 molecules of water than that with more
water. Thermal effect on dilution of nitric acid with
water reaches its maximum when there are 20 molecules
of water to 1 molecule of acid, and then it falls to rise
again when 320 molecules of water are added. The
thermal effect of dilution does neither bear anjr striking
similarity with other chemical properties nor with the
molecular weights. In the cases of the halogen acids the
thermal effects of hydrobromic acid are the highest instead
of being an intermediate one. In reviewing the thermal
effects of acetic acid Thornsen 8 on the ground of rise of
the figures from negatwe to positive considered that the
formation of hydrate in solution is improbable. There is
absolutely no reasoq to consider such deduction to be
correct, since such variation suits well with the associa-
tion theory of solution.
It is obvious that the solid salts melt and simultaneou-
ously or subsequently pass into solution including the
heat of fusion in the heat of solution in the first stage.
By extrapolating the curve of molecules of water used in
dilution an imaginary figure for heat liberated in calories
may be obtained when there is no water and such points
would indicate the apparent heat of fusion. The following
144 ASSOCIATION THEORY OF SOLUTION
results were obtained by extrapolating Thomson's
figures :
Substance. Heat of fusion.
NH 4 NO 3 ... .., + 450 c
Mg(NO 3 ) 2 ... ... - 500
Cu(NO 3 ) 2 ... ... -1400
NH 4 q>H 3 O 2 ... ... 4- 150
CuCJ 2 ... ... - 700
ZnCJ 2 ... ... +300
It would be of considerable interest to know how such
figures compare with those that could be actually obtained.
A solid passing into the liquid state suffers change in
molecular vibration and it is quite important to settle if the
solid must become liquid in order to mix with a solvent
to form a solution. It has not been properly dealt with,
whether the solid molecules become liquid before passing
into solution or only get covered with layers of solvent
molecules, and remain still solid in the centre of the
outter sphere of the solvent. Some of Thomson's figures
may be utilised for the purpose of elucidating whether the
solute molecules in solution are present as liquid or as any
other state, gas or solid, at which the pure substance
would have been, under the same conditions. He 9 deter-
mined the following heats of solution of the same subs-
tance at different states :
SO, NH 8
Heats of solution of gas 7,700e 8,4-30
Heats of solution of liquid 1,500 3,400
Heats of liquefaction of gas 6,200 5,030-
THERMAL EFFECTS 145
H 3 P0 4 H 3 P0 3 H 3 P0 2
Heats of solution of liquid 5,210c 2,940c 2,UOc
Heats of solution of solid 2,690 -130 -170
Heats of liquefaction of solids 2,520 3,070 2,310
Although these results are in support of the assump-
tion that solute molecules remain in a liquid state in solu-
tion yet more investigation on the subject is very much
welcome.
Thomsen in Chapter VII of his book on thermo-
chemistry dealt with the influence of temperature on the
magnitude of the thermal effect of chemical process. He
considered that "the thermal effect of a chemical reaction
is not a consistant magnitude, since it is dependent not
only upon the temperature, but* also upon the state of
aggregation and other conditions under which the subs-
tance re-act, as, for instance upon the degree of dilution."
He is only partially true in interpreting his results in
this way, the correct method of explaining his results
would be different. The thermal effect due to a parti-
cular chemical reaction is always the same no matter how
the phenomenon is brought about but if, however, such
reaction takes place in a medium (of solvent) then the
thermal effects due to the interaction between solvent
and solute would operate both before and after the re-
action whose thermal effects are under observation. The
thermal effects due to the reactions between a solvent and
a solute vary always with dilution and temperature. These-
two factors control formation and stability of the associa-
tions formed between a solvent and a solute. Such
10
146 ASSOCI \TION THEORY OF SOLUTION
thermal effects are unavoidably added; to those of the
original chemical reactions ; consequently the thermal
effects due to chemical reaction taking place in solution
are not due to the primary chemical reaction only but also
due to other simultaneous actions between solvent and
solutes before and after the reaction.
Double salts have heats of formation as chemical re-
action as well as heats of solution. Thomson (ibid 327)
obtained the following figures :
Reaction Heat of formation Heat of solution.
MgSO 4 ,K 2 SO 4 3,SOO c 10,600 c
ZnSO 4 ,K 2 SO 4 4,140 7,910
CuS0 4 ,K 2 SO 4 20 9,400
MnSO 4 ,K 2 SO 4 t 9*0 6,380
MgSO 4> K 2 SO t ,C>H 2 O ' 23,920 -10,020
ZnS0 4 ,K 2 SO 4 ,6H 2 O 23,950 - 11,020
CuS0 4 ,K 2 SO 4 ,6 HoO 22,990 - 13,570
MnSO 4 K,SO 4 ,4H 2 13,810 -6,440
HgCl 2 ,2KCl,H 2 O 6,130 -16,390
HgBr 2 ,2KBr 1,230 - 9,750
HgI 2 2KI 3,040 - 9,810
SnCl 4 ,2KCl 24,160 - 3,380
SnCl 2 ,2KCl,H 2 O 4,890 -13,420
AuCl 3 ,HCl,4H 2 O 32,130 - 5,830
AuBr 3 ,HBr,5H 2 O 35,280 -11,400
When anhydrous sulphates are mixed to form double
sulphates the reaction is attended with considerable evolu-
tion of heat but when their aqueous solutions are mixed no
thermal effect is noticed. Halides, however, interact under
THERMAL EFFECTS 147
the latter circumstances with considerable evolution of
heat :
HgC1 2 Aq,2KClAq = 1,920 c
AuCl 2 Aq,HClAq =4,530
AuBr>Aq,llBrAq -7,700
On the ground that when two sulphates capable of
forming double salts mixed in their aqueous solutions do
not show any heat effect, Thomsen concluded that such
gaits do not exist in aqueous solution as double salts.
Tammann 10 tried to show from a knowledge of the
heat of solution of one substance in another that it is
possible to decide whether or not a chemical reaction has
taken place during solution. The heat of solution is made
up of quantities of heat brought about by (i) the conversion
of an anisotropic substance into an iso-tropic condition,
(ii) the mixture of the isotropic substance with solvent, and
(iii) the chimical process, such as formation of compounds,
change in molecular weights, and ionisations. He con-
sidered that dimensions of the first two quantities can be
calculated theoretically, hence from experimentally
determined heat of solution it is possible to see whether
any heat change due to third cause is contained in the
experimental value, and so ascertain whether chemical
processes have taken place. This paper, though does not
assume any antagonistic view towards the association
theory of solution which propounds invariable reaction
between solvent and solute to effect solution, does not
properly consider the manifestations of all phenomena
attended by solution. When a substance passes into
solution thernaometric measurements indicate the thermal
148 ASSOCIATION THEORY OF SOLUTION
changes in eqDilibrium but if optical, electrical, etc.,
measurements are done before and after the solution, it
would be possible to know other work done during the
process of solution.
Thus studies in thermal change alone would neither
indicate the nature nor the magnitude of the chemical 1
reaction attended by the process of solution.
Tammann's 11 theory of concentrated solution is rather
based on conclusions drawn from studies of most of the
physical properties, specific heat, viscosity, electrical
conductivity, and optical rotatary power, that ordinarily
attend any chemical change. But he realised the material
support from the fact that the specific heat of a solution
is usually smaller than that of water i contains on
account of diminution of the specific heat of water under
pressure, The theory is mainly on the behaviour of
solutions towards pressure and temperature. The
condition of a solvent under a certain pressure is the same
as that of a solution of a certain concentration. This
result lead to the conclusion that the solution of a
substance keeps the solvent under a certain pressure
thereby causing the solution to behave in a similar manner
to the solvent under corresponding pressure which varies
with the nature and concentration of the dissolved
substance.
Among the many examples that Tammann brought
forward in support of his theory, Nernst considered that
the expansion by heat of water and alcohol under
pressure, and of certain concentrated solutions of calcium
chloride in these two solvents are useful for the purpose
THERMAL EFFECTS 149
of illustration in his text book. Increase of volume due
to increase of temperature from to 50 of pure water
under pressures of 1, 1000, and 3000 atmospheres, and of
aqueous solutions containing 10,20, and 30 per cent
calcium chloride are shown in groups of curves. Similar
expansion curves of pure alcohol under pressures of 1,500,
and 1000 atmospheres and those of alcoholic solutions
8.6, 25. 1 percent calcium chloride are shown. In these
groups of curves considerable similarity has been
established between expansion by heat of pure solvent
under pressure, and of solution of certain strengths. The
internal pressure, of the solution to which the pure
solvent must be subjected in order to make its coefficient
of expansion equal to that of a solution under pressure, is
practically proportional to the concentration of the
solution up to high concentrations, but varies with the
nature of the dissolved substance. Like expansion by heat
measurements of eompressibility agreed with the value of
the internal pressure of a solution. Deviations, however,
have been found in cases of very concentrated solutions ;
and the theory does not apply in cases with dilute
solutions.
Tammann's remarkable results suit well with the
association theory of solution ; and association theory
would explain the phenomena in its own way which is of
course slightly different. Dissociation theory assumes
that a considerable portion of calcium chloride breaks up
in aqueous solution into calcium hydrate and hydrochloric
acid, thus if there is any truth in the dissociation theory
of solution the internal pressure of solvent must be due
150 ASSOCIATION THEORY UF SOLUTION
to a mixture of CaClo, Ca(OH) 2 and HC1 and not due
to CaClo molecules alone.
It has not been shown why it would be rational that a
solvent would similarly behave with respect to volume
variation under the influence of heat or pressure when
alone or in a state of solution with some solute. In
studying changes in volume of solution it has been often
presumed that the solutes maintain fixed volume, but
whatever may be the state of solute before solution under
otherwise the same conditions than that as solution, while
present in solution they may be reasonably considered to
remain there as liquid and therefore liable to be influenced
by pressure or heat in the same way as the solvent. The
volume of the solute need not be considered uninfluenced
while that of the solvent undergoing change, unless very
definite proofs are available.
When a gram molecule of calcium chloride is mixed
with successively increasing quantitives of solvents like
water or alcohol the volumes of the solute in association
with varying molecules of solvent are not equal to the
sum of the volumes of solvent and solute before the
solution. The changes of volumes consequent on solution,
which in this case of aqueous solution of calcium chloride
are contractions might be according to Tammann's theory
of solution equal to diminution of volumes of water under
the pressures like what Nernst compared in graphs. The
following figures 12 have been calculated from the molecular
contractions of calcium chloride solutions, using
coefficients of compressibility of water at 20 from
Landolt,
THERMAL 1'FFKCTS 151
% w/sv sp. fr.@20/20 3 Molecular Contrac- Coef, of Pressure
contrac- tion per compres- in solu-
tion. 100 cc. sibilty, tion in
of water, atmos- atmos-
phere phere.
xlO.
1
1-00805
27'5
0'248
45'8
54
5
1-04000
26'9
1-22
42-4
288
10
107905
258
2-37
39-9
594
30
1-22540
21'5
6-28
39-9
1,574
If, however, change in volume by heat or pressure is
due to change in the molecular vibration with respect to
its amplitude or frequency then the increasing quantity of
solvent molecules being associated with a solute molecule
can always alter the volume of the final associated molecule
of the solvent and the solute, the associated molecule
acquiring new properties.
It is also necessary to consider in this connection how
the solute and solvent molecules remain associated with
respect to their inter-molecular relationship, whether the
solvent molecules come in between any of the atoms or
groups of atoms or simply form a spherical sheath round
the solute molecule one after another in proportion to the
dilution. Tammann's theory is not only inapplicable in
the case of very concentrated or dilute solution but it
needs its applicability properly considered when the solu-
tions are attended with expansions and when the contrac-
tions rising to maximum decrease with increasing dilution.
Although from the observed contractions in solution
pressures could be calculated which would produce such
152 ASSOCIATION THEORY OF SOLUTION
Change in volume of solvent assuming the solute present
remain unaltered with the change of condition, yet it is
not proposed to entertain usefulness of such figures for
want of rationality in comparing them with those obtain-
able by changing pressure or thermal conditions of the
pure solvent, The condition of the solvent in a state of
combination with solute may not be the same as that in
uncombined state.
Association theory assumes that solvent and solute
remain in solution always in state of combination and as
soon as they are brought to a condition of unstability
they begin to form stabler compounds with different pro-
portions of the one with the other and this phenomenon
is nicely illustrated in the following experiments. 13 Bod-
lander was the first to observe that on dissolving amonium
sulphate in mixtures of alcohol and water, at certain con
centrations, the liquid divides into two well defined
layers. Traube and Neuberg found a similar behaviour
with potassium and sodium hydroxides and carbonates,
sodium phosphate and zinc and magnesium sulphates and
other salts. They therefore examined this change in the
case of ammonium sulphate under varying conditions of
temperature and concentration.
With a solution containing 340 grams of salt per
litre, 750 cc. of which is mixed with 250 cc. of alcohol
{99*6 per cent.) it is found that with increasing tempera-
ture there is in the upper layer a decrease in the relative
amounts of water and salt, and an increase in that of the
alcohol ; in the lower layer, there is an increase of water,
but a decrease of salt and alcohol. The change in the
THERMAL EFFECTS 153
composition of the lower layer is, however, so small, that
within tolerabely wide limits of temperature it may be
looked upon as constant. Keeping the temperature
constant, and increasing either the amount of alcohol
or salt in solution, it was found that in the upper
layer there is a decrease in the relative amounts of
water and salt and an increase in that of alcohol, in the
lower layer there is a decrease in the alcohol and an
increase in the salt, the water first increasing and then
decreasing. In this way the addition of 40 grams of salt
to a litre produce about the same effect as addition of 100
grams of alcohol,
Experiments with K 2 CO 3 led to similar conclusions as
those above quoted. It was not possible in either case to
determine whether the components of the layers are
present in definite molecular proportions, but this appeared
to them to be likely, especially in the case of the lower
layer., the percentage composition of which has a great
tendency to remain constant.
Similar separation of layers have been found by the
author while an excess of codeine powder is heated in
flasks containing aqueous alcohol of different strengths.
Specific heal of solutions.
Equal weights of different substances experience very
different elevation of temperature with the same quantity
of heat j the term specific heat is applied to the thermal
capacity referred to the unit of weight, which is as a rule
different for different substances. Ordinarily the specific
154
ASSOCIATION THEORY OF SOLUTION
heat of a substance is the quantity of heat absorbed by 1
gram of the substance when its temperature is raised 1C.
The specific heat of liquids often vary very much with
the temperature at which it is determined and those of
water is particularly instructive in this sense.
Specific heat of water at 15C.
1-0093
35
0-9973
70
I'OOOO'
5
1-0049
40
0-9973
75
1-0008
10
1-0019
45
0-9975
80
1-0017
15
1-0000
50
0-9978
85
1-0026
20
0-9988
55
0-9982
90
10036
25
0-9980
60
9987
95
1-0046
30
0-9976
65
0-9993
100
1-0057
Specific heats of many substances have been deter-
mined by several authors and it would not be possible to
tabulate them all here. Only a few typical instances will
be briefly quoted below from Landolt, Castell-Evans, and
Thomsen.
CuS0 4
Substance. Temperature. Specific heat. Molecular
heat.
CuSO 4
0-20
01509
24-09
CuSO 4 ,H 2 O
0-1761
31-28
,3H 2 O
>r
0-2293
49-00
,5H 2
0-2690
67-17
,50H 2
12-15
0-848
,75'4H 2 O
15-49
0-849
THERMAL EFFECTS
ZnSO A
,75-4 H 2 O
19-89
0-871
,150H 2 O
1549
0-904
,150I1 2 O
1889
0-941
,200H 2 O
1214
0-951
,200 H 2
18-53
0-9516
,400H 2 O
13-17
0-975
MgSO,
4,
25-100
0-225
,H 2 O
9
0-2400
,6H 2
9
0-3482
,7H 2 O
9
0-3610
,7H 2
20-42
0-3615
,20H 2
19-24
0-755
,24-1 H 2 O
16-48
0-751
,24-1 H 2
18-90
0-796
,50H 2 O
14-18
080-2
,50H 2
19-52
0-8672
,157-8H 2 O
15-48
0-843
,157-8L-T 2 O
19-89
0-897
,200 H 2 O
18
0-952
ZnSO 4
22-100 .
0174
,H 2
9
0-1935
,6H 2 O
9
0-2996
,7H 2 O
9
0-3257
27-1
33-21
79-66
8997
89-11
28-1
34-73
80'75
9366
'56
ASSOCIATION THEORY OF SOLUTION
Substance. Temperature. Specific heat, Molecular
heat.
,18-OtfJ 2 O
15-48
0-685
,1805H 2 O
18-90
0-738
,45-1 H 2 O
15-50
0'814
,45'1H 2 O
19-90
0-828
,50H 2 O
20-52
0-8420
,200H 2 O
20-52
0*9523
NiSO 4
NiSO 4
15-100
0-216
,6H 2 O
18-52
0-313
,50H 2 O
25-56
0-8371
,200H 2 O
25-56
0-9510
PeSO 4
PeSO 4
,3H 2
-
0-247
,7H 2
19-16
0-346
,7H 2
46-100
0-357
,200H 2 O
18
0-951
MnSO 4
MnSO 4
21-100
0-182
,5H 2 O
17-46
0-323
,5H 2
22-100
0-407
,50H 2 O
19-51
0-8440
,200H 2 O
1951
09529
33-4
82-3
96'2
27-5
77-8
THERMAL EFFECTS
157
Substance.
NaO 2 C 2 H 3 (Solid) 14-59
,3H 2 O (Solid) 0-46
,25H 2 19-52
,50H 2 O
,100H 2 19-52
Temperature. Specific heat. Molecular
0-350
0-510
0-9037
0-938
0-9687
0-965
heat.
28'7
69*4
NaoSO,
jSOi
17-98
0-2312
,18H 2 O
24-100
0-731
,40H 2
20-23
0-843
,50H 2
0-894
,65H 2 O
18
0-892
,100H 2 O
18
0-920
,200H 2 O
18
0-955
,400HoO
12-15
0-977
3284
Na 2 CO :3
,25H 2 O
,50H 2 O
,100H 2 O
,200H 2 O
1698
21-52
18
18
18
0*2728
0-8649
0-896
0-933
0*958
28'92
158 ASSOCIATION THEORY OF SOLUTION
CH 4 O (Methyl alcohol)
Substance. Temperature. Specific heat. Molecular
heat.
CH 4 5- 10 0-5901 189
120/0 6-10 1-073
20o/ 7-11 1-073
3lo/ 3.7 0-980
50o/ 0-5 0-818
50o/ 21-27 0-801
Ethyl alcohol,
G>H G O 16-30 0-602 27'7
lOo/o 18-40 1-0324
20o/o 1-0456
WO/Q T0260
4()o/ 0-9806
50o/ 0-5 0-803
5()o/o 0-15 0-992
50o/ 20-26 0-912
50o/o 0-98 0-950
Great caution is needed in utilising the above figures in
disentangling the nature of the relation between solvent
and solute in solution. For the purpose of comparison it
seems more rational and convenient to consider molecular
heats than specific heats. The following points require
study in this connection :
(1) Whether the physical condition of the solute and
solvent remain the same (solid, liquid or gas) as they
THERMAL EFFECTS 159
would have been had they remain mixed under the same
conditions.
(2) Specific heat or molecular heat of a substance
depends on its state of existence, as well as the intervals
of temperatures and pressures between which the observa-
tions are made.
(3) Influence of the ratio of the quantities of solvent
and solute on the specific heat or molecular heat of the
either.
(4) If solvent and solute are not associated in
solution the molecular heats of the solution would have
been the average of those of the pure substances under the
same condition. Basing on a very limited number of
figures of densities and specific heats of solution,
Thomsen 14 concluded that there is a close relation between
these two properties of solution ; but the corresponding
figures for methyl alcohol or ethyl alcohol would not
support this view besides it is quite rational to think that
dilution and specific heat are simultaneously connected to
density, along with other properties of solution,
optical, electrical etc. And therefore Thomsen's con-
clusion could not be considered as general, though
favourable figures were obtained in a few cases by him.
Specific heat of substances had been found to vary
with the range at which such measurements were taken ;
different quantities of heat energy will be required to
raise 1C temperature of substances starting from different
temperatures. In ordinary text books it has not been
properly discussed how far the thermometer liquid could
give correct measurement in this reepect. Whether the
l6o ASSOCIATION THKORY OF SOLUTION
expansion of the thermometer substance, which is mercury
in the case of mercnry thermometers is regular enough to
indicate the correct measurement of specific heats at all
temperatures. Often variation in the specific heat is
considered to be the indication of change in the inter-
molecular region. Addition of successive instalments
of heat increases the molecular movements of the
substance, but sometimes portions of heat are utilised in
breaking up the molecules. In this connection it may be
argued that the unaccounted for heat may have been
utilised in producing other effects, e.g. electrical, optical,
etc. of the substance 5 more investigation on this line is
needed,
Freezing paints of Solution
It is known from time immemorial that suitable
withdrawal of heat from a liquid would render it a solid
and that this solidification takes place at a reasonably fixed
temperature. If the temperature remains constant from
the commencement of solidification till the whole of the
liquid is solid then the substance is considered pure.
Glacial acetic acid solidifies at 17 but when a small
quantity of water is introduced it must be cooled down to
about 10 before the freezing starts and the same is the
case with phenol and many other substances. The tem-
perature at which solidification starts is called the freezing
point. Solutions have different freezing points than any
of its components. The effect of the presence of solute in
a solvent is to produce a depression of freezing point, a
fact first noticed and studied by Elagden 15 , who found ia
THERMAL EFFECTS l6r
the case of several substances, that the depressions of
freezing points of acqueous solutions were proportional to
the quantity of solute. Blagden found when two solutes
are present together in a solution the depression of
freezing point was equal to the sum of the effects which
would be exerted as if each of them were present by
itself. Much credit should be given to this investigator
for the accuracy of his experiments considering the time
when he performed them. In persuance of his law,,
the depression of freezing point is proportional to the
concentration, he performed a number of experiments, and
found out that the law is not absolutely true : the lowering
of freezing points of solution containing large quantities
of solute is increased more rapidly than the quantity of
substance present in solution, and sometimes increased
more slowly than that expected from the contents of
solution.
It was known from early times that in freezing, solvents
leave behind solutes in the rest of the solution and this
problem was properly raised by Rudorff 16 and Dufour,
who performed quite a large number of experiments j the
subject was further investigated by some subsequent
experiments by a few others. 17 RudorfE performed quite a
large number of experiments with potassium chloride,,
sodium nitrate and potassium carbonate, and came to the
conclusion that the lowering of the freezing-point is
proportional to the quantity of salt present in solution*
He also found that this is not the case with several other
salts, e.g. calcium chloride, barium chloride, sodium chloride-
etc. The ratio of lowering of freezing point and the?
11
l62 ASSOCIATION THEORY OF SOLUTION
quantity of salt in 100 parts of water which is constant in
the cases with former classes of salts, increases with in-
creased concentration in the cases with the latter class of
salts. Rudorff tried to explain this abnormality by the
assumption that such salts are present in a sta f e of com-
bination with the solvent and on this hypothesis he made
several calculations as to the magnitude of hydration of
salts in solution. Experiments of Coppel 18 brought addi-
tional light on the subject. His conclusions were :
(1) Blagden's law of proportionality also holds good
for supersaturated solutions.
(2) The lowering of freezing point is proportional to
the number of molecules of solute present in solution and
not to its quantity in gross weight. Solutions containing
equimolecular concentrations of salts approximately freeze
at the same temperature.
(3) Substance which lower the freezing points of
solutions, in a decreasing degree exist in solution as
several partially decomposed hydrates by the action of
water or by the lowering of temperature.
Coppet's results showed that the molecular depressions
of the freezing points are nearly equal in groups of
similar compounds, which differ from group to group
though practically of the same dimensions. He, however,
devoted most of his labour to investigate the deviations
from the law of proportionality exhibited by certain
substances. The following figures of aqueous solutions
of earlier investigators seem still quite interesting, where
t= lowering of freezing point and m = salt content of the
.solution.
THERMAL EFFECTS 163
CaClo
NaCl
NH,C
Itudorff
Iludorff
Coppet
m
t/m
t/m
t/m
1
-0-400
-O'GOO
2
-0-450
-0-600
-0-415
4
-0-462
-0-600
6
-0-470
-0-600
- 0-400
<S
-0-487
-0-600
10
-0-490
-0-385
14
-0-490
- 0-GOO
18
-0528
-0-633
20
-0*555
-0-G47
- 0*345
The
ratio of increases
m
with
concentration of
chlorides of calcium and sodium but decreases with in-
creased concentration of ammonium nitrate. The nitrates
of sodium, barium, calcium, strontium, silver and lead,
sulphate and carbonate of sodium, amonium sulphocyanide,
and acetic acid lower the freezing points of their aqueous
solutions in a decreasing degree.
The most useful results on the determination of
freezing points of solution have been obtained by llaoult.
The extension of his investigations 19 to substances other
than salts won proper value to his work. He determined
molecular depressions of freezing point of many organic
substances basing his calculations for solutions of one
gram molecular weight of substance in 100 grams of
water. Generally he made experiments with solutions
containing one gram molecule of substance in one litre
of water. He examined more than 200 solutions of fairly
164 ASSOCIATION THEORY OF SOLUTION
representative nature. In very few cases 20 his solution,
contained 1/2 grammolecule or lesser quantity of solute
in 1000 grams of solution. From the results of his
experiments he laid down the following law :
" One molecule of any component, when dissolved in
100 molecules of a liquid, lowers the freezing-point of
the liquid by an amount which is nearly constant, viz.,
0'62 or its simple multiple."
Raoult was a man of considerable reputation and
declaration of such a law from him created material
interest amongst the contemporary investigators. He
and several other scientists subsequently tried to find out
the constant molecular depression of freezing points by
solutes in several solvents. Much accuracy of such
investigation is due to Beckmen 21 , the apparatus invented
by him is now in use in most laboratories and his thermo-
meter is almost indispensable. Numerous determinations
were made for finding out depression of freezing points
of many solvents assuming the law of proportionality and
results calculated for one gram molecular weight of solute
in 100 grams of solvent.
Van't Hoff- 2 worked out a theoretical method for the
determination of depression of freezing points of solutions
assuming its existence of a corresponding connection with
osmotic phenomena, The cryoscopic 23 constant, K, would
be the depression of the freezing-point of a solvent when
gram molecule of any substance (which does not dissociate
or associate) is dissolved in 100 grams of the solvent,
supposing the laws for dilute solution hqld good for such
a concentration. Raoult (1882) and Van't Hoff (1887)
THERMAL EFFECTS
bowed that K =
ET2
"100 L
vhere, R=gas constant = 002 (approximately),
T absolute freezing point of the solvent,
L latent heat of fusion of the solvent.
!L few typical figures may be quoted as an illustration in
his connection.
Solvent.
A r ater, EUO
Antimony chloride, SbCJ 3
formic acid, HCOOH
Icetic acid, CH 3 COOH
Benzene, C 6 H 6
5 henol,C 6 EI 6
Nitrobenzene, C 6 H B NO 2
Iniline, C G H 7 N
>.Xylol
Sthelene bromide, (CH 2 Br) 2 7 '
Following figures show the variation of cryoscopie
ionstant "K" with the nature of the solute :
Solvent. Solute. K
iVater Methyl Alcohol, CH 4 17'3
Ethyl Alcohol, C 2 H 6 O 17'3
Cane sugar, C 12 H22O n 18*5
Water Phenol, C G U G O 15'5
Acetone, CH 3 CO CH 3 17'1
Ammonia, NH 3 19*9
M.P.
Latent heat
Cdl.
calc.
K
obs.
o-o
80-025
18-57
18-5
13-37
177
18-4
-7'5
5738
28-4
27'4
16-5
43-2
338
39
5-5
3-0
51
49
39-6
26'9
70
74
-U21
22-3
69-5
70-7
-6-0
*
587
16
89-3
42-5
43
) 2 7'9
13
119
118
i66
ASSOCIATION THEORY OF SOLUTION
Solvent. Solute.
Aniline, C G H 5 NH 2
Acetic Acid, CH 3 COOH
Hydrochloric Acid, HCI
Nitric Acid, HNO 3
Sulphuric Acid, H 2 SO 4
Barium Oxide, BaO
Calcium Oxide, CaO
Sodium Hydroxide, NaOH
Potassium Hydroxide, KOH
Sodium Chloride, NaCl
Potassium Chloride, KCI
Sodium Nitrate, NaNO 8
Ammonium Nitrate, NrJ 4 NO 3
Sodium Acetate, NaOOCCH 3
Boric Acid, B 2 O 3
Borax, Na 2 B i O 7
Magnesium Acetate, M^ (CH 3 CO 2 )
Magnesium Sulphate, MgSO^
Water Copper Sulphate, CuSO 4
Zinc Sulphate, ZnSO 4
Acetic Ac^'d Chloroform, CHC1 3
Hexane, C G H 14
Camphor, C 10 H 1G O
Acetone, CH 3 COCH 3
Methyl Alcohol, CH 3 OH
Ethyl Alcohol, C 2 H 5 OH
Phenol, C 6 H 6 O
Ammonium Acetate, CH 3 COONH 4
Potassium Acetate, CH 3 COOK
K
15'S
19'0
39 1
35'8
3S'2
49'7
48'0
36'2
35'3
35'1
33 6
34
32'0
32'0
20'5
66'0
47'8
19'2
18'0
18'2
38'6
40*1
39'0
38'1
85'7
36'4
36'2
35'0
THERMAL EFFECTS
i6 7
Solvent. Solute. K
Stannic chloride, SnCl 4 4V3
Sulphuric Acid, H 2 SO 4 18'ft
Hydrochloric Acid, HCI 17'2
Magnesium Acetate,
Mg(CH 3 COO) 2 182
Formic Acid Chloroform, CHCI 3 26'5
Benzene, C 6 H G 29'5
Acetone, CH 3 COCH 3 27'8
Acetic Acid, CH 3 COOII 26'5
Ethylene-Bromide Chloroform, CHCI 3 118
Benzene, C G H 119
Acetic Acid, CH 3 COOH 58
Ethyl Alcohol, C 2 H 5 OH 57
Benzene Hexane, C 6 H 14 51'3-
Chloroform, CHCI 3 51'1
Nitrobenzene, C 6 H 5 NO 2 48'0
Camphor, C 10 H 16 O 51'4
Acetone, CH 3 COCH, 49-3
Aniline, C G H 5 NH 2 46'3
Stannic chloride, SnCI 4 48'8
Methyl Alcohol, CH 4 O 25.S
Ethyl Alcohol, C,H G O 28'2
Acetic Acid, CH 3 COOH 25'3
Nitrobenzene Benzene, C G H G 70*6
Acetone, CH 3 COCH 3 69'2
Methyl Alcohol, CH 3 OH 35-4
Ethyl Alcohol, C 2 H G O 35 6
Acetic Acid, CH 3 COOH 36 1
It is thus seen that a solvent is differently influenced
l68 ASSOCIATION THEORY OF SOLUTION
by different solutes with regard to its freezing point. An
abstract of a few representative figures are tabulated to
show how differently a solute is influenced by solvents
when experimented with for the same purpose :
Solute "K'' in different solvents
>-i
j
ctf
13
*u
s<
u
<
u.-2
1<
o
ttt
Ethelene
Bromide
Benzene
<u
o
"II
Methyl alcohol
17-3
35-7
-
-
25-3
35'4
Ethyl alcohol
17-3
36-4
-
57
282
35-6
Acetone
17-1
381
27'8
-
49'3
692
Acetic acid
19-0
26'5
58
25-3
36-1
Hydrochloric acid
39*1
17-2
-
-
-
-
Chloroform
-
38-6
26-5
118
51-1
69-9
Hexane
40-1
51-3
Camphor
-
39-0
-
51'4
-
Naphthalene
-
39-2
-
_
50-0
73-6
Solute and solvent remaining the same, dilution has a
fundamental influence on the cryoscopic constant 'K'. A
large number of figures on this subject was recorded by
numerous investigators from time to time, the bulk of
which, however, have been included in Landold Bornstein
Tabellen, 1923 pp. 1424-1460, from which a few represen-
tative ones are given below. These were determined by
Beckman's method by dissolving about O'l to 0'2 gms. of
substance in about 15*20 gms. of solvent, the depression
of freezing point observed and results calculated for the
molecular depression by use of the formula.
T.L.M.
lOO.g
THERMAL EFFECTS
169
where,
M = molecular weight
G = grams of solute
L = grams of solvent
T = depression of the freezing point of the solvent
K = molecular depression of freezing point when
one gram-molecule of solute dissolved in 100
grams of solvent.
(a) Molecular depressions of freezing points remain
practically unchanged at varying concentrations :
Potassium chloride
in
water.
Concentra-
T
K
tion in
grams per
100 gms.
of solvent.
1
- 0-45
33'3
2
-0-9
33'3
4
-1-8
33'3
1
~3'55
33-4
12
-5'33
3299
Potassium carbonate
in water.
Concentra-
T
K
tion in
grams per
100 gms.
of solvent.
1-41
-4-5
441
3-06
-0-95
42-9
5'29
-1-7
444
12-20
-3'9
44-1
14-86
-4-7
43-7
(b) Molecular depression of freezing points increases
with increasing concentration.
Calcium chloride in water.
1 -0'4 44-40
-0'9
49-95
Camphor in benzene.
0'411 48'7
1*253 48-37
170
ASSOCIATION THEORY OF SOLUTION
Calcium chloride in water.
Camphor in benzene,
8 - 3'9 5412
2-791 49-05
10 - 4-9 54-39
5 897 49-61
14 - 7-4 58-66
12-11 50-40
18 -lO'O 61-66
23-12 51'40
2659 52-08
Sodium chloride in water.
1 - 0-6
35-08
15 - 92
35-85
18 - Ll'4
3701
20 -12-8
37-41
(c) Molecular depression
with increasing concentration.
of freezing points decreases
Ammonium nitrate in water.
Barium nitrate in water.
2 -0-83 33-21
0-01002 -0-00214 56-0
10 -3853 0-80
0-2236 -0-04311 50'4
20 -6-90 27-60
2-175 -0-363 43-5
30 - 9-35 24-90
4-375 -0654 391
40 -11-75 23-50
50 -13-60 21-76
60 -15-60 20*80
70-24 -17-40 19-82
Molecular depression of freezing points increases
THERMAL EFFECTS
171
passes through maximum and then decreases with increa-
sing concentration.
T K
Methyl acetate in water,
2-288 -0-566 18'3
7-198 -1-704 18-5
12-65 -3-123 1828
T K
Ethyl alcohol in water.
0-001851 -0-000670 16*7
01332 -0-04936 17*07
2-418 -0-9645 18'34
17-96 -7-49 19-2
51-06 -23-6 21-2
86*22 -33-9 18'2
i- Potassium tartrate in water.
2-428 -0-40 37-0
4-855 - 0-83 39-0
9-710 -1-64 38-0
19-42 -318 370
(e) Molecular depression of freezing points decreases
passes through minimum and then increases with increa-
sing concentration
Sulphuric acid in water.
^-Tartaric acid in water.
0-00299
-0-00161
52-75
0-1504
-0-0234
23-3
0-04095
-0-02102
50-3
1*522
-0-209
20-6
0-6364
-0265
409
7-633
-1-000
19-7
1-989
-0765
37-7
34-66
-4-79
20-7
3-618
-]-37
37-0
9-397
-3-80
397
22-685
-11-83
51-1
172
ASSOCIATION THEORY OF SOLUTION
Barium chloride in water,
0-00446 -0-00119 55'7
0-2379 -0-0577 50'5
2-3659 -0-5319 46*83
20-52 -510 52*0
25-1 -7-85 651
Citric acid in water.
Aluminium sulphate in water.
0-1924
-0-0226
230
0-4474
-0-073
56-0
3-929
-0-3978
19-4
2520
-0260
346
13-45
-1-350
191
12-600
-1-531
41-2
27-85
- 2'849
19-6
5224
-5-792
21-3
Cobalt chloride in water.
0-0225 -0-0093 538
-0-0457
-0-2930
-0-6134
-1-3934
-21900
0-1159
1 0342
1-6314
3-601
5-477
51-3
49-1
48-8
50*3
51-93
Maganese sulphate in water,
1-941 -0293 22-8
,5-120 -0-687 20-3
18-572 -2591 2M
Maganese nitrate in water.*
1-611 -046 51-5
3-222 -0-88 49-0
9-670 -2-98 55-2
5641 -38-50 122-2
A critical examination of all the figures quoted above
on the depressions of freezing points will show that all
kinds of variation are noticeable in them ; although it is
true that there is a certain amount of constancy in the
* Concentrations given are in the volume.
THERMAL EFFECTS
sense that they do not vary in any large extent the
minimum is almost always more than half that of the
maximum for the same solute and solvent. This low
range of variation made such figures accepted as constants
and largely employed as such in the determination of
molecular weights of substances by means of the formula
given above by many investigators.
Many figures are not available on the influence of
temperature and solvent on the constant *'K" and investi-
gation on such line are needed. Beckmann and Maxim 24
however, found by using phenol as solute and carbon tetra
chloride as solvent that the freezing point data give values
for the molecular weight of phenol which increases from
90 in a 0'019<V solution to 313 in a 2'31o/ solution ; and
the same substance in bromoform, m.p. 8, gave molecular
weight rising from 97 in 0'296/o solution to 190 in 4'33o/
solution. These authors have explained the fact in a
different way but they may be due to the difference in
the formation of compounds between solvent and solute.
According to the association theory a solution is a
compound of solute and solvent in molecular ratio identical
as dilution. On cooling a solution below the temperature
of freezing of the solvent, the molecules of the solution
may get vibration similar as the molecules of pure solvent
at the same conditions but its solidification could not take
place on account of the solute molecules keeping them in
association with a certain amount of force and thereby
maintaining them in a state of solution. To displace their
force to effect separation of the solvent in a solid state a
corresponding amount of energy is needed, And this
174 ASSOCIATION THEORY OF SOLUTION
negative counterbalancing energy can be imparted to a
solution by the application of cold, under which circums-
tances the freezing of solvent would take place. The
difference of temperature' and the total quantity of heat
needed for the solidification of the solvent from the solution
are functions of the force with which solute and solvent
are kept attracted in solution. Evidently then the "solu-
tion force'' is dependent on the nature of solvent and
solute, and on their proportion of combination.
The behaviour of aqueous solutions of salts or electro-
lytes is somewhat different from other solutions or non-
electrolytes which is due to the difference in properties of
solution molecules of two kinds of solutions and not due
to the dissociation of solutes into ions as has been
assumed by the dissociation theory.
Mention has already been made about the assumptions
of Van't Hoff in working out his formula for the deter-
mation of molocular weight in solution which are only
true as long as the solutions are dilute enough to obey
osmotic pressure Jaws within certain limits, Explanation
of results of osmotic pressure determinations on the light
of association theory has been given already. This theory
is against any such assumption that solute molecules could
remain in solution as gas molecules in space, the former
are bound with solvent molecules and are not as free to
move about as the latter. In a solution the movements
of molecules of solute and solvent are in their state of
combination and therefore Van'fc HofPs assumptions are
not supported on the grounds of rationality.
Variation of "K" has been explained by many
THERMAL EFFECTS T 75
scientists by basing multitudes of assumptions but only
three principal ones may be tabulated here j
(1) Solute forms complex molecules amongst
themselves
(2) Solute gets partially associated with the solvent.
(3) Solute gets dissociated.
In explaining all variations of "K" association theory
does not require the help of such assumptions.
Considerable difficulty is experienced in the compara-
tive study of variation of "K" of a solute in different
solvents on account of recording such figures after
calculating for 100 grams of solvent containing one gram
molecule of solute. 100 grams of all solvents neither
contain the same number of molecules nor occupy the same
volume. It would be very convenient or rather correct
to work out the constants in terms of one gram-molecule
of solute dissolved in 100 gram-molecules of solvent or
&ny such fixed number of molecules.
Comparative study of many figures becomes also
irrational on account of the fact that they are mostly
obtained by performing the experiment at some dilution
and then mathematically calculating the same for another
dilution, one grain molecule of solute for 100 grams of
solvent. "K" varies with dilution and the figures
quoted cannot in such cases represent the actual fact.
In order to perform comparative study of "K" it would be
only rational to have the figures determined with proper
consideration to concentration in molecular proportions o
solute and solvent.
Van't Hoff in working out his formula for the lowering
1 76 ASSOCIATION THEORY OF SOLUTION
of the freezing point of solution assumed that the frozen
solvent of sufficient quantity to keep one gram-molecule
of solute in solution, melts at the normal freezing point of
the solvent and passes into solution just as solvent passes
through a semipermeable membrane in an osmotic
pressure determination experiment. In doing this, Van't
Hoff assumed, the solvent does the osmotic work, which
i equal to the osmotic pressure of the solution under the
same conditions. Now the question arises when the pure
solvent passes into solution does it only 'work 1 which could
be measured by osmotic pressure, or which could correctly
be represented by the sum of all kinds of changed
properties of solution, electrical, thermal, optical, etc.
It seems reasonable that when any solute or solvent is
introduced or withdrawn from a solution the actual
amount of work done may not be measured by the
measurement of one of the simultaneous change of
properties of solution but may be done by determining all
such accompanying changes.
Some interesting results were obtained in studying
thermal effects of binary mixtures by Madgin 25 and his
coworkers but these being obtained by cryscopic methods,
though very useful otherwise could not be used as a direct
evidence in establishing the association theory of solution,
Because the compounds separated by extreme cold may
not be the same that are present in solution before freezing.
On the whole their results are very helpful in connection
with the association theory of solution and further develop-
ments are anxiously awaited.
THERMAL EFFECTS 177
Vapour pressures and boiling points of solutions.
It was known from very early days that the presence
of foreign matter in solution affects the temperature of
boiling of water. Experiments on the subject were started
by Faraday 20 and others 27 since 1822 with non volatile
salts with the object of studying their influence on boiling
point of water. These early investigators performed
quite a number of experiments but did not succeed in
arriving at any generalisation and their work was
restricted in the determination of the temperature
at which solutions of different strengths boiled, or in
other words, the temperature at which the vapour
pressure of the solution becomes equal to that of the
atmosphere. The pressure remaining practically constant,
the variation of temperature with the strength of the
solution was recorded.
Determination of vapour pressure at a fixed temper-
ature of solution containing varying quantities of solute
was started by Gay-Lussac and Prinsep 28 , and conducted
to a fair extent by Von Babo 29 . Wullner's 3Q experiments
on this subject afforded a generalisation. From his results
he concluded that the lowering of vapour pressure of water
due to the existence of solutes, having no appreciable
pressure at the temperature of the experiments, in
solution is proportional to the quantity of the solute.
He also noted that it would not matter if the solute
be composed of one salt only or a mixture of more than
one. This view however did not stand long, Pauehon 33 ,,
and afterwards Tammann showed that Wullner's law of
12
1 7 8
ASSOCIATION THEORY OF SOLUTION
proportionality is not strictly accurate so far as aqueous
solutions of salts are concerned :
(1) Sodium sulphate, ammonium sulphate, magnesium
sulphate, (MgSO 4 ,6H 2 O), and ammonium bromide agree
fairly with the law of proportionality of lowering of vapour
pressure with proportionate increase of concentration.
(2) Sodium nitrate, potassium nitrate and chlorate of
potash cause relatively decrease of vapour pressure as the
concentration increases,
(3) Instances have also been found by Tamrnauu where
vapour pressure increases at first, reaches maximum and
then decreases with increased concentration.
(4) Most other salts gave increase of the relative dimi-
nution of vapour pressure as the concentration increased.
Effects on vapour pressure by a molecule of non-vola-
tile solute present in solution in different concentrations
have been subsequently studied by several other investiga-
tors. A few figures of Emden 32 and Walker 33 are very
interesting in this connection as a fair representation of the
most instances of effects of dilution or concentration :
Substance. % Present in Relative lowering of vapour pressure
solution. caused by one molecule of solute
dissolved in 100 molecules of water.
NaCI
CO(NH 2 ) 2
By Walker.
By Emden.
5.96
2.07
2.12
18.60
2-18
2-14
32,265
2-29
2-25
6667
111
13-333
1-07
26*667
0'91
THERMAL EFFECTS 179
From the results of Caven and Ferguson 34 it may be
easily concluded that water in the solid hydrated salts is
differently bound with different salts, and Sidgwick 34 has
shown that same is the case with aqueous solutions by
means of determinations . of vapour pressures. The
binding force is influenced by the chemical nature of salt.
Tammann made an extended series of experiments on the
determination of lowering of vapour pressures of aqueous
salt solutions of different concentrations at a fixed tempera-
ture of 100C. His figures seem to be very systematic and
some of which are quoted below ; where N number of
grammolecules of salt dissolved in 1000 grams of water,
and figures lowering of vapour pressures in mm. at
different concentrations diminutions of vapour pressures
in mm. when N = 0*5.
(a) Molecular lowering of vapour pressure remains
constant at all dilutions :
Substance N = 0-5
KCI
KSCN
KBr
Ki
(b) Molecular lowering of vapour pressure increases
with increased concentration.
N^o'5 i 23 45 6 8 10
KF ro 1*05 ro8 i 18 1*25 1*29 131 1*37 -
Nal ro 1 06 1-24 1-37 1-41 147 1*62 1-55 153
LiBr ro 1-07 1-23 1*32 1-43 1-53 r6$ 175 179
o'S
I
2
3
4
5
6
8
I O
I O
I
'O
I
QI
1-03
i
05
1-04
-
10
1-05
I
II
1'
'17
ri6
i
19
1-17
n6
I'O
ro8
I
06
I
07
no
i
ii
no
-
I
roi
I
04
I
*IO
1*12
i
-I 3
1-14
i'i3
180 ASSOCIATION THEORY OF SOLUTION
(c) Molecular lowering of vapour pressure decreases
with concentration,
No*5 123 4 5 6 8 10
NH 4 NO 8 ro o'86o 0-822 0816 o'Si 0-81 079 074 0-70
KNO 8 ro 0*02 0*97 0*93 090 0*86 0-83 077 072
(</) Molecular lowering of vapour pressure increases^
reaches maximum and then decreases with increased con-
centration.
N = Q'5 i 23 4 5 6 8 10
NaSCN 10 ro6 i'2O 1-28 r8i 1*35 140 138 1*35
NH 4 Br ro 1*004 1*025 1*038 1*044 1*021 1*019 *999 '96
(e) Moleoular lowering of vapour pressure decreases,
reaches minimum and then increases again with increased
concentration.
No*5 123 4 5 5 8 ro
N(C 2 H 6 )H 8 C1. i*o 080 0^38 1*105 1*107 1*119 1*109
K a S a O 3 ro 0-95 098 1*00 1*04 105 107 17
Na a W 4 Oi3 ro 0*84 3-84 1*07 1*05
CdCl a ro 098 0-96 099 i 01 103
More investigation under each of the following heads
seem still quite welcome :
(1) Influence of temperature. Molecular lowering of
vapour pressure of solutions at rising temperatures.
(2) Influence of concentration. Molecular lowering of
vapour pressure of solutes at different concentrations.
(3) Influence of nature of solute. Molecular lowering
of vapour pressure of different solutes in same solvent
under similar temperatures, pressures and concentrations.
THERMAL EFFECTS l8l
^4) Influence of nature of solvent Molecular lowering
of vapour pressure of the same solute in different solvents
under similar temperatures, pressures and concentrations.
It has already been mentioned that some experiments
on these lines have been made by Von Babo and
Wnllner, and Raoult 35 tried to do elaborate experiments
on the subject. Unfortunately, however, he rushed into
generalisation with quite insufficient data. It does not
seem worth while to discuss much on his figures specially
as they are more or less repetitions of his previous
workers. The rapidity with which he started generalisa-
tion throws considerable doubt on the interpretations of
his results. He also admitted that he had some
difficulty in obtaining accurate results.
Elevation of boiling point.
Some of the above mentioned results created con-
siderable interest in the subject and attention was drawn
to perform experiments in a different 36 way. Atmospheric
pressure was kept constant at 760 mm. and elevations of
boiling point of solutions were studied by Beckmann 37 .
Difficulties were experienced in obtaining concordant and
accurate results in such determinations by Raoult,
Tammann, and Beckmann for the purpose of finding out
molecular weights of solutes on the basis of elevation of
B. P. produced. Beckmann, however, over-come the
difficulty by the discovery of his method and his
thermometer, which are now applied in almost all
laboratories as his freezing point method. Constant
182
ASSOCIATION THEORY OF SOLUTION
K--
"K" the molecular elevation of boiling point for different
solvents have been determined by numerous investigators
by taking solutes of known molecular weights. The
following formula is used ; -
M x t x L
"10x~g~
Where, K=MolecuJar elevation of boiling point at
760 mm.
M=* Molecular weight of the solute.
t = Elevation of boiling point observed.
L = Weight of the solvent in grams.
g = Weight of the solute in grams.
Thus the constant U K" represents the elevation of
boiling point of a solvent that could be produced by the
solution of 100 grams of solvent with one gram-molecule
of solute. Actual experiments are to be done in a very
dilute solution and by calculation figures are to be obtain-
ed as above. A few typical instances of "K" are given
below : -
Substance.
HC1
II Br
HI
H.OH
CHg.OH
C 2 H 5 OH
C ;J H 7 OH (Normal)
C H B OH
B. P. Latent heat of
"K"
Vapourisation.
~82'9
105-5
6-4
-68-7
51-4
15-0
-35-7
38-7
28-3
100-0
5-15-7
5-2
6'7
267-5
8-4
78-4
207-0
12-0
97-3
162-6
17-3
182-1
114-3
36-0
THERMAL EFFECTS 183
Substance. B. P. Latent Heat of "K"
Vapourisation.
CgHn'OH (iso-amyl
alcohol) 131-5 125-1 25*8
C 5 H n 'OH (tertiary
amyl alcohol) 102'0 1061 22'6
C 2 H 5 C1 12-0 83-1 19-5
C 2 H 5 Br 377 Cl'65 25-3
C 2 H 5 I 72-2 47-6 50'1
These figures indicate probable relationship inherent
on the chemical nature on account of their gradation
with change of radical in the molecule.
The two sets of figures given above for two amyl
alcohols are instructive amongst themselves and show how
intia-molecular or atomic adjustment in a molecule could
influence such properties.
These so called constants "K" of solvents have been
determined under special conditions of dilute solutions and
are not really constants when strictly examined. These
are only approximately true for dilute solutions and do
not hold good for concentrated solutions. For practical
purposes, however, they are extremely useful in rough
determination of molecular weights of substances.
This constant "K" not only varies with the nature of
the solvent and solute but may do in 5 different ways with
variation of concentration as was the case vapour pressure.
Beckmann 38 and his eoworkers have done a large number
of determinations of molecular weights by observing
elevation of boiling point of solutions, amongst which his
work on the influence of temperature and solvent on the
1 84
ASSOCIATION THEORY OF SOLUTION
molecular weights of the dissolved substances seems very
interesting in this connection. Although his figures are
too few to afford any generalisation yet they show how
further investigation is needed on such line. The influence
of temperature on the molecular weight determination of
phenol by the elevation of boiling points of carbon tetra-
chloride at temperatures of 75, 60 and 54 has been
examined. The results of such investigation show that
the variation of temperature of determination has no
appreciable influence on the molecular weight or on "K"
at a given concentration. But the same constant varies
with concentration of solute. Following tables show the
influence of concentration in aqueous solutions :
(a) Molecular elevation of boiling point does not
appreciably change with concentration.
Concentration t "K"
in gms.
per 100 gms.
of solvent.
Cadmium iodide.
CdI 2 = 361
4-54 068 5-4
14-31 0212 54
22-58 328 5-3
Concentration t
in gms.
per 100 gms.
of solvent.
Boric acid.
2-35
0-186
49
2-99
0-241
5-0
5-02
0-450
5-0
7-69
0610
4-9
10-92
0-900
5-1
1727
1390
50
26*50
2-130
5'0
36'41
3-010
5-1
THERMAL EFFECTS
(b) Molecular elevation of boiling point increases with
increased concentration.
Barium chloride.
Potassium chloride
BaCL
a- 208-3
KC1
= 74-6
3-397
0-208 12-8
0-376
0'05()
10-0
8777
0-525 12-5
0-752
0-091
9-0
18-619
1-174 13-1
2-279
0-288
9-4
35-036
2-517 14-9
6-191
0-768
9-3
54-191
4-157 16'0
18-44
2-376
9-6
27-17
3-75
10-3
48-94
7-60
11-6
Cobalt sulphate.
CoS
O^- 15-51
4-446
O'llO 3'8
9-596
0-262 4-2
20-60
0-568 4-28
32-84
1-055 4-98
Potassium iodide
Cane sugar.
KI - 166*0 C 13 H 22 O U = 345
1-2
4*32
2-256 9-8
4-316
0-1964
5-1
11-22
0-656 9'7
7-25
0-212
4-9
18-20
1-076 9-8
11-02
0-322
5-1
29-24
1-812 10-8
21-66
0-638
5-H
47-61
3*159 11-0
36-15
1*056
6-2
104-80
8-02 12-0
65-97
1-93
5-9
100-95
2-95
6-3
1751
5-12
7-5
276'2
8-07
8-3
i86
ASSOCIATION THEORY OF SOLUTION
Sodium
chloride.
NaCl
-58-5
0-4388
0-074
99
2*158
0-351
9-5
4-386
0-717
9-6
1217
2-182
105
1877
3-866
12-0
31-242
6-82
12-8
(c) Molecular elevation of boiling point decreases
with increased concentration.
Silver nitrate.
Potassium nitrate,
KN0 3 = 101-1
0-804
0-044
9-3
0-505
0051
10-0
1-543
0-087
9-6
1-010
0-095
9-5
3-893
0-197
8-6
2-789
0-248
9-0
7-495
0-382
8-7
9-22
0-797
87
15-545
0-741
8'1
1974
1-603
8-22
35-08
1-526
7-39
53-37
3-795
7-2
86-43
3-143
6-18
70-76
4-677
6-69
136 36
4415
5-50
Barium nitrate.
Ba(N(V 2 = 261-5
1-205 0-065 140
2-270 0-104 12-0
23'25 0-911 102
Lead nitrate.
PbiNO 3 N 2 = 331-0
1-569 00 7 15-0
13-816 0-418 10-0
29'10 0-824 9-4
THERMAL EFFECTS
I8 7
Sodium nitrate.
Rubidium chloride.
NaNO 3 = 85
RbCU1209
0-3931 0-044 9'5
0-4943 0-039 10
0-7250 0-080 94
1-1420 0089 9*4
3-785 0-398 9-0
2-502 0-190 9-2
7-343 0771 8*9
6-385 0-478 91
11-383 0-860 9-1
(d t Molecular elevation
of boiling point increases,
attains maximum and then
decreases with increased con-
centration
Mercuric chloride.
Mannite.
HgClo- 270-9
C H 14 O -182i
3-341 0-056 4-5
2-38 065 5'0
8-68 0-159 5-0
4-298 0121 51
16-54 0-268 4-4
6-501 192 5-4
34-90 496 3-8
1267 0-360 5-2
52-59 0-645 3'3
19-67 0-535 5-06
Magnesium sulphate.
Fructose.
MgSO 4 = 120'4
Co H 12 O = 1801
2-733 0-097 4*3
1016 0-294 5*2
7-236 0-281 4-7
16'12 488 5-5
43'47 1-455 403
27-52 0807 5-28
lodic Acid
Urea.
HI0 3 := 175-9
co;NH 2 ) 2 -eo-i
3-39 0116 60
1-118 0-090 4'8
5-51 0-190 6-1
3-361 0-269 4-8
1074 0-385 6-3
6'60 0'549 4'99
29-94 0-772 4*53
16-59 1-169 4-23
l88 ASSOCIATION THEORY OF SOLUTION
(e) Molecular elevation of boiling point decreases,
attains minimum and then increases with increased con-
centration.
Calcium chloride.
Lithium nitrate.
(
3aCl 2 -lll-0
LiNO 3 -69'l
0-585
0-091 1 7
1-96* 0-278
9-8
2-405
0302 1-39
6-36* 0-830
9-0
535
0-643 1-34
13-99* 1-516
7-49
10-89
1-481 1-51
23-29* 2-916
8-66
31-91* 4-428
9'58
Copper sulphate
45-03* 8'496
1303
CuSO 4 - 159-7
3356
0-091 4-3
7-811
0-189 39
Manganese sulphate
15-952
0374 3-7
MnSO 4 -151'
1
32-36
0-874 4-3
3-713 0114
4-6
56-95
2-283 6-37
14-46 0-373
3-9
7377
3-768 8-16
24-21 0-678
423
Nickel sulphate
D 4 ~ 154-8
2-766
0-096 5-4
11196
0-396 4-6
23*143
738 4-94
34-461
1-389 6'24
37-735
1-734 7-11
Volatile solvent and solute. Vapour pressure of
solution containing both solvent and solute as volatile
substances has neither been much determined nor
* Concentrations are in gms. per 100 c.c. solvent.
THERMAL EFFECTS 189
available data properly interpreted so well as they should
be. I could only try to deal with a few useful figures to
show how their study may disentangle the nature of the
phenonrenon of solution.
Planck 32 worked out a formula establishing a relation-
ship between lowering of vapour pressure with con-
centration of the dissolved substance in the liquid and in
the vapour, which was subjected to some experimental
test by Wirikelmann 40 who found considerable deviations
in several cases. It is not worth while establishing much
importance on such formula for want of its testing with
sufficient experimental data.
Vapour pressures of some binary mixtures of volatile sol-
vents and solute have been determined but they are also not
comprehensive enough to allow much discussion. Amongst
these a few only are given below which may be good enough
to show that the solute and solvent remain in solution in
a state of combination. Variation of vapour pressure with
concentration of any of the constituents of the solution
always do not depend on their quantity only but also on
the particular association they form at that condition.
(a) Increase or decrease of vapour pressure with concen-
tration depends on the temperature of determinations :
Acetic Acid and Benzene.
Molecular percentage Pressure in milimetres
of acetic acid. of mercury.
at 49'99 at 85-05
10 26-0 31-9
20 24-6 32-9
1 90 ASSOCIATION THEORY OF SOLUTION
Pressure in milimetres of mercury.
at 49-99 at 85-05
30 23-6 33'4
40 21-9 33-1
50 32-7
CO 18'6 31 7
70 165
80 13-8
90 8-7
(/;) Vapour pressure increases or decreases passes
through maximum or minimum, and then decreases or
increases with concentration.
Acetone and chloroform.
Chloroform, Pressure in milimetres of mercury.
10
20
30
40
50
60
70
80
90
100 2218 37-18 63*28
at 28-15
at 40-40
at 3510
2588
42-50
7418
24-5
40*6
699
33'1
38-6
66'2
21-9
35-6
631
20'8
34'9
60*3
19-8
33-2
60'0
19-0
321
56'4
18-9
32*2
56'8
19-6
33-4
586
20-7
35-2
60-7
THERMAL EFFECTS 191
Carbon disulphide and acetone.
Pressures in mm. of mercury.
CS 2
at 24-78
at
3,5-17
2316
34 38
10
321
48-2
20
38-0
56']
30
413
60-0
40
43-3
633
50
44-8
64-7
GO
45-2
05-3
70
45-5
65-5
80
44-7
642
90
439
61'5
100
35-85
51-2:3
Nitric acid and
water.
o/o w/w
Pressures in
mm. of mercury.
Nitric acid.
at 75
at 85
at 95
at 100
289
434
634
760
20
260
390
554
663
30
230
350
497
583
40
195
300
435
510
50
155
250
375
450
60
135
225
330
405
70
115
195
300
370
80
170
250
375
450
90
295
440
625
745
100
524
725
I 9 2
ASSOCIATION THEORY OF SOLUTION
Ethyl alcohol and water.
o/o w/w
Alcohol.
Pressure in mm.
of mercury
at 39-76.
o/o w/w
Alcohol
Pressure in mm,
of Mercury
at 74-79*
100
129-8
100
653-0
99-13
131 1
99-00
653 2
98-20
1314
97-93
654-0
97-52
131-5
95-68
6543
96-37
130-9
94-57
652-8
90-04
129-2
90-88
651-0
80*00
125 5
82-46
640-5
40-85
107-6
41-80
549-0
22-00
90 -5
30-25
515-2
15-92
81-4
20-0
468-8
00
54-7
o-oo
286-7
Propyl alcohol
in water.
o/o w/w
Propyl
alcohol.
Pressure in mm.
of mercury
at 49-92.
% w/w
Propyl
alcohol.
Pressure in mm,
of mercury
at 79-80
100
, 90
100
374-6
93-83
121-3
93-83
479-2
9042
129-3
90-42
506-6
80-65
138-4
80-65
541-7
75-34
138-9
75-34
545-7
69-98
140-4
71-46
5497
69-51
139-7
70-72
548-5
39-36
139-1
69-98
548-5
38-79
138-7
58-96
547-0
2488
136-4
38-09
539'6
O'OO
92-05
23-79
530-0
o-oo
352-2
THERMAL EFFECTS 193
Cryohydrates : In attempting to separate solvent
from aqueous solutions of certain substances it lias been
found that practically pure ice separates out till a certain
concentration is reached when separated solid water becomes
a mixture of solvent and solute in a ratio practically srme
as the mother liquor from which the solidification has
taken place. On further application of cold the rest of
the solution solidifies gradually containing the solute and
solvent in the game ratio as the solution. When such
solids of constant composition separate out the temperature
of the solution remains practically constant. For the
reason lhat the solution behaves like a pure liquid, and
has a definite freezing point the separated substance was
supposed to be a definite compound of salt and water and
called a CRYOHYDRATE. Similar phenomena of freez-
ing of a mixed binary liquid at constant temperature and
conversely melting of mixed solid at the same temperature
were noted by Guthrie 41 . This investigator called such
combinations an "eutectic mixture/* He found that a
mixture of Pb(;NCV 2 = 46-86 and KNO 3 = 5314, melts at
constant temperature of about 207 and that by altering
this proportion in any way a mixture of higher melting
point is obtained. Laterly several such mixtures were
discovered. Cryohydrates are also instances of eutectic
mixtures and those of NaCI, Nal, NH 4 CJ, NaNO 3 , FeCI 3
have formed subjects of investigation by Guthrie 42 ,
Mazotto 43 , and Roozeboom 44 .
Constant boiling points : Boiling points of solutions
containing volatile liquids only have also been much less
investigated and formerly whatever experimental facts
13
194 ASSOCIATION THEORY OF SOLUTION
were available have been rather improperly applied in
connection with the theory of solutions. The following
aqueous solutions have approximately constant boiling
points at 760 mm. pressure :
Approximate strength Boiling point,
of solute.
96o/o ethyl alcohol ... ... 78C
75o/ propyl alcohol ... ... 88
68o/ nitric acid ... ... 86
20'2o/ hydrochloric acid ... ... 110
75o/ formic acid .. ... 107
These do not mean that these mixtures are definite
compounds but only indicate the magnitude of vapour
pressures relative to each other under those conditions.
Subjecting a binary solution to the action of gradually
decreasing or increasing heat 'the combination between
solvents and solute is broken at respective temperatures.
At such conditions, solvent, solute, or mixture of solvent
and solut6 may separate out in the form of matter other
than liquid, i.e , solid or gaseous state. The difference of
thermal condition needed for such separation is to work
against the force with which the solute and solvent are
bound together in order to remain in a state of solution
and to convert solute, solvent or mixture of both solute
and solvent into solid or gaseous state. Each of the
molecules of solvent and solute after becoming released
from the tie of solution forces as a result of corresponding
thermal action, higher or lower than the limits of ^existence
in a state of solution, subjects themselves simultaneously
THERMAL EFFECTS 195
to the action of the temperature of existence and to the
rest of the solution. If they are not sufficiently attracted
by the rest of the solution they leave it as a solid or gas
totally or partially according as the case may be. The
cases when both the ingredients of the binary liquid
mixture separate out in the same proportion as the compo-
sition of the solution they do so at a fixed temperature as
has been said already. Interesting instances of this
phenomenon could be found in the following pairs of
mixtures 45 :
Substance. M.P.
{Hexachloro-cyclopentenone 97* t 7C
Penta chloro-monobromo-cyclopentenone 8*7 '5
/ Mercuric bromide. 236'5
{ Mercuric iodide, 255*4
Mixtures of given definite ratio of these two pairs will
separate from their liquid states as a solid mixture, having
the same composition as the solution, at fixed unchanged
temperatures In majority of cases, however, such constant
freezing point mixtures are obtained in limited ratios of
the components of the solution. One of the components
gaining concentration as a result of the separation of the
other by the application of cold, and it would do so having
more affinity for passing into solution than the tendency
created for separating out by the application of cold.
Some of the binary mixtures composed of both volatile
components could vaporise as has been stated already in
the same rate as they are in a quantitative ratio in solution
at 760 mm pressure, Lecat in his book "La Tension de
196
ASSOCIATION THEORY OF SOLUTION
Vapeur des Melanges de liuides : L'Azeotropisme", Lamer-
tin 1918, Brussels, gave instances of some cases of binary
mixtures which boil at constant temperatures producing a
distillate of same composition as a the original solution.
Following are a few interesting examples 46 :
(a) Minimum boiling point of the binary mixture is
lower than that of the pure substance of low boiling point.
Minimum
B.P. of
mixture.
76-5
77-5'
72-0
B.P.
Cyclohexane 80-8
and Carbontetra
chloride.
Benzene
Methylethyl
ketone.
B.P.
76-8
80-2
79-6
Carbontetra
Chloride
Mesitylene
76'8 Ethyl acetate 77 2 74 8
164-0 Chlorotoluene 161-3 160'5
Propionic acid 140'7 139'3
(b) Maximum constant boiling point of the binary
mixture is higher than that of any of the pure substance
of high boiling point.
Maximum>
B.P. of
B.P. B.P. mixture.
Chloroform 61'0 and Acetone 56'3 63'4
Water 100* Hydrochloric
acid 80-0 110
Nitric acid 860 120'5
THERMAL EFFECTS
I 97
Formic acid 100'8
Maximum
B.P. of
B.P. mixture.
105-8
Glycol 197-4 199 3
Benzaldehyde 1792 185'6
Aniline 1844" 1 186-2
Etbylbutyrate 178'6 185-6
Benzyl alcohol 205'5 206
Diethyl ketone 102'2 105'0
B.P.
Pyridine 115'5 and Propionic acid 1407
Phenol 181-5
The magnitude or the degree of the lowering or the
elevation of boiling point depends on the force needed to
work against the energy binding the substance present in
solution. Ternary mixtures, known to give constant
boiling points, 46 come under this law also.
General Remarks : Having described the representa-
tive experimental facts regarding the removal o solvent
or solute from solution by the presentation of suitable
thermal conditions it behoves now to consider to what
extent such phenomena of their removal from solution
could tell the secrecy of the mechanism of formation and
state of existence of solution. Van't Hoff and numerous
other physical chemists have indirectly or directly stated
that the relation-ship between solute and solvent in dilute
solutions is not identical in nature with that in the
concentrated solutions without making or establishing a
^clear line of demarcation betweea the two kinds of
solutions. Properties of solutions vary considerably with
concentrations and it is easy to find out ranges of
198 ASSOCIATION THEORY OF SOLUTION
concentrations where data of one solution agree with
those of another of different ranges of concentration. For
the purpose of comparison, it is therefore necessary to
have similarity in molecular concentrations of both solvent
and solute. Apart from the irrationality of distinguishing
electrolytes from non-electrolytes by considering that
solutes of electrolytes break up into its components in
solution Arrhenius 47 as well as several other scientists
have no justification in accepting Kirchhoff's and Yon
33abo's results that at high dilutions of salts a further
addition of water has no thermal effeet. This assumption
has unfortunately, been utilised in explaining many pheno-
mena of dilute solutions and sometimes extended towards
concentrated ones also. It is quite worth while repeating
the experiments of the earlier investigators who had neither
sufficiently accurate methods of measurements nor quite
sensitive instruments. Even admitting that no thermal
work is done by the introduction of additional solvent to a
a dilute solution it remains to be proved whether any
work is done against other properties of it, e.g. optical,
electrical, etc., under the circumstances.
Van't Hoff in establishing gas laws for dilute solutions
and in its application to explain phenomena connected
with deviations from boiling points and freezing points of
solution assumed that the solution is so diluted that
relationship between the solvent and solute to remain in a
state of solution continue to exist uninterfered with tha
change of dilution. Throughout his calculation he never
considered fully what work is done in removing solvent
from the solution against the force by which solvent and
THERMAL EFFECTS 199
solute are kept in state of solution. The association theory
of solution puts forward that in solution any changes in
the ratio between solute and solvent always accompany
corresponding changes in energy which may express
itself in terms of thermal, optical, electrical, etc.,
alterations. Ignorance of this fact renders Van't HofPs
theory useless in elucidating any thing about the
phenomenon of solution. Some of the text book writers 48
paid much importance in spreading out this erroneous
idea about solution without properly ascertaining if any
work, other than that of thermal, is done in changing
dilution of a dilute solution. There does not seem much
justification in accepting, Van't HofPs mathematical
Calculations and deductions in connection with theories of
solutions because he only drew similarity of some figures
with those of gases but did not mean to express any
mechanism between solute and solvent in dilute solutions,
"Van't HofE's theory of solution" becomes a misnomer
after the author has said 49 in this connection "simply
leave the question of mechanism alone altogether/'
The association theory of solution assumes that
solute and solvent always remain in solution in state of
combination and in attempting to remove one of them
from solution by freezing, vaporising or boiling work
must be done first to separate them and then to change
the condition from liquid to solid or to gas. This work
may be perceptible in, thermal, electrical, etc., forms of
energy. Deviations from boiling points vapour tensions
and freezing points of a solvent containing a solute in
solution is due to additional work that must be done to
20O ASSOCIATION THEORY OF SOLUTION
separate the components of the mixture. Magnitudes of
deviations somewhat apparently indicate the strength of
binding force between solute and solvent in solution, and
this, however, needs experimental confirmation.
REFERENCES.
1. Ostwald's Klassiker, No. 9.
2. Thomsen, Thermochemische Unterschungen, 1882.
3. Thomsen, Thermochemistry, English Translation
by Burke, 1908, 53.
4. Ibid, 56.
5. Thomsen, Thermochemische Unterschungen, 1882.
6. Thomsen, Thermochemistry, English Translation
by Burke, 1908, 67-72.
7. Ibid., 73, para 2.
8. Ibid., 82.
9. Thomsen, Thermohemische Unterschungen, 1882.
10. Tammann, Zeit, Anorg. Chem., 1920, 109, 215-
220.
11. Nernst, Theoretical chemistry, Eng. Edition. 1923.
268.
12. Rakshit, Zeit. Elektrochem., 1925, 321 ; Landolt,
1923, 98.
13. Traube and Neuberg, Zeit. phys. Chem., 1888, 1,
509.
14. Thomsen, Thermochemistry, Eng. Trans., 1908,
164, 168 j Vrebski and Kaigorodov, Jour. Russ. Phys.
Chem. Soc., 1923 54, 335, 348, 360, 376.
15. Blagden, Phil. Trans., 1788, 78, 277.
THERMAL EFFECTS 2OI
16. Ostwald, Solutions. Eng. Trans., by Muir, 1891,
200.
17. Fritzsche, Peterob. Akad. Bull., 1863, 6,385,
495; Kries, Schweigger's Jour., 1814, 11, 26 ; Rudorff,
Pogg., 1861, 114, 63 ; 1862, 116. 55 ; 1871, 145, 599.
18. Coppet, Ann. Chim. phys., 1871, (4> 23,366 ;
1872, 25, 502 ; 26, 98.
19. Raoult, Compt, rend., 1882, 94, 1517 ; 95. 188,
1030.
20. Raoult, Ann. Chim. Phys., (6) 2 66.
21. Backmann, Zeit. phys. chem., 1888, 2. 638 ; 189,
7. 223.
22. Van't Hoft', Phil. Mag., 1888, 5, 26, 81 ;
23. Ibid. Zeit. fur phys. chem., 1887, 1, 481 .
24. Beckmann and Maxim, Zeit phys. chem., 1915,
89, 411.
25. Madgin and Briscoe, Jour. Soc. Chem. Ind , 1927,
46, 107 T ; Madgin, Peel and Briscoe. Jour. Chem. Soc.,
1927, 2873 ; 1928, 707.
26. Faraday Ann. Chim. phys., (2), 20, 324.
27. Griffiths, Jour, of science. 184, 78, 90 ; Le Grand,
Ann. Chim. phys., 1835, 59. 423 ; Caven and Ferguson.
Jour. Chem. Soc,, 1922, 1412.
28. Ostwald, Solutions, Eng. Trans, by Muir. 1891.
157.
29. Von Babo, Jahresberiehte, 1848-1849, 93 ; 1857,
72.
30. Wullner Dissertation, Pogg., 1856-60, 103, 529 ;
105. 85 ; 110, 564.
31. Pauchou, Compt. rend., 1879, 89, 572.
2O2 ASSOCIATION THEORY OF SOLUTION
32. Emden, Wied. Ann., 1887, 31, 145.
33. Walker, Zeit, phys. chem., 1888, 2, 302.
84. Caven and Ferguson, Jour. Chem. Soc., 1922,
1406 ; Sidwick, ibid, 1920, 1340 ; 1924, 2268, 2273 ;
Tammann, Mem. Acad. Petersb., 1887, 35. No. 9 ;
Ostwald's solutions, Eng. Trans, by Muir, 1891, 190.
85, Eaoult, Compt. fend., 1886, 103. 1125 ; 1887,
104, 1430.
36. Tammann, Wied. Ann., 1887, 32, 683.
37. Beckmann, Zeit. fur phys. chem.,1889, 4. 352.
38. Beckman and Maxim., Zeit. phys. chem., 1915,.
89, 411.
39. Planck, Zeit, fur phys. chem., 1888. 2. 405; Wied
Ann., 1887, 32, 489.
40. Wikelman, W r ied. Ann., 1890. 39. 1.
41. Guthrie, Phil. Mag., 1884 (5) 17, 462.
42. Guthrie, Phil. Mag., 1884 (4) 16, 446; (5) 2, 211;
6, 35, 105.
43. Mazotto, Beibl., 1891, 15. 323.
44. Roozeboom, Zeit. Phys. Chem., 1892, 10. 477.
45. Walker, Physical chemistry, 10th Edition, 1927,.
66-67.
46. Hilderbrand, Solubility, 1924, 125-127 ; Attack,
Chemist's year book, 1923, 605-608.
47. Arrhenius, Theories of solutions, 1923, 131.
48. Senter, Physical chemistry, 1923, llth Edition
103 $ Larmor, Encyclopedia Britanica 10th Edition,
Vol. 28. 1070. Findley, Osmotic Pressure, 2nd Edition,.
1919, 8.
49. Van't Hoff, Zeit. phys. chem., 1890, 5, 174.
CHAPTER VIII.
OPTICAL PROPERTIES OF SOLUTIONS.
Measurement of work done in changing optical
properties of a substance has not been done in such a way
as to render its application in this book. It would have
been convenient if such data were available regarding the
measurement of changes in potential energy by the
alteration of optical properties of substances. Such re
searches would have been useful in forming accurate
inferences regarding the relationship between solvent and
solute in solution, and in changing their dilution.
Experimental results on optical properties of solutions
may be taken \n this book under the following branches :
(1) Refraction.
(2) Polarisation.
(3) Absorption.
(4) Fluorescence.
Each one of these branches is very comprehensive and
it is beyond the scope of this book to give any full treat-
ment of them. Only those portions will be considered
which will help expressing any knowledge of the relation-
ship between solvent and solute in solution.
1. Refraction.
The velocity of propagation of light through different
substances depends on the nature of the medium including
its compositon, temperature, pressure etc. This variation
2O4 ASSOCIATION THEORY OF SOLUTION
of propogation of light through a medium is ordinarily
measured basing on the fact that when a ray of light
passes through a suhstancc in an inclined angle, the sine
of the angle gets changed, The velocity of light through
a medium has been taken to be proportional to the sine
of the angle between the normal and the refracted ray.
The velocity of light through a medium may be expressed
in terms of comparison with that through a standard
medium. Light attains its maximum velocity in vacuum
which decreases when it passes through matter. Vacuum
or air is ordinarily taken as standard. The index of
refraction is the ratio of the velocity of light in the two
media and is known by the ratio of the sine of the angle
of incidence to the sine of the angle of refraction and
often expressed by the symbol n.
Sin i Vj
n ~Sir7^~ Vo
where i = angle of incidence; r = angle of refraction;
V x = velocity of light in a standard medium, ordinarily
vacuum or air ; V 2 ==> Velocity of light in medium under
observation. The speed of light through space containing
matter depends on the wave length of vibration, the angle
of incidence remaining constant, rays of different wave
lengths will be refracted differently in the same medium.
Thus a composite ray of light gets differently refracted
and the phenomenon is known as dispersion. To obtain
comparative values of the refractive indices of substances,
they are usually measured with lights of standard wave
lengths, and often A or D lines in the solar spectrum are
chosen for the purpose.
OPTICAL EFFECTS 205,
The refractive index has been found to vary with the
density of the medium and these two properties of matter
have been connected by different authors by means of the-
following formula :
(1) by Laplace 1 ,
?^. 1 = R'= Constant,
d
(2) Gladstone and Dale 2 ,
2-^ = R"= Constant,
d
and (3) by Lorentz 3 and Lorentz 4 .
n 2 -! 1 - o ,. i
-~ rt x ^sR = Constant.
n 2 2 d
Although it is desirable to have 'n' and 'd' determined
at a standard temperature for the purpose of accurate
measurements yet variation due to variation of tempera-
ture does not seem to affect these formulae appreciably.
The constants R', R" R'" are called tf specific refractive
power" and the product of this with molecular weight
of the substance is called the "molecular refractive power".
Refractivity of solutions has been determined by many
investigators and references on which have been fairly
collected by Smiles 5 . Specific refractive power of any of
the components of a binary mixture in liquid state has
been determined in numerous cases by the following
formula derived from that of Gladstone and Dale :
Ps-l^ "i~l P , "2-1 100- P
d a di 100"*" d 2 100"
where, n x> n 2 and n 3 are the refractive indices of solute,
solvent and solution respectively and d 1} d 2 and d 3 the
206 ASSOCIATION THEORY OF SOLUTION
corresponding densities at the same temperature, and p is
the percentage by weight of the solute in solution.
Using Lorentz and Lorenz's formula the following is
obtained :
n 2 3 - 1 l_ n 2 !-! _JP__ nV^J- 12^LP
X " * + X
_ ___
nV+2 ds " nV 1 ^ 1 - d j nV+2
In determining refractivity of solutes by means of
these formulae it has been found in some cases that the
refractivity varies with the solvent used. Although this
method is not so valuable in determining refractivity of
solutes yet results obtained by them are quite useful in
establishing fundamental principles of association theory
of solution especially in view of the fact that more diver-
gency is noticeable in cases where more alteration in
volume takes place on mixing the components. Pulfrich 6
proposed a correction for the change in volume but; this
part of the subject needs more investigation to show how
solvent and solute are related in a state of solution so far
as this property is concerned.
It has not been considered properly in working out the
above formulae if any reaction sets in between solvent aud
solute in solution. If the following figures of refractivity
of acetic acid in benzene, toluene, and pyridine at different
dilutions are studied it will be at once seen without any
doubt that the relationship between acetic acid and
pyridine in solution is not the same as that the acetic acid
forms with benzene or toluene, the relationship of acetic
acid in solutions with benzene or with toluene seem to
be nearly the same. The following table is prepared from
ZawidzkiV figures taken from Landolt, 1923, 994.
OPTICAL EFFECTS 207
Approximate Difference of refractive indices
w/w% Acetic (n D ^ at 25*2 of acetic acid
Acid. solutions in different solvents.
Benzene Pyridine Pyridine
Toluene. Toluene. Benzene.
0-00428
0-01329
0-00901
10
0-00213
0-01299
0-01086
20
0-00697
0-01489
0-00792
30
0-00060
0-01617
0-01557
40
0-00076
0-01875
0-01799
50
-0-00015
0-02111
0-02126
70
-0-00070
0-02620
0-02690
80
-0-00087
0-02582
0-02669
90
- 0-0066
0-01649
0-01715
100
-0-00009
000012
0-00021
It is seen in the figures of columns 3 and 4 that the
maximum differences are in the mixtures when acetic acid
in the solutions is about 70% ; the compounds formed
with two components of the solution at this dilution differ
in refractivity most widely. Had there been no such
formation of compounds between solute and solvent in the
>case with pyridine and acetic acid solutions, the differences
given in columns 3 and 4 would have been more or less
uniform. The compounds of acetic acid with benzene or
toluene in solution are very much alike in refractive
properties.
A few typical instances are given below to show
the variation of refractive indices of solutions with
-dilution.
208 ASSOCIATION THEORY OF SOLUTION
(a) Refractive index increases or decreases practically
uniformly with concentration of solute or solvent.
o/o Aqueous solution of HC1 Aqueous solution of
By Wagner. By Wagner.
1-33320 1-33320
1 3551 3455
2 3779 3589
5 4449 3980
10 5528 4598
15 6565 4616
20 5032
Aqueous solutions of glycerine ; By Henkel and Roth.
% Glycerine** i '226 6^320 9*308 12746 14*178 19*843
n^ 75 aB| '33463 r34075 1*34000 1-34868 1-35041 1*35765,
Solution of ethelene dibromide and propyl alcohol.
C 3 H 4 Br t % 10-0084 209516 407320 600940
pis-oT . ,-386161 1-391892 r399i3 6 i'4i58i5 1-439013.
C i H 4 Bra%8oo893 90-01912 loo'oooo
n^ <or 1475796 1503227 1*540399
Solution of acetone and benzene.
C 3 H 6 Oo/ = 9-8 20*0 31'0 40'0
nJJ = 1*5036 1-4885 1-4723 1-4558 1-4426.
C 3 H 6 Oo/ =49-5 69-4 84*7 lOO'O
n 6 = 1-4284 1-4011 1-3803 13609
OPTICAL EFFECTS
2O9
(b) Refractive index increases, reaches maximum and
then decreases with increased concentration.
Methyl alcohol
in water.
Ethyl alcohol
in water.
Acetone in
water.
By Wagner.
By Wagner.
By Drude.
1
1-3339
1-33379
1-3335
2
1-33359
1-33444
4
1-33404
1-33571
10
1-33565
1-33997
20
1-33858
-.
25
1-35132
1-35135
40
1-34292
1-35968
42'25
1-34313
49-8
1-34313
50
1-34311
1-36318
13637
60
1-34154
1-36525
65
1-33990
1-36577
66-9
1-3671
69
1-36584
70
1-32748
1-36572
89-9
1-3649
100
1-3606
Aqueous solution of sulphuric acid. By Hess.
H,SO 4 %w/w-o 19981 39757 59-980 80-096 100-
% 6 -I-33364 L35782 1*38169 1-40653 1*43083 1-4277*
14
210 ASSOCIATION THEORY OF SOLUTION
Aqueous solution of acetic acid in water.
By Buchkrneiner.
CH 8 CCOH%w/w
o i4'339 44*43 1 71*194 83-828 100
1*34380 1-36362 i*3749 6 1*37722 1-37265
Refractive indices of mixtures of sulphuric acid and
water in various proportions for all principle lines of
solar spectrum from A to H inclusive have been
determined by Van der Willigen 8 , who has shown that a
progressive increase in refraction and dispersion takes
place with every addition of ET 2 S04 molecule until a
maximum is reached at a point. Some of his results are
given below :
SO 3 % = 7127 81-41 85-93 86-97 91-43 9472
n^ 8 8 i -42466 i '43596 1-43806 i '43669 i '43426 1-43163
Sp gr. at O/O= 1*64925 176066 1-80676 1*83123 1-84485 1-84995
Although the specific gravity steadily increases with
concentration of the acid yet the specific refractive index
remains maximum at a concentration of 85'93% SO 3 .
Van der Willigen concluded that the formulae connecting
refractive indices of solvent, solute and solution are not
applicable to mixtures of sulphuric acid and water.
Cheneveau 9 in studying the variation of specific
refraction of salts in dilute aqueous solutions has shown
that specific refraction -~ of a dissolved salt is con-
N
stant for solutions down to a dilution equal to . He 10
OPTICAL EFFECTS 211
however, subsequently found that at extreme dilutions the
refractive power may diminish or increase, with the
decreased concentration according to the nature of salt
used. Results are plotted for magnesium nitrate,
potassium chloride and ammonium nitrate and it is seen,
in very dilute solutions, the value of - decreases with
the pressure of the dissolved substance for the two sub-
stances first named but increases, as the pressure decreases
in the case of ammonium nitrate.
Studies in the refractive properties of substances in
solution with increasing quantities of solvent, show that
compounds are formed afc each stage of dilution refractive
properties of which are not necessarily always gradual
with the ratio of increased solvent. Association of solute
and solvent takes place at each dilution producing com-
pounds which may differ considerably in refractive
properties from the compounds formed at any other
dilution. Proper attempts have not been made by the
followers of the dissociation theory of solution to explain
these phenomena in light of their assumptions that
electrolytes like KCl breaks up into KOH and HC1 in
.aqueous solution. It seems almost impossible to explain
reasonably by the dissociation theory of solution the
mechanism taking place in cases of solutions stated above,
which are easily explained by the assumption that the
solvent and the solute remain in a state of combination at
all dilutions.
Gladstone and Hibbert 11 carried out a large number of
experiments and found that the influence of solution on
212 ASSOCIATION THEORY OF SOLUTION
the salt is a very complex. The behaviour of different
salts is not uniform ; some increase and other decrease in
refracfcivity on passing into solution. These facts also
could not be reasonably explained 12 by the electrolytic
dissociation theory. Although in many cases the
refractive index of a substance, can be calculated from the
refractive power of its solutions, other cases, not a few in
number, have also been met with where the refractive
index so obtained, not only differs from that obtained
with the solid substance, but depends also on the solvent.
At first this behaviour was considered to be due to
ionisation, but it was found later that many cases can not
be explained on this view. Recently, the subject has
again been studied by Walden, in pursuance of his
previous work on non-aqueous solutions. This author has
determined the refractive indices of tetraethylammonium
iodide, tetrapropylaramonium iodide, and phenyldimethyl-
ethylammonium iodide in a large number of different
solvents. Whilst the refractive indies of these substances,
calculated by the mixture formula, have been found to
vary only slightly with the concentration of the solution,
the values calculated from solutions in different solvents
have been found to differ considerably from one another.
These differences cannot be ascribed to variation in the
degree of ionisation, because it has been found that they
occur even in solutions in which the degree of ionisation is
the same, and further in solution in which the degree of
ionisation is different, the same value for the refractivity
has been obtained. In considering other physical
properties of the solute and solvent, however, the author
OPTICAL EFFECTS 213
shows that the molecular volume of a substance in
solution is a variable quantity according to the solvent
employed. Thus for tetraethylammonium iodide in
aqueous solution, the molecular volume is ISG'9, whereas
in acetonitrile solution it is only 157"1. The molecular
volume, however, varies only slightly with the concentra-
tion in any given solvent On comparing the values of
the molecular refractivity with those of the molecular
volume in different solvents, it is found that the greatest
value of the refractivity is found in that solvent in which
the molecular volume of the solute is the least, and vice
versa. Moreover, it is found that the solute has the highest
value, of molecular volume in that solvent which has the
smallest co-volume. It would therefore appear that
variations in the refractivity of a dissolved substance are
probably due to variations in its molecular volume in
different solvents, and not to variations in its degree of
ionisation. Two points, however, may be raised in this
connection, (1) it still remains to be settled properly
whether solute, solvent or both separately or in associated
form change, in volume, and (2) if change in volume
could counterbalance the change in refraction how do the
other properties stand which undergo alterations
simultaneously with the formation of solution, ft may
also be noted here that the determination of contractions
in solution have shown that Walden's assumption that the
molecular volume varies only slightly with the con-
centration in any given solvent is not general.
The effect on the refractive property of a substance by
its association with a solvent could be studied by
2 14 ASSOCIATION THEORY OF SOLUTION
determining the change in refractive properties taking
place when a molecule of it is allowed to combine
gradually with increasing number of molecules of solvent.
Each addition of a molecule of a solvent may introduce
changes in property of the resultant substance which may
differ from any of the original components. This final
property depends on the adjustment of all other
properties e.g., thermal, electrical etc., a loss in one may
introduce an equivalent gain in one or more of the other
properties.
At present tabular statements are available in per cent
by weight or by volume which are rather inconvenient
in studying the effect of solvents on solute. Determination
of properties of a substance by gradual addition of
another substance in molecular proportions are considera-
bly desirable.
2. Polarisation.
A light ray passing through a Nicol's prism prepared
from Iceland spar gives emergent ray differing from the
original. One of the planes is polarised and if this ray is
examined by another NicoPs prism, it will be found that,
on rotating the latter, the field of view appears alternately
light and dark, the minimum of brightness following
the maximum as the prism is rotated through an angle
of 90". Ordinarily the first prism is called the polarizer
and the second the analyzer.
If a tube containing a solution of cane sugar is placed
between the two prisms, after making the field of view
dark by placing the axes of the two prisms at right
OPTICAL EFFECTS 215
angles to each other, the field ligthts up. By rotating
one of the prisms through an angle the field darkens
again. Substance like turpentine having the property
of rotating the piano of polarized light is called optically
active. An optically active substance is called dextroro-
tatary when the analyzer has to be turned to the right
i.e., clockwise to obtain darkness, and lievorotatory when
the analyzer should be turned to the left for the purpose.
The refractivity of substances and their mixtures is
general, but the polarisation of light is a property
possessed by a few selected ones. This property is entirely
dependent on the arrangement of the atoms in the molecule.
The magnitude of this property of a substance is
measured in teims of angle of rotation mentioned above
by means of instruments called polarimeter. This angle
of rotation for pure substance depends on :
(1) nature of the substance.
(2) length of the layer through which the light passes,
(3) wave length of the light used (the shorter wave-
length geneially gives greater angle of rotation).
(4) temperature.
When the substance is examined in solution the in-
fluencing factors, in addition to above, are :
(1) nature of the solvent.
v2) concentration of the active substance.
The standard of measurement used in this connection
is the angle of rotation produced by a liquid which in the
bulk of 1 c. c. contains 1 gm. of substance, through a
length of the column of 1 dcm. This angular deviation
of light is called the specific rotation of the substance
2l6 ASSOCIATION THEORY OF SOLUTION
and is represented by [a] 1 = for pure substance and
r -|t lOOct
L a J , -- tor solutions.
Where [a] specific rotation of the substance.
t = temperature of determination.
D = D line (sodium light),
a = observed angle of rotation.
1 = length of the tube in decimetres containing
the liquid through which the light
passes.
d=density of the pure substance,
c number of grams of active substance in
100 cc. of solution.
" Molecular rotation " or " molar rotation " is the
product of the specific rotation and the molecular weight.
In considering the factors influencing the property
of substance which can turn the plane of polarisation of
light it may be noted that this property is derived from
the position of an atom or atoms contained in a molecule.
Carbon has been ascribed to give this property to the
molecule of many commonly occurring organic compounds.
And its magnitude depends considerably on the position
of the rest of the atoms or groups of atoms in the
molecule. Magnitude of specific rotation is considerably
influenced by the nature of the radical which created such
property. Several investigators have worked on this sub-
ject, but only a few typical instances will be quoted for the
purpose of illustrating the nature of the influence that
could be created by the introduction of different radicals
OPTICAL EFFECTS 217
in a molecule on its optical activity. Gave 13 considered
that the degree of the asymmetry of the molecule of an
optically active compound with mass of each radical and
the distance of its centre of gravity from the centre of the
figure of the tetrahedron ultimately influence the sign and
magnitude of the specific rotation. He, in support of his
theory, gave numerous results amongst which the follow-
ing are interesting :
(1) Specific rotatary power increases with the molecular
weight of radical introduced in the place of acidic
Jiydrogen of tartaric acid.
(a]
Methyl tartrate ... ... + 2-14
Ethyl tartrate ... ... ... -f7*66
Propyl ... ... ... +12-44
Isobutyl,, ... ... ... -f-19'87
(2) Substituted benzoyl group in dibenzoyl tartarie
acid has been assumed to move the centre of gravity to
the other side of the plane of symmetry which is situated
between the COO II and OH groups ; this replacement
reverses the sign of the rotary power. The centre of
gravity of the molecule moves back towards the above
mentioned plane of symmetry and thereby rotatary power
is diminished by the substitution of H in COOH group
with an alkyl group :
[]
Dibenzoyl tartaric acid ... 1177
Methyl f , ... _ 88-8
Ethyl .. ... _ 60-0
Isobutyl ... - 42-0
2f8 ASSOCIATION THEORY OF SOLUTION
If acetyl radical is introduced in the place of benzoyl
radical in the above compounds the effect on the rotatery
power is similar but the magnitude of the laevorotation is
less on account of the molecular weight of the radical
being less than that of the other :
Diacetyl tartaric acid ... 23'1
Methyl ... -14-3
Ethyl ... + 1-0
Propyl ... + 6'5
Isobutyl ... 4-10-3
Crum Brown H considered that the constitution of
substituting radicals has an influence on rotatary power
of the final molecule in addition to its mass as proposed
by Guye. Examples have been shown contradicting 15
these hypotheses and modified ones proposed, but nothing
seem to be completely decisive except that of the existence
of a relationship between molecular structure and its
optical activity.
Influence of change of temperature on the optical acti-
vity of substances has been noticed by Biot and subse-
quently more data published by several other investi-
gators. Optical activity of a substance may increase,
decrease or remain constant with variation of temperature.
Formulae have been proposed by several investigators to
establish the relationship between specific rotation and
temperature but none of them unfortunately seem to
attract any general importance.
Alkyl esters of tartaric acid afford considerably inter-
OPTICAL EFFECTS 219
esting examples. Methyl tartrate gives [a] = + 2'07,
no rotation at 6 C and negative rotation below C. And
others give maximum 10 at higher temperatures, some oi'
them are given below ;
Ester. Temperature. Maximum molecular
rotation.
Ethyl tartrate 175 +309
n-propyl 150 +41*65
Sec-propyl tartrate 144 + 58'03
iso-Butyl 120 +539
iso-Amyl 151 +4872
Allyl 130 +4324
Sec-Octyl 160 +51-40
Di-trichloioacetyl derivatives of ethyl and iso-butyl
tatrates give minimum 17 rotations at elevated tempera-
tures.
It has already been mentioned that specific rotation
of a substance can be determined by taking polarimetric
readings of its solution of known strength by means of
the formula given theiein. But the figures obtainable by
this method are materially influenced by the nature of
the solvent and generally different from what it is in the
pure condition. Waldon 18 has studied this problem with
considerable thoroughness but the following typical 10
examples will serve the purpose of illustration of the
phenomenon :
220
ASSOCIATION THEORY OF SOLUTION
Solvent.
l^ormamide
Water
Methyl alcohol
Ethyl alcohol
Benzene
Ethene dibromide
Glycerol
n-propyl alcohol
iso-Butyl alcohol
Sec-Octyl alcohol
Toluene
O. Xylene
m. Xylene
p. Xylene
Mesitylene
Chloroform
[a] at infinite dilution.
Ethyltartrate.
+ 30-4
+ 26-85
-f 9'13
-f 61
-191
-f 10-57
+ 7-4
+ (V53
-f 5-24
-f 4-6
-f 2-7
+ 1-8
+ 0-7
- 30
- 3-2
Nicotine.
- 70
- 77-4
-1294
-1401
-163-5
-183-5
Determination of effects of concentration on specific
rotation of substances in solution is very useful in studying
the relationship between solute and solvent. Influence of
concentration on rotation of substances in solution has
been appreciably found in many cases - f malic acid C 4 H ( .O 5 ,
however, affords most interesting results. Schmeider' 20
found that the rotation of an aqueous solution of malic
acid was left handed in dilute solutions and right handed
in concentrated solutions passing through a solution
having no rotatary power at about 34% strength.
OPTICAL EFFECTS 221
The following figures may be examined in this
connection :
(1) Specific rotation increases with concentration.
Malic acid. C 4 H G O 5 in water. 21
2165
-0'44
-090
-T43
28-67
+ 0-33
-0-35
-0-83
40-44
4-131
4-0-54
-0-12
53'75
4-2-52
4-1-73
4-0-94
64-00
+ 4-10
4-272
4-1-99
d.Camphor C 10 H 1G O, in acetone. 22
>/o -15-11 22-29 32-29 46-56
13*7
x] =48-77 49-13 49'66 5055
Quinine (anhydride) CootL^NoOo in ethyl alcohol. 23
10
w."
20
I
-171-4
-1696
-168-2
4
-166-1
-1644
-163'2
6
-162-4
-160-9
-159-8
(2) Specific rotation decreases with concentration.
Barium d.Methoxyl succenic acid C 5 H 5 O 6 Ba in water. 2 *
o/o = 1-149 5746 12-42 26-12
[af =+316 -2-21 -7-36 -14'27
222 ASSOCIATION THEORY OF SOLUTION
Cane sugar C 12 H 2 2O n , in pyridine. 25
% =1 2 4 625
[a]^ =4-86-7 85-9 847 83'fi
(3^ Specific rotation increases, passes through maximum
and then decreases with concentration.
Nicotine C 10 H 14 N 2 in water. 20
% ro6i 5*700 8-307 1026 1559 ico
laf D --77'66 -76-95 -7674 -76*89 -7759 -1640
Unfortunately work has not been sufficiently extended
to afford enough instances of variation as could be found
and described in connection with other physical properties
of solutions. But the variations shown above would
be able to give conclusions needed for the present
purpose.
Specific rotation of a substance in solution may be
considerably interfered by the introduction of a third
substance 27 . Boric acid, molybdates, tungstates, arsenates,
antimoniates, and alkaline uranyl nitrates have been used
by different investigators, in altering the rotation of
solutions of tartaric, malic and lactic acids. Magnitude
of interference, of course is dependent on the quantity of
the added substance, Geruez found that about 10 per
cent solution of malic acid having [a] =0*189
OPTICAL EFFECTS 223
indicates +13*26 with an addition of 2'017 gins, of
sodium molybdate Na 2 Mo0 4 ,2H 2 O per 100 cc., whereas
inactive solutions are produced with additions of 1 4, 2 85
and 4*25 gms of the substance, maximum and minimum
rotations occurring at intermediate concentrations.
According to Boeseken and Convert, the rise in specific
rotation shown by certain sugars in the presence of boric
acid is always accompanied by a marked increase in
electrical conductivity. Probablity has been proposed
that there is a particular configuration of the terminal
hydrogen atoms and hydroxyl groups which favours the
formation of compounds with boric acid. Most sugars,
on dissolution in water, undergo a transformation which
can be followed with the polarimeter, and is due to the
transition from the a- to the /5- form, or the reverse,
until equilibrium is attained. Such a change may
involve an increase or a decrease in the power to combine
with boric acid, and hence a corresponding change in
conductivity. The behaviour of a- and /> lactose in
solution, in the presence and absence respectively of
boric acid, has been examined. The efCect of boric acid
on the rate of change of rotation of both forms is
practically negligible. The presence of a minute trace
of impurity greatly affects the result and such experi-
ments have been made at 12'9, 15'5', 20'0 and 25.
Muta-rotation : Association theory of solution con-
siders that a reaction takes place between solvent and
solute, and like chemical reactions this combination occurs
with some velocity, Majority of the optically active
substances have been found to combine with solvents very
224
ASSOCIATION THEORY OF SOLUTION
quickly and attempts have not been made to measure them*
But many sugars, oxyacids, lactones of oxyacids, nicotine,
amines and nitro-camphor when freshly dissolved and
polarized give rotation values which gradually change
and become constant after a period depending on the
condition in which they are presented to react on each
other. Rotation may increase or decrease towards a
constant with a velocity of reaction which could be
influenced by catalysts. For sugars, dilute alkaline
solutions even 0'1% ammonia may cause immediate change
of rotatary power to the constant value Like chemical
reaction this is also enormously accelerated by increasing
the temperature. The following specific rotations 26 of
sugars in different conditions only indicate the effects of
association of solvent and solute.
[a]
[a]
[a]
;able modifi-
First labile
Second labile-
cation in
modification
modification
solution.
in solution.
in solution.
+ 52-7
+ 105-2
+ 22-5
+ 816
+ 135-0
+ 52-3
+ 52-5
+ 86-2
+ 344
4-138-0
+ 118-2
- 92-5
-104-0
+ 104-4
+ 156-7
+ 19-2
+ 94-4
Dextrose
Galactose
Lactose
Maltose
Levulose
Arabinose
Xylose
Experiments on muta-rotation have been studied with
some thoroughness by only a limited number of investi-
gators 29 and resutsso far obtained can hardly be considered.
OPTICAL EFFECTS 225
to be exhaustive. Although investigation on this subject is
very difficult on account of the uncertainty of purity of com-
pounds to be dealt with yet there seem to be much known.
Lowry obtaining muta-rotation with camphor in non-
aqueous solutions opposed the views of Fischer that the
phenomenon is due to the hydration of solute (sugars)
with the solvent (water) and considered the process to be
due to some iso-dynamic changes. Association theory
of solution, however, explains phenomena both in aqueous
and non-aqueous solutions by the assumption that each
pair of solvent and solute form compounds in molecular
ratios same as the dilution which may differ in properties
from any of the components or from what could be
formed at any other dilution. This is true for all
combinations so long as they can form solutions.
Phenomena of muta-rotation and other changes in
physical properties of. solutions with time have been
explained by Riiber and others 30 by assuming that the
molecules of the solute only undergo changes under
the circumstances, but it would be interesting to know
how the subject would stand if all such phenomena
are explained in light of the association theory of solution
and the change with time is due to velocity of the re-
action between solute and solvent at those particular
dilutions. It may be possible to explain all the facts
without assuming the existence of any such new modi-
fications of the solute which could not be isolated.
Magnetic rotation : Faraday 31 discovered that any
transparent body being placed in a magnetic field acquires
the property of rotating the plane of polarisation of light
15
226 ASSOCIATION THEORY OF SOLUTION
in the direction of lines of force. This magnetic rotation
is proportional to the strength of the field, the thickness
of the traversing medium, and the nature of the light used.
Perkin 32 did lots of experiments on the subject some of
which are very useful in considering the relation between
solute and solvent in solution. His determinations of
molecular rotation 33 in magnetic field of organic and
inorganic bases and acids, and some ammonium salts in
solution may be used in support of association theory of
solution, namely the solvents and solutes form compounds
in proportions same as their dilutions, all properties of
which may differ from any of its components and from
any other formed in other proportions.
The optical rotatary property of a molecule is acquired
and altered by changing the constituents in the molecule,
by action o solvent, by altering the dilution of the same
solvent, by changing the temperature and by introducing
it in a magnetic field. Now a question arises whether any
change in optical rotatary property indicates simultaneous
corresponding change in the structure of the molecule or
indicate only a change in molecular vibration which would
respond to the ray of light. The ultimate result is that
the light ray passing through such medium receives a
property imparted by the molecules of the medium. Thus
whenever any rotation of light is observed it may be
presumed that it must have come from the peculiar
movements of the molecules of the medium through which
the light has traversed. Consequently the molecules of (he
medium must prepossess a property of imparting such one
to the light ray passing through it. Now it remains to
OPTICAL EFFECTS 227
*be considered how a molecule can acquire this property,
If it is true that the molecules remain in a state of vibra-
tion and it is this property that could interfere with the
ray of light which may come in contact while passing
through it, one may not be irrational to think that the
vibration of a molecule as a whole could be possibly inter-
fered with without changing the intra-molecular relation-
ship amongst the atoms or radicals composing the molecule.
When a molecule is placed in a magnetic field it acquires
such property of rotation which may not interfere with
intra-molecular arrangement.
It is also rational to suppose that a molecule may acquire
this property by undergoing some intra-molecular change,
and effects of solvents may be considered amongst this class.
Solvents combining with solute molecules interfere with or
impart this property. When a third substance is introduced
in a binary mixture a combination of three things takes
place and the properties of the final product differ from the
components and from any other that may form in a different
ratio. The effect of combination of solute and solvent might
change or not the internal condition of the associated
molecule to affect all or any other properties, and the final
resultant molecule reaches a state of equilibrium after
neutralising all forms of energy, thermal, electrical, etc.
Attempts have been made to correlate the optical
properties of solutions with osmotic and other properties
and although most of them have failed yet it may be
some time interesting to know them 34 , but it is not
worthwhile to describe them here. Scheuer 35 determined
several physical properties of solutions of diacetyl tartrate
228 ASSOCIATION THEORY OF SOLUTION
and menthol each dissolving in a number of solvents..
His investigations included determinations of viscosity,
dilation, melting point, and rotation for light of different
refrangibilities. From these results this investigator
failed to notice, either in the liquid or in the solid phase,
any sign of complex formation between solute and solvent
molecules in solution. It would not be correct to draw
conclusions from these results only that solute and
solvent do not combine because formation of such com-
pounds is attended w*th readjustment of all forms of
energy arid these few observations are- too incomplete and
inexhaustive to bring forward such generalisation.
Association theory of solution has succeeded in
explaining above phenomena connected with optical pro-
perties of solutions. The electrolytic dissociation theory
of solution seems to have totally failed to explain the
magnetic rotatary properties of electrolytes. Very
systematic attempts have been made by many eminent
investigators 30 to establish relationship between this
property and ionisation in electrolytes but the only
rational conclusion that may be drawn from their
results is that there can be no electrolytic dissociation in
electrolytes. Smiles has summarised the results of
experiments on the effect of solvent and dilution on the
magnetic rotation of salts showing that this property
increases, decreases, or remains constant with dilution
depending entirely on individual nature of the substance.
These results obviously do not harmonise with the con-
ception that the solutes in an electrolyte partially break
up into ions and number of the broken molecules increases
OPTICAL EFFECTS 229
with dilution. There is no doubt that the electrical
conductivity increases with dilution but the explanation
put forward that that is due to the breaking up of the
molecules in solution does not suit other properties.
Effects of dissolved state and dilution present strong
evidences in favour of the association theory of solution.
Perkin's work on the subject with amonium and sodium
salts of formic and acetic acids when compared with that
with ammonium nitrate affords interesting illustration.
The rotatary power of ammonium nitrate is nearly equal
to sum of the rotations of ammonia and nitric acid but the
ammonium salts of halogen acids are quite abnormally
high. In the cases of double salts" of Na 2 SO 4 ,MgSO 4 ,
Na 2 SO 4 ,CdSO 4> and Na 2 S0 4 ,MnSO 4 the rotations were
found to be equal to the sum of the rotatary effects of
the individual salts present there and in the cases of
NaCl.HgClo, and 2KI,HgI 2 larger increase in rotatary
power has been noticed than that of the sum of the rotatary
effects of each of the salts. These results have been inter-
preted to be the consequence of no rearrangement of
atomic affinities in the former cases , and in the latter cases
atomic combination taking place, rotation gets so much
changed. It is undeniable that the large increase in
rotatary power produced by mixing of salts indicates
some redistribution of the atomic relationship inside the
molecules of the components, but in the cases where no
such increase is noticed it need not necessarily be conclu-
ded that no reaction has taken place, on the contrary it
may be concluded that the result of the reaction did not
produce complex molecules of different rotatary activity.
*30 ASSOCIATION THEORY OF SOLUTION
Adsorption.
When rays of light of different vibration are allowed
to pass through gaseous, liquid or solid media of different
substances, some of them pass through while others
are absorbed. Presently absorption by liquid media wilt
be needed in trying to disentangle the mechanism of the
state of existence of solvent and solute in solution.
Hartely 38 , Julius 39 , and Drude 40 did not quite agree in
their opinions about the exact dynamic nature of this
property. There are two kinds of absorption in solution,
continuous and selective. The former decreases with
dilution and ultimately disappears. A medium showing
continuous spectra may show only selective one under the
same conditions if the thickness of layer traversed is
sufficiently decreased. Thus it will serve the purpose of
the theory of solution to deal with the phenomenon of
selective absorption which could be influenced by solvent
and solute whereas the other is dependent only on the
number of molecules present per unit area in the path of
the ray of light. Hartley considers that the selective
absorption of light is caused by sub molecular particles
vibrating synchronously with the incident light waves.
Julius gave a strong support to this view by showing
that the absorption and emision spectra of simple substan-
ces are indentical. It seems reasonable from the results
obtained by numerous investigators that the selective
absorption is caused by intramolecular vibrations arising
from atoms and groups of atoms.
The molecules in the medium thus absorbing the
OPTICAL EFFECTS 23!
light energy may get it converted into (1) floursecent
light (2) heat, and (3) chemical energy,
Solutions of substances, having strong absorption-
power, are often dealt with in these studies instead of pure
samples. Solvents are selected in a way so that it has
not- got any absorption band in the region where the-
solute under investigation would show any. Ordinarily
ethyl alcohol has been used by many investigators. While
studying the absorption spectra in solution it has beei>
noticed that although a solvent may not exert any
absorption in the region of the spectra in question, yet it
often influences on such absorption of the solute. Kundt 41
found that the absorption bands of the dissolved substances
are pushed towaids the red legion of the spectrum by
increasing the refractive index of the medium and there-
fore he recommended that when a comparative result of a
series has needed they should be obtained with the same
solvent and with the same molecular dilution ; unless
study on the effect of dilution has needed. Variation
of the thickness 42 of the layer of solution of uniform
molecular concentration serve useful purpose for com
parison of one substance with another. Photographic
determination of absorption bands of camphor in
alcoholic solutions in different concentrations and different
thickness of layer traversed by light serve as important
illustration for proving the action between solvent and
solute. The effect of solvent on solute so far a&
absorption band is concerned is influenced by dilution, and
in case of alcohol-camphor it is over 10 units on the-
logerithmic scale of reletive thickness of solution.
33 2 ASSOCIATION THEORY OF SOLUTION
Miller 43 , and Soret and Rilliet 44 tried to find out the
relationship, if any between chemical constitution and
absorption band, but the problem, however, Lad been
exhaustively tackled by Hartley 45 and subsequently by a
few others, who finally came to the conclusion that
absorption spectra and chemical constitution of organic
compounds were related. Position iso-mersitn 46 in the
benzene nucleus was found to influence absorption of
light, the addition of meth^lene group caused the
absorption bands to shift towards the red position of the
spectrum in alkyl nitrates, alcohols 45 and amines 47 .
Homologous alkaloids 48 like morphine and codeine,
and quinine and cupreine have almost identical obsorption
curves Although constitutive nature of the absorption is
admitted yet Dobbie and Lander have shown that a given
substitution has less influence on absorptions of complex
bodies than on those of simpler compounds. Piperonylic
acid and veratric acid are simple benzene derivatives.
COOH <ZI> O COOH <II> OCH 3
O J OCH 3
CH 2
Piperonylic acid. Veratric acid,
have appreciably different curves whereas complex mole-
cules of alkaloids of tetrabydrohereberine and corydaline,
A
C 13 H 10 N(V''| I \ CH 2 CH 3 C 12 H 1B NO,
Tetrahydrobererine Corydaline
\\hioh possess the same relations to one another, have
OPTICAL EFFECTS 233
practically the same absorption curve. Similar examples
have also been noticed in the cases with styrol (C (; li 5 CIi
- OH 2 ) and benzoic acid (C G H 5 COOII), and cinchonine
(C 17 H 19 N 2 O.COOH) and cinehotenine (C 17 TI 19 N 2 O.COOH)
very different spectra in the case with the first and almost
identical in the latter was found.
These results indicate how the absorption spectra
are due to the components constituting the molecule and
to what extent this property is influenced by the remaining
components of the same molecule. A valuable collection
of all results of this nature has been made by
Hartley 47 in discussing the relations between the type of
ultra-violet absorption and the structure of compounds.
Effect of dilution on the absorption power lias
been noticed in the cases of dextroracemic-, and rne&o-
tartaric acids no , in-active-, and dextro-, corydalineb 51 ,
tetrahydrobere-berine and canadine, and diben^oyl succinie
esters 52 .
There are two bands shown by quinone, the one in the
ultraviolet region with its head at about 4000, and the
other in the visible region at about 2300 units The
former is benzenoid band and the latter, isorropic band, is
caused by dicarbonyl system. It will be found that in
alcoholic solution the benzenoid band is weak and the
isorropic band well marked. Hartley and Leonard 58 have
found that the two bands are nearly equally distinct in
etherial solutions. Kehrmann 54 has shown that, similarly
as the solvent, substitution of hydrogen atoms with
methyl group or halogen atom in p-benzoquinone would
influence the absorptive power of its carbonyl groups. In
234 ASSOCIATION THEORY OF SOLUTION
determining absorption of derivatives of quinone in
alcoholic solutions the following steady increase in the
persistence of the benzenoid band had been noticed.
Change of dilution over which the band persists with
%
Monochloro benzoquinone 42*0
2*6 Dichloro benzoquinone 55*0
Trichloro benzoquinone 77 '0
Trichloro toluquinone 88'0
Intramolecular change undoubtedly influences tha
absorption of light but the exact nature of the relationship
is not properly known. The absorption by nitrocamphor 54
and its derivatives in solvents of varying composition
afford splendid examples of intramolecular change caused
by the solvent only. Lowry has also found that the
speed of change between normal and pseudo-nitro-camphor
can be easily controlled by the regulation of the solvent or
by the addition of a suitable third substance. A deep
band is developed, with its head at about 3100 units by
the addition of an alkali which it has been considered,
may not be due to isomeric change that takes place in
other non-alkaline solutions of nitrocamphor. These facts
serve useful purpose of illustrating the states of existence
of a substance in solution and in pure condition.
There are two factors by which a substitution may
influence the absorption of the parent compound, (i) the
mass of the substituent 55 and (ii) the residual affinity
of the substituent 56 . Hartley thinks that the increase in
the mass "of the molecule caused by the insertion of a
substituent tends to retard the intra-molecular vibrations
OPTICAL EFFECTS 235
and thus brings the absorption towards the red region of
the spectrum. The influence of substitution with a parti-
cular group is greater on substances of low molecular
weights than on compounds of complex molecular weights.
Dobbie and Lander 57 have shown that insertion of
methyl and other light groups has no effect on the absorp-
tion spectra of many alkaloids.
In studying the influence of the residual affirmity of
the substituent it is necessary to eleminate or minimise
the disturbing influence of the mass of the group. Thus
it will be found that in mono-substituted benzene the
absorption band due to benzene are not to any great
extent disturbed if saturated atoms are attached to the
nucleus, but with unsaturated atoms the disturbance is
quite distinct 58 . In anisole C G H 5 OCH 2 , the benzene
spectrum persists but in a modified form ; the benzene
absorption, however, becomes quite indistinct in benzal-
dhyde C 6 W 5 COH, aniline Co^NETo, and nitrobenzene
C G H 5 NO 2 . In such cases a broad band appears near the
visible spectrum and an additional interest is created to
find that simultaneously with these effects refractive power
and magnetic rotation are altered.
Effects of solvents on the absorption of light by sub-
stances are well illustrated and known in the cases with
nitrophenols and cotarnine. Ortho-, para-, and meta-
nitrophenols 59 present lots of differences in neutral solu-
tions from those that could be found under similar circum-
stances in alkaline solutions. The solutes in neutral
and in alkaline solutions differ in molecular structure.
The nitro-group changes its consitution in different
236 ASSOCIATION THEORY OF SOLUTION
iso-merie molecules and forms different compounds in alka-
line solutions. Cotarnine is known to be capable of under-
going iutra-molecular changes in different media of sol-
vents. Their distinctions are manifested in their power
of absorption of light. Coiarniue as an alkaloid when
present as a salt possesses of (i) ammonium type 00 , as a base
liberated from these salts with alkali behaves like an (ii)
aldehyde 01 and secondary base, and when obtained from
isoquinoline it is said to possess (iii) carbinol 02 structure.
Spectrographic 03 studies have proved very useful in
establishing identities of these modifications. Dobbie 64
and his coworkors made extensive researches on this
change of structure according to the state of existence in
solution of phenyl acridine derivatives, cotarnine, hydro-
cotarniiie, eyanhydroeotarnino and hydrastinine. These
results furnish a great evidence as to how the relationship
amongst the components of a molecule is dependent
on the other molecules or groups of molecules with
which they will come in contact during the state of
existence. "
The molecular structure of cotarrine also varies from
the carbinol to the ammonium type according to the nature
and composition of the solvent. Ether or chloroform
keeps it as the carbinol form while alcohol converts it
into the ammonium type. Conversion of one form into
the other has been studied by taking a chloroformic or
ethereal solution and then adding alcohol gradually. The
following quantitative results were obtained by adding
methyl alcohol to an ethereal solution of the base at
odinary temperatures :
OPTICAL EFFECTS 237
o/o methyl alcohol 25 40 50 100
o/o Carbinol form
of cotarnine 100 97'5 92'5 85'0 750
% Ammonium form
of cotarnine 2*5 7 5 15'0 25'0
These reactions 05 causing changes of molecular structure
have velocities like all other chemical reactions and this
speed is influenced by light and heat. Colourless solutions
of hydrazones do not change speedily when kept in the
dark but very rapidly become deep yellow on exposure to
sunlight. 6 * 5 This reaction, however, is not so pronounced
in dilute solutions. The spectrum of the exposed solution
gives the characteristic hand of the hydrazone* 57 group
being thus changed into azo group :
Bally found that in undergoing isomeric change produced
by the action of light on the aldehyde hyrazones the
phenomenon is accompanied by the appearance of well
marked band in the visible region of the spectrum. The
velocities of these reactions in solutions haye been found
to depend also on several other factors. (
When substituted ammonium salts of halogen acids are
converted into their corresponding hydroxides by the
action of silver hydroxide in aqueous solutions strongly
alkaline solutions having electrical conductivities same
as those of the common alkali hydroxides are formed.
In course of the determination of electrical conductivities
of such substituted ammonium base, Hantsch 08 found
that the value gradually decreased on keeping. Phen)l
238 ASSOCIATION THEORY OF SOLUTION
aeridine methiodide gives a good illustration of this
phenomenon. Influence of time and temperature in the
transformation of the carbinol form to the ammonium
base type of contarnine in alcoholic or aqueous solution
has been noticed by Dobbie 69 .
The brief account given above about the phenomenon
of absorption of light by solutes in solution do not seem
to contain any instance that would go against the
association theory of solution as described elsewhere.
Formation of compounds amongst solute and solvent may
be assumed whenever this optical property presents any
change in the solution. The disturbance of energy at
particular conditions of solutions, where absorption of
light is interfered with, assumes its equilibrium after
acquiring such property. When the substances lose
their such property after separation from the solvent it is
reasonable to assume that they acquired the same by dint
of their combination with the solvent.
4. Fluorescence.
From very early days attemps have been made by
several scientists 70 to discover the theory of the pheno-
menon of fluorescence but they do not seem to have come
to an unanimous definite conclusion as yet. Ml that can
be gathered from the discussions made on the subject
is that the phenomenon may be due to or influenced by
both intra-molecular reactions in pure state and in state
of solution. The phenomenon of fluorescene has been
associated with the existence of certain atomic groups
in the molecule, e.g. isocyclic benzene, anthracence, aeridine,
OPTICAL EFFECTS 239
azine, oxazine, thioazine and pyrine rings, of course effects
due to these groups of atoms are influenced by the pre-
sence of other groups in the same molecule.
A substance after absorbing light energy produces
fluorescence, and in order that a substance could fluoresce
it should be placed in a light of suitable wave length.
A solution of quinine salt fluoresces in violet and ultra-
violet light but does not do so when illuminated by a red
light. A solution of eosin absorbs the green rays and
fluoresces greenish-yellow. An acid solution of a quinine
salt fluoresces pale blue, whilst it absorbs a portion of
the ultra-violet light concluding obviously that the wave
lengths of the absorbed light are shorter than those of
the fluorescent light. Since it has been found that the
fluorescent light does not enter into the substance it is
concluded that the wave length is changed during its
reflection from the surface, and this is known as Stokes's
Law 71 . In studying fluorescence in solution Stokes noted
that there is an optimum concentration for each fluores-
cent substance, the intensity of the fluorescent light
increases, reaches maximum and then decreases again
with concentration.
Attempts have been made by some investigators to
make use of this property for analytical purposes and
attention of the scientists is drawn to this branch of
research since not only the results will be useful for
industrial or technical purposes but also bring many
conflicting theories into proper test.
Most of the investigations on fluorescence are done
Jby taking the substance in solution. It has been noticed
240 ASSOCIATION THKORY OF SOLUTION
that the effect of the solvent is often very pronounced.
Thus a solute may give strong fluorescence in one solvent
and much less or none in another ; and the colours of
the fluorescence substances in different solvents very
seldom agree. Solutes form compounds with solvents
in ratio same as their dilution, and the property of
fluorescence of such compounds composed of solute and
solvent in different ratios need neither agree nor should be
proportionate, since these compounds are likely to differ
in properties from other similar ones formed at different
dilutions and from the calculated average of those of the
pure solute and solvent present there. The solvent alcohol
forms fluorescent compounds with s-phenyl-di-p-nitropen-
azothionium hydroxide and non -fluorescent with diphenyl-
pyrone, on the other hand, sulphuric acid gives non-
fluorescent solution with the former and bright fluorescent
one with the latter. Some substances do not form
fluorescent compounds with solvents in concentrated
solutions but do so when the portion of the solvent
becomes qtfite large 2 -methyl- 3 -amino 4- oxy-quinoline 72
is fluroscent only in dilute alkaline or acid solutions.
Flourescence of eosin solutions in acid or alkali increases
on dilution up to a certain limit. This behaviour lead
some chemists 73 to think that it is the formation of ions
that causes the increase of fluorescence on dilution and
when it is too much diluted the weakening effect
of decreasing concentration operates and fluorescenc
diminishes with concentration at higher dilution. It does
not seem necessary to think that ionisation does take
place at all in such solutions as the pheonomenon is also
OPTICAL EFFECTS 241
found in non -electrolytes The association theory explains
the plenomenon in its own way, the compounds formed
with solvent and solute in different proportions have their
properties which need neither agree nor vary in the same
ratio in which they are constituting the molecule. In
view of the fact that majority of the fluorescent substances
either contain a mobile atom or are capable of undergoing
a change in constitution simply by a movement of the-
valencies of the component atoms it seems reasonable to
think that there exists a connection between tautomerie
change 74 and fluorescence. Hewitt thinks that a fluorescent
substance must exist in more than one interchangeable-
forms ; one of them absorbing energy from the incident ray
becomes converted into any other form, which performs
the function of fluorescence and gets reconverted into its
original state. This theory, however, is not inconsistent
with the association theory of solution.
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242 ASSOCIATION THEORY OF SOLUTION
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OPTICAL EFFECTS 243
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244 ASSOCIATION THEORY OF SOLUTION
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OPTICAL EFFECTS 245
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246 ASSOCIATION THEORY OF SOLUTION
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CHAPTER IX
ELECTRICAL EFFECTS OF SOLUTION.
St udies on electrical properties of solution attracted
considerable attention on account of its wide application
for practical purposes in science, and in industry. Results
connetcted with theory of solution may be mainly divided
into two following classes :
(1) Generation of electric current in solution.
(2) Conveyance of electric current through solution.
Generation of Electric Current in Solution.
Electrical equilibrium is disturbed by the production
difference of potential when a substance passes into
solution and when two different substances are placed in
the same solution. If these are suitably connected by
means of a metallic conductor electric current flows from
the terminal of the higher potential to that of the lower
and the reaction between solvent and solute proceeds
with increassed activity. Such system in which changes
of energy are associated with changes of matter
chemically in producing electro-motive force is called a
galvanic element.
Galvanic elements can be prepared either by means of
substances which conduct electrolytically or with the
combination of these and carbon or metal electrodes*
The former cluss has been investigated since a long time
by Bois-Reymond (1867), Worm-Muller (1870), Paalzow
248 ASSOCIATION THEORY OF SOLUTION
and a few others. If such an element is constructed
with both metallic poles of the same metal' and each
immersed in solutions of a salt of metal of different con-
centrations, the two solutions being separated by suitable
porous partition or by any other means so that they may
not get mixed too soon, the ends of the metallic conductors
on being connected by means of a wire a current will flow.
Such a cell is called concentration cell. Here the
electromotive force is generated by the osmosis of the
solutions of different concentrations in mixing to form a
homogeneous liquid. The molecules of solute in a con-
centrated solution is in combination with lesser number of
solvent molecules than those in dilute solution and
therefore there is a tendency to form compounds of solute
and solvent in uniform ratio thus causing disturbance of
electrical equilibrium. If C x and C 2 are the concentrations
of the two solutions then the electromotive force or the
difference of potential is expressed by the following
equations at 15 9 C,
E = 0-057 log^ 1 volts.
^2
and this equation, however, varies with temperature, which
at 25C becomes j
E 0-059 log^ volts.
v^o
It has already been noted in page 11 that the
solubility of sparingly soluble substances could be
measured by the measurement of electromotive force in a
concentration cell system. The determination of solubility
of silver chloride in decinormal potassium chloride
ELECTRICAL EFFECTS 249
solution was done by setting up the following com-
innation :
Oln AgNO 3
O'l n KNO 3
O'ln KCl
Ag.
saturated
with AgCl
One pole was made up of a silver electrode in decinor-
mal silver nitrate and the other of a silver wire coated
with silver chloride, in a decinormal solution of a potas-
sium chloride which was saturated with silver chloride by
the addition of a few drops of silver nitrate. To prevent
the two solutions precipitating each other a decinormal
solution of potassium nitrate was interposed in an U
tube. The electromotive force observed was 0*45 volt at
25C. This was of course composed of the electrode poten-
tials, and the diffusion potentials at the junctions of the
various electrolytes. In this case the latter may be neg-
lected as they fall within the limits of experimental error.
The observed difference of potential O45 volt was due to
the difference in concentrations of the silver ion in the
silver nitrate solution on the one hand and the silver
chloride solution on the other. Applying this E. M. F.
to the above mentioned formula,
0-45-0-059 log- 1 .
^2
The concentration Ci of Ag in decinormal silver nitrate
as about 0*084 according to dissociation theory,
0-45 = 059(log 0-084 -log C 2 ),
.'. C 2 = 1-95x10-9 normal.
The solubility in pure water may be obtained from
250 ASSOCIATION THEORY OF SOLUTION
this solubility of silver chloride, in decinormal potassium
chloride by the application of constant solubility product,.
(A of) X (01') = constant.
On application of the above results to this the
following value is obtained,
(1-95 x lO- 9 ) x (8-4 x 10- 2 ) - 1-64 x lO' 10 .
The concentration of each of the two ions in pure
water is the same and is equal to the square root of the
above figure i.e. 1/28 x 10~ 5 , Thug the saturated solution of
silver chloride in pure water at 25* C is 1'28 x 10~ 5 normal,
which is equal to 1*82 mg. per litre. Kohlrausch obtained
this solubility figure, 1*34 mg. per litre by conductivity
measurements at 18 C.
Although the agreement of result obtained by this
method with that of Kohlrausch from conductivity measure-
ment is a great mutual support about the accuracy of
the two methods yet at the present moment such con-
clusion need not be considered absolutely true because,
(1) this is rather a result of limited experiment and
should be corroborated by results of many other substances
at varying conditions,
(2) silver chloride is generated by the addition of a
few drops of silver nitrate solution. In forming silver
chloride a corresponding quantity of potassium nitrate is
also formed, which remains in the field and it is not
reasonable to think that this substance does not interfere
with electric measurements, and
(3) validity of application of rule of constant
solubility product inspite of the presence of a third
substance, potassium nitrate, properties of which are not
KLECTRICAL EFFECTS 25 I
identical with any of the other two solutes under the
circumstances of the Qxperiment, has not been properly
established.
Generation of electricity in these cells has been explain-
ed by Nernst 2 on the assumption that all metals possess
a property which he calls solution pressure or solution-
tension and that this property tends the metal to drive
ions* (positively charged) from itself into the surrounding
solution. In these systems of concentration cells there occur
two kinds of chemical reactions, (1) passing of the metal
into solution and () formation of homogeneous solution
by the mixture of different solutions used. Electrical
property of substances concerned such as conductivity is
changed as a result of the above mentioned reactions,
therefore their electrical equilibrium within themselves
must have been changed during the occurrence of the
phenomena. It is quite rational to consider that when
electrical equilibrium is disturbed with a change in con-
centration in solutions, such change is indicated by
potentiometric measurements. Mostly tbe electrical
properties of such resultant mixture are neither equal to
those of any of the original substances nor equal to their
arithmetical mean.
The ionic theory while calculating P. D. explains the
generation of electricity in concentration cells consisting
of simple
Liquid /Liquid,
by the assumption that the positive and negative
ions generally tend to diffuse from the place of high
concentration to that of low. If their velocities differ
25 * ASSOCIATION THEORY OF SOLUTION
across the boundary, the two other ends of the solutions
being connected by electrodes of common metal and by
means of metallic connection the current of electricity
would flow till equilibrium is reached.
The same phenomenon of generation of electricity may
be explained by the association theory of solution on the
assumption that when two solutions of different con-
centrations are brought into contact,
Solution
N
Solution
N
5 7 5
Solution,
osmosis or diffusion will commence at once to form
N N
solution of uniform concentration. Thus if y and , _r
solutions are brought in contact the strength of the
N
mixture after a lapse of time will be =-^. Now the electri-
5*5
cal properties of the resultant mixture are not arithmetical
means of those of the original solutions j therefore a
certain quantity of electrical energy is liberated or absorbed
during the process. Thus a disturbance in the electrical
equilibrium is created during the formation of association
of solute and solvent in a different molecular ratio.
Conveyance of Electricity through Solution.
For the purpose of studying conveyance of electric
current through solution the subject may be divided into
two classes (1) non-electrolytes and (2) electrolytes. In
disentangling theory of solution the former do not help
ELECTRICAL EFFECTS 253
much as they do not allow electric current to pass through
them. In dealing with the latter, which convey electric
current the laws of Faraday and Ohm may form the chief
basis. These laws are expressed in the following
formulae
(1) C-g.
(2) W=ExCxT=HxC 2 xT.
(3j Wt=ZxCxT.
Where, C = intensity of electric current,
E = electromotive force,
R = resistance,
W = work done,
T = time,
Wt = weight of a substance deposited at any
electrode as a result of conveyance of
electric current through a solution,
Z electrochemical equivalent of the substance
deposited.
When an electric current passes through a metallic
conductor there are manifestations of disturbances of
equilibrium of thermal, magnetic, energies, etc. and when
it passes through an electrolyte in addition to these some
chemical reaction takes place. This chemical reaction is
called electrolysis and it attracted more attention and
study than others. Whether any simultaneous generation
or absorption of other form of energy takes place when
electric current passes through a solution causing ultimate
decomposition of solvent need mo^re systematic and
quantitative study.
254 AS5OCIATION THttORY OF SOLUTION
It is presumed from the fact that electricity is generat-
ed or absorbed by chemical reaction that atoms in a
molecule are in a state of combination by electric force,
which practically means that chemical and electrical
forces are either the same or nearly so. Definite proofs,
however, are not available if these two forces are identical
although it has definitely been established and could never
be denied that the one could be often converted into the
other. The chemical force, with which an atom or groups
of atoms remain in a state of combination in a molecule
of a compound, is the combination of all kinds of
energies which are manifested during the formation or
decomposition of such compounds.
Conductivity of electric current through solutions of
a few salts like tetraethyl ammonium iodide in some non-
aqueous solvents have been determined without properly
establishing the exact nature of the chemical reaction that
may have taken place (if any) as a result of passage of
such current. But results have been considerably studied
when an electric current is allowed to pass through aqueous
solutions of substances like salts, acids and basses. Thus
when aqueous solution of sodium chloride is used the
.products of decomposition may be shown as,
NaCl=Na+Cl (1)
2Na + 2H 2 0~2NaOH-j-H 2 (2)
4C1 + 2H 2 = 4HC1+0 2 (3)
NaOH + HCl~NaCi + H 2 (4)
The sum of all these reactions is, however, the simple
decomposition of watjer ;
KLHCTRICAL EFFECTS 255
Such solutions are usually called electrolytes, and ac-
cording to Faraday the components of electrolytes, that
is, on the one hand hydrogen, metals etc., and on the other
hand the halogens, the acid radicals ete., are called ions.
The first named which travel down the current, are called
cations, and those which travel up anions. Metallic
conductors which touch the electrolytes are called
electrodes, and the surface at which the anions appear is
called the anode and that whereat the cations appear is
called the cathode.
After performing a very large number of experiments
Faraday formulated the general law to which all move-
ment of electricity in electrolytes is subject :
"In every electrolyte the quantity of ion separated out
is proportional to the quantity of electricity which has
passed through, and the same quantity of electricity
passing through different electrolytes separates quantities
of different ions that are in the same ratio with the chemi-
cal equivalents of those ions."
Faraday introduced into the same circuit dilute sul-
phuric acid and tin chloride, lead chloride, or lead borate
and he led the same current through different beakers
with sulphuric acid, using electrodes of different metals j
in all cases he found his law verified. It has also been
proved by other investigators that in every case the electri-
city passed through has been strictly proportional to the
quantity of ion separated out.
Fused inorganic salts and aqueous solutions of salts,
bases and acids convey electric current producing chemical
change which is called electrolysis. The conduction of
256 ASSOCIATION THEORY OF SOLUTION
electricity through such media is dependent (1) on the*
nature of the substance and (2) on the condition in which
it is presented for such reaction.
The nature of the ultimate products of decomposition
followed by the passage of electric current through an
electrolyte varies widely of which the simplest, NaOl
+ H 2 O = NaCl + H 2 + O, has already been noted. This is
an instance of electrolysis of binary compounds where the
acidic or basic radicals could not be further decomposed.
Complications, however, easily arise when any or both
these radicals are capable of further change. The follow-
ing are a few instances of such complicated products :
Sodium formate gives :
Sodium acetate gives :
2NaOOCH 3 + 2H 2 O - 2NaOH 4- 2CO 2 + C 2 H 6 .
Salts of the higher fatty acids give the corresponding
paraffins. Although in many cases the reactions are
simple yet the few complicated instances are quite im-
portant. Nature of the chemical decomposition that
would accompany depends on external circumstances,
temperature, concentration, current density, etc.
When a solution containing a complicated molecule is
electrolysed the products of electrolysis may change with
the progress of the reaction as the current produces re-
distribution in concentration of the solution by the
migration of solute molecules or their components. On
electrolysing potassium silver cyanide, silver separates at
the cathode, but by using small electrodes and strong
currents the salt near the cathode is soon used up by
ELECTRICAL EFFECTS 257
migration and hydrogen will evolve in the place of
separation of silver.
Aqueous solutions of salts, acids and bases have
attracted considerable notice of many investigators of
electrochemistry in studying conduction of electric current
through them. Pure water, however, does not appreciably
conduct electricity and there are substances which are
nearly non-conductors but whose solutions in water are
quite good conductors. From this it follows that electro-
lytes have a special constitution with which their special 1
property is connected. After the discovery of a number
of organic compounds soluble in water producing non-
conducting solution, Hittorf experimented over the-
subject very carefully. He drew attention to the fact
that the power to conduct electricity and the power to
exert chemical reactions were outcome of the same
cause.
Hittorf accepted the ionic conceptions and nomencla-
ture of Faraday and assuming a considerable mobility of
ions in solutions stated that electric conduction is brought
about by the positive and negative electricities moving
through the conductor bound to their ponderable carriers,,
the ions. Clausius 3 pointed out that in every electrolyte
there should be present number of ions capable of moving
freely being split up to produce electrolytic conduction.
He, however, did not apply any method for the determina-
tion of amount of such ions in the solutions.
Clausius having based on the kinetic hypothesis
conceived that, "Owing to the collision of the molecules-
of the electrolyte with each other and with those of the-
17
-258 ASSOCIATION THKOKY OF SOLUTION
solvent, one or other occasionally splits into its con-
stituents, and so gives the free ions" His contemporaries
brought further support from the comparative chemical
phenomena of the reactivity of gaseous substances at the
ordinary temperature and that of the aqueous solutions of
acids, bases and salts under the same conditions. A
mixture of hydrogen and oxygen does not form water
until it has been heated to between 400 and 500 ; a
mixture of hydrochloric acid and potash, however, when in
aqueous solution passes almost instantaneously into
potassium chloride although in the first case 68,000 Cal.
of heat are produced and only 13,700 Cal. in that of the
second. An instance of very slow action in solution has
also been quoted in this connection. The formation of
acetic ether from a mixture of alcohol and acetic acid
that is, for a process much analogous with that of the
formation of salts at least ten years are needed, at the
temperature of the room, to complete the reaction
parallel to which would be done almost instantaneously.
Thug an. apparent case was made that it is necessary,
from chemical point of view, to suppose that compounds
which react instantaneously, that is the electrolytes
possess a special mobility of their parts or ions ; and that
those are the constituents of "salts*', which are the ultimate
cause of electrolysis as well as of chemical reactions.
These 4 arguments of Clausius, Hittorf, and others are
not quite rational and the reactions of (1) combination of
hydrogen and oxygen, (2) neutralisation of caustic
potash with hydrochloric acid, and (3) ester ifieation of
alcohol with acetic acid should hardly be brought in
ELECTRICAL EFFECTS 259
the same field for the purpose of comparison. The
heats of reaction need not indicate the quickness or
the velocity of the reaction. If "owing to the collison
of the molecules of the electrolyte with each other and
with those of the solvent, one or other occasionally splits
into its constituents, and so give the free ions" then such
ions should also be present to help the esterification in the
solution for the formation of ethyl acetate by the
reaction of alcohol and acetic acid. It may be argued
that the above spliting up takes place only in aqueous
solution, but such reasoning should have hardly any value
unless it is made clear why aqueous solution would have
this special property. Besides if such assumption has to
be made in the case of aqueous solutions it may be better
assumed or rather the assumption will be less com-
plicated that one of the chemical properties of some
aqueous solutions is the acceleration of chemical reactions.
Explanation of such behaviour through the assumption of
formation of ions are not only superfluous but erroneous iu
the absence of any reasons why the same is developed in
certain selected aqueous solutions only.
Again the velocity of chemical reaction in nou-
electrolyte solutions are quite instantaneous in many cases.
Alcoholic solutions of amines and acids are practically very
feeble conductors but they react to form salts just as
instantaneously as aqueous solutions of caustic potash and
hydrochloric acid. Gaseous acids and gaseous ammonia,
or any volatile bases when brought into contact would
show quite a different type of activity in chemical com-
bination than that of hydorgen and oxygen. It will be
260 ASSOCIATION THEORY OF SOLUTION
explained later on that Bakers' 5 researches on the influence
of presence of even a trace of moisture in a chemical
reaction does not help the theory of ionisation in aqueous
solution in any way. If ions had anything to do with
the chemical reaction it might have been proportional to
its quantity available for the purpose. At any rate the
influence of the quantity of ions over the chemical
reactions on which the ionic hypothesis is based needs
proper establishment in order to prove the validity of
such assumption.
It is now necessary to describe the broad principles of
Van't HofE's 6 theory of solution to consider the electrolytic
dissociation theory because much mutual support has
been brought to bear on each other. The fundamental
principles of Van't Hoff's theory are based on the
analogy that the molecules of solutes in a solution behave
in the same way as pure gas molecules in respect of the
relationship between pressure, volume and temperature.
These gas laws are true because the molecules of gases are
always at such a distance apart that they are not capable
of exerting an action on each other. In liquid or in solid
substances the molecules being in close proximity exert
specific reciprocal actions. In solutions the solute molecules
are sufficiently apart from each other and therefore they
behave like pure gas molecules. Like gases the characteri-
stic property of solutions is the power of extending
uniformly through any given space containing their
solvents. When a solvent is placed in contact with a pure
substance or its solution in another solvent a molecular
movement sets in at the partition, which continues till no
ELECTRICAL EFFECTS 261
more distribution is possible, This subject is dealt with in
Chapter VI on 'Osmotic Pressure'.
Van't Hoff considers the agreeable analogy of 'Osmotic
Pressure 1 with 'Gas Pressure* very important, the pressures
in respective cases being set up by the bombardment of
solute molecules to the semi-permeable membrane and by
the gas molecules to the sides of the vessel containing it.
He also showed that some results of Osmotic Pressure
compare very well with those of gases. Boyle's Law
enunciates that pressure and volume are inversely
proportional and Pfeffer's Law states that the Osmotic
pressure and concentration are directly proportional. Gay-
Lussac's law states that volume remaining constant
pressure increases uniformly with the rise of temperature
and a similar law has been proved by Van't Hoff from the
results obtained by Pfeffer. The gas law of Avogadro
being that the temperature and volume remaining the same
equi molecular quantities of different gases exert the same
pressure. A gram-molecule of a gas when occupying one
litre volume at OC exerts a pressure of 22'37 atmos-
pheres. Pfeffer observed that the osmotic pressure of
one per cent cane sugar solution is 0'649 atmosphere at
0C. Therefore a gram-molecule of cane sugar (C 1 2H 22 O U
= 342) contained in a solution occupying litre volume
342 x 0*649 . 4 , ^,.
exerts pressure = ^ =22*2 atmospheres. This
agreement has been held to be of fundamental importance
in proclaiming that Avogadro's law also holds good for
solutions. This subject, however, has been subsequently
verified in a representative manner without obtaining a
262 ASSOCIATION THEORY OF SOLUTION
desirable satisfaction to justify such a great generalisation.
It is rather unfortunate that this limited agreeable
analogy has been used by earlier investigators as one of
the main pillars of the dissociation theory of solution and
thereby made a mutual support.
Van't Hoff's theory of solution could not explain the
abnormal results obtained in the determination of osmotic
pressure, lowering of vapour pressure, elevation of boiling
point, and lowering of freezing point by the electrolytes.
The difficulty, however, was afterwards overcome by
Arrhenius. This powerful and celebrated investigator
ascribed the deviations to dissociation of solutes present, in
the electrolytes into their ions. He also determined from
the magnitude of the deviation the number of molecules
which are dissociated, and thus claimed to have solved
the problem left unfinished by Clausius. He conceived that
ions are free by dissociation in an aqueous solution of
potassium chloride, but the chlorine does not escape into
air as a greanish yellow was and the potassium does not
act on the water because it is considered that the greenish
yellow gas is the electrically neutral molecule C1 2 and
does not consist of separate atoms of Cl which are charged,
with a large amount of negative electricity, and similarly
potassium is not present as compact metal, but in the
form of strongly positively charged ions. These differ-
ences in properties of the same element in different forms
have been considered to be similar to those exhibited by
allotropic forms of the same element e.g., oxygen and
ozone etc. Attempts have been made to explain con-
siderable number of physico-chemical phenomena in light
ELECTRICAL EFFECTS 265
of this dissociation theory which have already been treated
in previous chapters,
Kohlrausch 8 discovered a method based on laws of
Ohm and Faraday for the determination of electrical
conductivity of solutions which has the speciality of using
alternating currents, and up to this time this method
stands better than any other. After this discovery
numerous investigators started their investigations on this
line using the following nomenclature
Specific resistance is the resistance in ohm offered
by a cube of one centimetre dimensions to a current of
electricity.
Specific conductivity is the inverse of specific resistance.
Molecular conductivity is the conductivity of a solution
containing one gram-molecule of solute when placed
between electrodes of indefinite dimensions exactly one
centimetre apart.
Equivalent conductivity is the conductivity of a
solution which contains one gram-equivalent of solute,,
when placed between two electrodes one centimetre apart.
In cases like potassium chloride KC1, a molecule of
which contains two simple monovalent ions the equivalent
conductivity becomes equal to molecular conductivity.
Kohlrausch and his followers established the following,
facts for dilute solutions in which one gram-equivalent of
solute is dissolved in more than one litre of water.
(1) The equivalent conductivities of normal salts are-/
of the same order of magnitude, but are not identical.
(2) The conductivities of all salts increase slowly
with increase of dilution, which reach maximum
264 ASSOCIATION THEORY OF SOLUTION
value at dilutions of 20,000 to 50,000 litres per gram-
equivalent.
(3) The increase of conductivity is the least for salts
which consist of two monovalent ions, nearly twice as
great for salts containing one divalent and one monovalent
ion, and nearly four times as great for salts containing
both divalent ions.
(4 The equivalent canductivity of equally concentra-
ted solutions of the most different salts can be represented
as the sum of two constants, which are solely determined
by their constituents, negative and positive ions.
Daniell 9 has been said to have observed differences in
concentrations near the two electrodes in a solution which
underwent electrolysis for sometime. Hittorf 10 made a
thorough study of these changes in connection produced
by electrolysis and tried to explain the phenomena on the
assumption that the positive and negative radicals of the
solute while remaining in dissociated states in solution are
carried away in opposite directions by the electric current
causing thp electrolysis. When a current is passed
through an electrolyte the numbers of positive and nega-
tive ions discharged at respective electrodes in a given
time are equal but the velocities of the two are not equal.
The speed of the anion and cation are often different,
which causes the increase of concentration of the faster
ion round the electrodes towards which it travels. The
velocities of cation and anion are usually represented by
u and v respectively ; and the total amount of electricity
passed through the solution is proportional to the sum of
the velocities of cation and anion, i.e. u and v.
ELECTRICAL EFFECTS 265
If n be the fraction carried by the anion, then 1 n
will be the fraction carried by the cation, from this it
follows that.
and 1 n
u + v u -h v
The values of n and 1 n are called transport numbers
of anion and cation respectively. If the total amount of
electricity which passes through the solution and the
amount of one of the ions which have passed from the
solution in the immediate neighbourhood of one of the
electrodes, that is, the change of concentration of one of
the ions round one electrodes be determined the transport
numbers can be calculated. The total quantity of electricity
is measured by the usual methods and change of con-
centration is easily determined by analysing a portion of
the solution round one of the electrodes.
Determination of transport numbers has been a
subject of considerable 11 study leading to the establish-
ment of following facts :
(1) Current strength has no influence on the ratio of
the migration velocities,
(2) Variation of temperature has also very little
influence.
(3) Concentration has a great changeable effect,
varying from too small to too high depending on the
nature of the solute. t
Potassium chloride solution does n:>t give much change
in concentration at the electrodes after electrolysis thus
indicating the migration velocity of potassium and chlorine
to be the same. Basing on this fact as also on the assump-
266 ASSOCIATION THEORY OF SOLUTION
tion that chlorine in potassium chloride has the same
migration velocity as on sodium chloride or in any other
similar salt, many transport numbers were calculated
which were sufficiently true for very dilute solutions, but
several deviations were noticed with concentrated solutions,
which also changed with concentration. Generalisations in
this respect became further difficult on account of the fact
that different salts behave differently, salts consisting of
two monovalent ions show the smallest deviations, salts
with one monovalent and one divalent ion show greater
deviations and salts with two divalent ions show the
greatest deviations.
Association theory of solution does not admit any
dissociation or ionisation of the solute in solution but
assumes combination of solute with solvent. Any energy
consumed during the process of solution is not due to the
ionisation or dissociation but due to the combination of
solvent and solute, which varies with dilution. Such
change in energy may be expressed in the shape of change
in, thermal,, optical, electrical, properties etc. of solute,
solvent and solution. Conduction of electricity through
solution is not due to the dissociation or ionised molecules
of salts and bases in solution because the fused salts and
bases also conduct electricity without any dissociation or
ionisation through the intervention of solvent. Fused
silver chloride conducts electricity and is itself decomposed
simultaneously. Davy discovered metals of the alkalies by
electrolysing the fused bases of potassium hydroxide and
sodium hydroxide. Lithium and magnesium may be
easily obtained by passing electric current through their
ELECTRICAL EFFKCTS 267
fused anhydrous chlorides. Aluminium is manufactured
on a very large scale by the electrolysis of fused aluminium
oxide. Thus ionisation or dissociation in a medium is not
necessary for conduction of electric current or electrolysis.
It is not reasonable to say. when a substance conducts
electricity, that it must have contained ions simply on
account of this property.
Results obtained from the study of the reflection and
refraction of X-rays by crystals 12 have been utilised in
assuming the existence of ions in solid crystals. It has
also been noted that these ions are responsible for the
electrical conductivity of such substances. The subject
however needs more experimental verification before
considering the acceptance of the theory that ions present
in a solid convey the electric current. Some of the non-
conductors 13 mercuric cyanide, arsenic chloride, stannic
chloride seem to need more examination in this connection.
It would be beyond the scope of this book to discuss
the merits of the wave theory and the corpuscular or
electron theory of electricity, but probably the latter is
not quite suitable for the association theory of solution.
Corpuscular theory is based mainly on the results of
researches during the electrical discharge through a highly
exhausted tube. Rays shot off from the cathode may be
stopped by the interposition of some material placed in
their path and when they strike the walls of the tube
cause a vivid green fluorescence upon soda glass, blue on
potash glass. These rays can be deflected in certain
directions by a magnetic or electric field, or both simultane-
ously and therefore they have been assumed by th&
208 ASSOCIATION THEORY OF SOLUTION
corpuscular theory to consist of negatively charged
particles. According to this theory, negative electricity
consists of extremely small particles called corpuscles or
electrons, which are all identical in size, and carry the
same charge, and molecules and atoms are partly built up
)f them and of others containing an equal amount of
positive electricity. This latter assumption is necessary to
prevent spontaneous disintegration of the atoms due to the
nutual lepulsion of a number of similarly charged electrons.
Thus corpuscular theory assumes that a neutral atom
:onsists of numbers of corpuscles moving in various orbiis,
bhe number of such corpuscles and the kind of the motion
jhey possess being the ultimate course of chemical and
physical properties. The experimental evidence described
ibove in favour of the real existence of corpuscles may
not be enough for the assumption of such fundamental
importance, but these, however, do not render it impossible
to discuss the question of electrolysis without entering
into the topics of constituents of atoms.
When two electrodes possessing a suitable difference of
electrical potential are introduced in an electrolyte electro-
lysis takes place. Variation of products of electrolysis,
however, with the variation of conditions under which
electrolysis is carried out does not encourage acceptance of
the simple view of the phenomenon proposed by the
electrolytic dissociation theory.
Association theory of solution does not consider that
the solutes undergo dissociation in an electrolyte in the
absence of any electric current, All phenomena of
electrolysis could be explained by the assumption that the
ELECTRICAL EFFECTS 269
associated molecules, between two electrodes provided with
constant supply of difference of potentials, arrange them-
selves along the lines of force present in the field in the
same way as the particles in a magnet. The transmission
of electric current from one pole to another takes place by
vibration received and delivered by actual contact of the
associated molecules with the electrodes. The series of
consecutive associated molecules of solute and solvant
between the two poles along the lines of force behave like
an elastic rod, which receives vibration from one end and
delivers through the other in the same direction as the
current. Both the ends of such rods and those of the
electrodes meet while vibrating in the same way as the
ends of the electrodes of an electric arc. Disintegration
of associated molecules takes place on account of vigorous
vibrations at the junctions. After disintegration some of
the components of the associated molecules are set free and
the rest combines with a portion of the molecule, next
towards the other electrode forming the conducting rod,
to rebuild a complete molecule ; and the fraction liberated
from this second molecule repeats the process with the
third and the propagation of the operation is continued
till the terminal one is affected, when a corresponding
fraction is set free. All these reactions take place with
considerable speed and facility on account of the existence
of the state of vibration during the conduction of electric
current. The portion of the rod consumed by decomposi-
tion is replaced from the rest of the solution by the
natural tendency of fluidity to fill up internal gaps.
The decomposition of solute or solvent molecules
270 ASSOCIATION THEORY OF SOLUTION
occurs according to the conditions present for the purpose.
A part of an associated molecule is liberated at one
electrode and the balance at the other. While this de-
composition takes place the total amount of undecom posed
solvent molecules, that formerly remained associated with
the decomposed molecule, will be set free ; and their
distribution between the two portions of the original
molecule liberated at the electrodes, will take place
a cording as their comparative affinities for the solute
molecules. If the solution is KC1,100H 2 O and the
products of decomposition are K and Cl, the distribution
of 100 H 2 O between K and Ci will be as 50H 2 O, K and
50H 2 O,C1, if there are no changes of concentration at the
two electrodes. But in other cases where there will be
changes of concentrations at the two electrodes, the
phenomena will be due to carrying of unequal number of
solvent molecules by the two portions of the decomposed
molecules liberated at two poles.
Electrical conductivity in solution is due to the con-
duction of electricity in the same way as the solid con-
ductors 14 , and investigation on electrolysis by Vollie and
Chassagny 17 needs extension and amplification in this
connection.
Ordinarily when weak current is passed through
dilute solutions the reaction is very mild and products are
often uniform and if these two conditions are varied with
varying solutes considerable variation in products of
electrolysis are obtained. In the electrolysis of aqueous
solutions of platinic chloride 15 , it has been shown that
only hydrogen is liberated at the cathode when weak
ELtCTRICAL EFFECTS 271
curr.ent is used and a deposit of platinum is obtained
with strong current. Such difference of prdoducts of
decomposition is chiefly due to the variation of vibration
available for the purpose. Development of heat in electro-
lytes 16 during electrolysis has not been well studied and
needs considerable experimental results in this connection.
John experimented with copper sulphate and zinc sulphate,
and stated that the quantities of electricity used up, or
rather converted into heat f in overcoming the resistance to
conduction and other secondary influences 17 , are inversely
preportional to forces of affinity of the ions of the
electrolyte. Influence of concentration on the products of
electrolysis has been well illustrated in the case of hydro-
chloric acid 18 , a subject, deserves consideration in this
connecion,
Ostwald and Nernst 19 described experiments in favour
of their statements that the ions are present in the
solution in a free state and that no part of th# charge is
used up in their liberation. The fact that no charge is
used up in the formation of ions suit very well with the
association theory of solution in respect of the assumption
that no ions are formed and if there be any formation of
ions in solution there would be some disturbance of energy
in some form or other. Consumption of no energy in the
formation of ions in electrolytes would be a fact against
the dissociation theory of solution. Their experiments
using mercury electrode contained in a tube for observing
small quantities of hydrogen seem very useful and need
good deal of amplification. It seems probable that the
associated solute molecules in solution form a flexible
272 ASSOCIATION THEORY OF SOLUTION
chain, as stated already between two electrodes along the
line of forces arranging themselves in a way so that the
basic radical of one faces the acidic radical of the other.
This chain behaves like an elastic rod, and when receives
an impact from one or more molecules on account of
kinetic motions drifts away from the line of force causing
passage of sparks at its both ends. As soon as a chain
is displaced another is formed as long as difference of
potential is maintained.
Complicated products of electrolysis of salts of organic
acids were obtained by numerous authors 20 and attempts
hove been made by several of them to explain the pheno-
mena of such complications. All these experimental
observations are quite highly ineresting but the conclu-
sions drawn by various investigators are not quite illu-
minating and perhaps could be better explained by the
association theory of solution. Ib is possible that ordinary
text book writers were not quite sure of the conclusions
drawn by the investigators, and therefore, it may be, that
they could not pay proper importance to such subject in
their books. It is always necessary to draw possible
generalisations and conclusions from experimental facts
but the sagacity shown by some of the investigators on
this subject does not bring so much credit. Many of them
have been done without reasonable experimental veri-
fication. Conclusions drawn by several were proved to be
wrong by subsequent investigators by means of fresh
facts.
Tho association theory of solution, finds no difficulty
in explaining such phenomena; the passage of sparks
ELECTRICAL EFFECTS 273
or vigorous vibrations at both ends of the hypothetical
rod cause violent decomposition, nature of which depends-
on the factors which could influence an electric spark,
and on the properties of the associated molecules of
solute with solvent. Unstable nature of the organic
acid radicals leads to the production of considerable
variation in the products of electrolysis depending on the
(1) concentration, (2) temperature, (3) current density,
(4) anode potential, (5) nature of solvent, (6) presence of
another substance in solution, and (7) material of anode*
Although some experiments have been done by Gordon,
Murray, Gibson, Robertson and Fairweather and Walker
to show the variation of products with the variation of
above conditions of electrolysis yet it appears that
exaggerated generalisation has been done in the absence
of reasonably representative data.
Mycroscopic examination of polished metallic elec-
trodes after electrolysis showed a peculiar crater-like
formation on the surface, suggesting that the surface of
the metal had been blown open by an internal explosion 21 -
This observation gives a leglitimate support to the
hypothesis that electrolysis is the result of passage of
spark during the conveyance of current. It is rather too-
early to lay considerable importance on the results of
Wien, though his researches showing deviations from
Ohm's law for electrolytes add strong arguments against
the dissocation theory of solution in respect of the fact
that it does not provide any assumption for the purpose.
On the contrary the association theory accommodates quite
easily such results since it assumes that conduction of
18
274 ASSOCIATION THEORY OF SOLUTION
electricity in solution is dependent on the nature of the
associated molecules and on the energy available from
the vibrations that pass through the solution. This
investigator found that for certain electrolytes the
conductivity increases with increasing voltage at a rate
greater than that may be expected on the basis of the
increased temperature when the temperature-coefficient is
normal. The increase in conductivity is made up of two
separate effects, the Joule heat effect and an increment
which is proportional to the voltage. The latter
increases rapidly with increasing valency of ions and
corresponds with departure from Ohm's law, also ap-
proaches a limiting value when the voltage is very high
or the concentration is very low. The valency of the
ions exerts a marked effect on the attainment of this
limit, since both increasing concentration and higher
valencies shift the limit in the direction of higher
C5
values.
It is -now necessary to consider what other similar
properties are acquired by substances in coming in contact
with water and its allied substances. Salts, bases and
acids acquire variable electrical properties in aqueous
solutions and this subject has been representatively
described. Manifestations of considerable new properties
are shown by some matters by the presence of water even
in small quantities. Development of such properties
according to the association theory of solution is
due to the formation of new compounds with two brought
in contact. Such associated molecules set up a change in
the kinetic movements of the whole body including them-
ELECTRICAL EFFECTS 3f5
selves and those that are not so associated, if any such
be present there, causing aquisition of corresponding new
properties. Behaviour of matters in the presence of very
small quantities of water have been studied by Baker and
Dixon 22 . Combustion of carbon monoxide, dissociation
of ammonium cloride vapour and action of sulphuretted
hydrogen on salts of heavy metals are not successful in
the absence of water. Explosion of a mixture of carbon
monoxide and oxygen by electric spark did not take place
ordinarily in the absence of water vapour. The explosion
in the absence of moisture was effected wheo a third gas
containing hydrogen, e.g. H 2 S, C 2 H G , H 2 CO 3 , NH 3 , C 5 H 12
or HC1, were present instead. And traces of other gases
like CS 2 , SO 2 , CO 2 N 2 O, C 2 N 2 or CCI 4 did not help the
same explosion like water vapour. Baker's studies in the
change of properties of substances on drying seem
very useful and investigators should follow them very
carefully. Boiling points of trioxide and tetraoxide
of nitrogen were raised by 44 and 47 respectively
when they were allowed to stand for a long time in
contact with phosphoric oxide. He prepared a number
of liquids in a high state of purity and sealed them up in
vessels containing purified phosphoric oxide. In many
cases, direct contact of the liquids with the drying agent
was avoided on account of possible chemical reaction, the
drying being then dependent on the removal of water
from the continually changing vapour. The substances
were thus dried for eight or nine years. He obtained
the following results of different types.
ASSOCIATION THEORY OF SOLUTION
Period of Original New-
drying boiling Boiling
Rise-
in years.
point.
point.
8
63
118 3
55
9
358
420-425
62
84
68'4
82
14
84
80
106
26
49'5
80
30*
9 9
78
above 112
34
9
35
83
48
9
66
above 120
54
9
73-5
138
60'
9
95
134
39
Bromine
Mercury
Haxane
Benzene
Carbon disulphide
Carbon tetrachloride
Ethyl ether
Methyl alcohol
Ethyl ether
Propyl alcohol
He 23 also found afterwards that melting points and
vapour densities of substances when extraordinarily dried
considerably differed from those obtainable from the same
substances' when not dried by means of any lengthy
process. A definite fractional distillation of dried 1
benzene was done, the highest temperature observed being
87. The melting points of sulphur trioxide (dried for
years), bromine (dried for 10 years), and benzene (dried
for 10 years) have been found to be 61, 4-5, and 6,
respectively. The vapour density of ether (dried for 10-
years) has been found to be 81*7, more than double the
normal, and of methyl alcohol, dried for the same length
of time, to be 45, compared with the normal value 15.
These changes of properties of substances on drying have
been ascribed by the author to be a confirmation of the-
ELECTRICAL EFFECTS 277
theory of allotropy of Dr. Srnits of Amsterdam with
whom he discussed the results.
Thus instances are available regarding the influence of
water in changing various properties of matter. Smits 24
assumes that every phase contains two different kinds of
molecules, an active and an inactive varieties, and that
these are in equilibrium. In intensive drying the
equilibrium is shifted to the inactive side, so that the
molecules which remain after drying are only inactive It
is rather difficult to say if such results of Baker and
others should help the "theory of allotropy" of Smits,
since such extremely dry substances are not very stable as
the case with other instances. It may be difficult to
change a substance allotropically yet if it is once changed
it often does not go back to its original condition easily. In
one of Smits's experiments (Jour. Chem. Soc., 1924, 125,
1074) a sample of nine month's dried benzene, which
(liquid) boiled at 87 had a chance of coming in contact
with a small quantity of moist air, gave (liquid) boiling
point 80'9. Consequently the effect attained after 9
months' intensive drying had almost completely disappeared
by the introduction of a minute quantity of water. At any
rate it is not yet proved so conclusively how the change of
properties acquired by prolonged intensive drying could
disappear by the introduction, of moisture and whether
such properties are stable against original conditions
-containing moisture. The ^reason why moisture should
take part in the allotropic formation is neither properly
suggested nor understood as yet, nor it has been proposed
-bow these minute traces of moisture are present with
2)8 ASSOCIATION THEORY OF SOLUTION
such substances, in a state of solution, in a state of combi-
nation or in a state of mechanical mixture. It is-
true that the properties are changed, but it is not quite
definitely established that the change of properties is due
to the substance passing into another modification.
Some scientists 25 have argued that chemical reactions
take place in aqueous solution on account of ionisation
but there is no need to do so as there are sufficient number
of cases where chemical reaction would take place in non-
aqueous media with practically the same vigour. Pre-
cipitation of silver iodide from alcoholic solution of silver
nitrate and methyl iodide is just as quick and quantitative
as that would be the case in aqueous^ solutions of potas-
sium iodide and silver nitrate.
Various ways in which water affects the various pro-
perties of matters are now mentioned in a representative
manner, in large quantities it causes electrical properties
strikingly changed, and in small quantities it makes them
chemically active, and boiling point, melting point, etc.,
altered. The electrolytic dissociation theory took advan-
tage of representing water as H.OH and put forward
many explanations combining "H" or ''OH" with some
part of the substances. But any such explanation is not
useful in explaining the part played by gases containing
"H." causing the explosion to take place in dry mixture
of CO and O.
A matter may exist in three states, solid, liquid and
gaseous, and the difference is often attributed to the
different motions of its molecules. As the matter
changes from solid to liquid and from liquid to gas, its
ELECTRICAL EFFECTS 279
molecules get more and more motion. In this connection
it would not be irrational to presume that different subs-
tances move with different kinds of motion, varying in
frequency, and amplitude. If two such substances are
mixed to produce solution the resultant product will have
a resultant kinetic motion which may differ from any of
that of its components and that of the mean oF them. It
is reasonable that the change of property exhibited by
solutions or by substances in the presence of small quantity
of water is partly or wholly due to the change in kinetic
motions. It may be true that electrical and other proper-
ties of matter depend on the internal movements of
electrons contained in its atoms or molecules yet it is not
unreasonable to presume that the same electrical and other
properties are changed owing to the change in movement
created by the introduction of water or any similar mole-
cules in small or large quantities. This hypothesis may
lead to a reasonable assumption that the phenomena of
solution and solubility are functions of nature of the
kinetic motions possessed by the solvent and solute.
The influence of the mass of the substance reacting
per unit weight of another substance had been a subject
of study by Wenzel, by Berthollet, by Pe'n de St. Gillers,
and by Guklberg and Waage. The last two investigators
found that the chemical activity of a substance is not
proportional to the quantity present, but to the amount
present in unit volume of the reacting mixture, or to its
concentration. Thus they enunciated the law of mass
action that, the amount of chemical reaction is proportion-
al to the active mass of each of the substances reacting,
tfSo ASSOCIATION THEORY OF SOLUTION
active mass being defined as the molecular concentration
of the reacting substance. In attempting to apply this
law of mass action to the properties of various electrolytes
considerable discordant results were obtained and much
more divergent opinions were expressed. It does not seem
useful to enter into such discussion as no satisfactory
conclusion could be arrived at till more experimental data
are available.
Bodenstem's 20 researches on the equilibrium in gaseous
system between hydrogen, iodine, and hydroiodic acid
increased considerable importance of the law of mass
action. A similar instance of the decomposition of phos-
phorous pentachloride into phosphorous trichloride and
chlorine,
being reversible in a gaseous system afforded an useful
study of this matter. Investigators are requested to work
on the suitability of the association theory of solution
regarding the similar reversible re-action between solvent
and solute with reference to the law of mass action.
Aqueous solutions of different salts react differently
on the indicators. A solution of an aluminium salt is
distinctly acid and a solution of carbonates of alkali
metals is distinctly alkaline. The dissociation theory
explains such phenomena stating that the salts dissociate
in solution and the indication ot acidity is due to the
presence of hydrion and that of alkalinity due to the pre-
sence of hydroxion. Aqueous solutions of some salts when
sufficiently diluted deposit oxides or hydrated oxides leaving
behind acidic radical as free acid in solution. According
ELECTRICAL EFFECTS 28 I
to association theory these are due to the fact that when
.large quantity of water is added the compounds formed
with solvent and solute become unstable on accouut of
overbalance of the chemical affinities amongst the final
compounds formed over those of the decomposed com-
pounds. The reaction of acidity or alkalinity in aqueous
solutions of salts on indicators may be explained without
assuming the pre-existence of decomposed components of
solute. When an indicator substance is introduced in such
solutions it subjects itself to action by ail the acidic and
basic radicals present in the field. The final selection of
radicals for the reaction between the indicator and the
components of the solute takes place according to the
comprative affinities they offer. Thus the pre-existence of
the components of the solute in decomposed state in
solution need not be assumed. The real decomposition
takes place after the introduction of the indicator when
selective chemical reaction takes place ; and this pheno-
menon, however, is quite different from what takes place
when oxide or hydroxide is separated as result of dilution
of solutions of salts like aluminium sulphate, lead
acetate, etc.
Ostwald believed in the dissociation theory of Arrhenius
and thought that the reaction is universal for solutes in
electrolytes, and obeys the law of mass action. He worked
out the following formula :
^
(l-a)v'
Where,
K= equilibrium constant,
282 ASSOCIATION THEORY OF SOLUTION
v = volume of solution containing unit mass of
the solute,
a = fraction of the solute molecules contained in v
volume of solution dissociated into ions.
This relationship has also been expressed in terms of
electrical conductivity measurements. If,
fS = conductivity at a molecular dilution v,
^oc = conductivity at an infinite dilution,
then, .=-.
Substituting this equivalent of a in Ostwald's original
formula,
U, 2
] __ _ v
^oc(^oc-/\)v'
Considerable number of experiments were done to
verify the accuracy of this law which is commonly known
as the Ostwald's dilution law ; both agreeable and dis-
agreeable results were obtained. This variation is due
to the difference in properties of compounds formed at
different dilutions. The results would have agreed
well had the properties of associated molecules of solute
and solvent formed at different dilutions borne any ratio
with the quantities of their components. Any agreement
observed is due to the approximate variation of their
properties with dilution. An interesting disagreement
was shown by Tansley 27 ; his results of acetoxime hydro-
chloride are given below, v = volume in litres containing
one gram equivalent of the substance and K Ostwald's
bydrolytic constant,
v= 8 16 24 32 40 48 80 100 120'
Kxio 8 i8'o 12*6 9-52 7*58 6*32 5-67 3*81 3*29 278
ELECTRICAL EFFECTS 283
This author tried his results with several other formula
and found one of them suitable for the purpose. It
remains to be seen how other results suit with this
formula.
Among the numerous investigators who worked on
the validity of dilution law of Ostwald, Van LaarV 28
researches need mention. This author, in considering the
causes of the divergences from Ostwald's dilution law
which are exhibited by many aqueous and alcoholic
solutions, is led to the important conclusion that al-
though the ratio yields a degree of dissociation, yet
this value is not that which one obtains in the absence
of the current ; the dissociation thus determined is,
therefore, incorrect. The cause of the alteration of
the dissociation during the passage of the current is, in
the author's opinion, the great difference which exists
between the temperature of the actual conducting ions and
the salt molecules, and that of the liquid as a whole
Armstrong and Worleby 29 published an elaborate paper
where the view is put forward that hydrolysis is essentially
an associative process which involves the association and
direct interaction of two complexes, one of which consists
of the hydrated hydrolyte and the other hydrated catalyst
Such associated systems are being constantly produced,
broken down and reformed in such a manner that while
some gh e rise to the original components, others are
resolved into the products of change, They ventured to
claim that the explanation given by them of the process
of hydrolytic change as simple, consistent, in harmony
with the facts, in accordance with chemical experience, and
.284 ASSOCIATION THEORY OF SOLUTION
generally applicable. In their opinion, the ionic dissocia-
tion hypothesis does not afford an explanation of the facts.
They <^o so far as to assert that there is now sufficient
evidence that the hypothesis is a false one.
Attempts have been made by some investigators to
explain contractions in solution by a theory of electrostric-
tion 30 . But it does not seem suitable to take up the study
of the validity of this theory till a definite conclusion is
arrived at regarding the relationship of the volume of the
solution with the volumes of its components. It has
already been said (see page 37) that the volume of the
solution is related with the kinetic movements of the
associated molecules.
REFERENCES.
1. Nernst, Zeit, Phys. Chem., 1889, 4, 372.
2. Nernst, Zeit. Phys. Chem., 1889, 2, 613 ; 4, 129.
3. Clausius, Pogg. Ann., 1857, 101, 338.
4. Ibid., 339-347.
5. Baker, Trans. Roy. Soc., 1884, 175, 617 ; Jour.
hem. Soc , 1886, 49, 94 ; 1894, 603 ; 1922, 568.
6. Van't Hoff, Zeit. Phys. Chem., 1888. 1, 481.
7. Arrhenius, Zeit. Phys. Chem., 1888, 1, 631.
8. Kohlrausch, Wied, Ann., 11, 653 ; Zeit. Phys.
Chem., 2, 565.
9. Hittorf, Pogg. Ann., 89-106.
10. Hittorf, Pogg. Ann., 89, 177 ; 98, 1 ; 103, 1 3
106, 337.
11. Kohlrausch, Wied. Ann., 6, 164 } Ostwald, Zeit.
Phys. Chem., 1, 74 ; Loeb and Nernst, ibid., 2, 948.
ELECTRICAL EFFECtS 285,
12. Debye and Scherrer, Phys. Zeit., 1917, 18, 291 ;
1918, 18, 23 ; 19, 74 ; Bragg, Phil. Mag., 1920, 40, 169 ;
Bragsj and Bragg, X-Rays and crystals spectra, 1918.
13. Clark, Phil. Mag., 1886, (5), 20, 37-47.
14. Bouty, Compt. Rend., 99, 30 j Ostwald, J. Pr.
Chem., (2), 31, 433.
15. Kohlrausch, Ann. Phys. Chem., 1897, ii. P, 63,.
423.
16. Plank, Ann. Phys. Chem., (2), 39, 161 ; Jahn,
Montsch., 4, 679 Ber., 1883, 2449.
17. Voille and Chassagny, Compt. Rend., 108, 284.
18. Haber and Grinberg, Zeit, Anorg. Chem., 1898,
16, 198.
19. Nernst, Zeit. Phys. Chem., 3, 120.
20. Kolbe, Annalen, 1849, 69, 279 ; Crum Brown
and Walker, ibid., 1891, 261, 107 ; Jahn, Wied, Ann.,
1889, 37. 420; Bunge, J. Russ. Chem. Soc., 1890,21,
525. Kekule, Annalen, 1864, 131, 79; Loeb, Zeit.
Elektrochem., 1896, 3, 43 ; Schall, ibid., 1896, 3, 83 j
Murray, Jour. Chem. Soc., 1892, 61, 10 ; Foersterend
Pignet, Zeit. Elektrochem., 1904, 10, 727 ; Hot'er and
Moest, ibid., 1904, 10, 833 ; Fichter, and Krummenacher,
Helv. Chiv. Acta., 1918, 1, 146 ; Fichter, Fritsch and
Muller, ibid., 1923, 6, 502 j Gibson, Jour. Chem. Soc.,
1925, 127, 475 ; Robertson, ibid., 1925, 127, 2057 ;
Fairewather and Walker, ibid., 1926, 3113 ; Gibson, Proc.
Roy. Soc. Edin., 1924, 44, 140; Gordon, J. Physical. Chem.,
1914, 18, 55 ; Prenuer and Ludlam, Zeit, fur. physical.
Chem., 1907, 59, 682 ; Bonnet and Thomson, J. Physical.
Chem., 1916, 20, 300 ; Bencrot't, ibid., p. 376 ; Zeit. Ele-
286 ASSOCIATION THEORY OF SOLUTION
ktrochem., 1899. 6. 40 ; Lewis and Jackson, Zeit. physical.
Chem., 1906, 56, 207 ; Salauze, Bull. Soc. Chim , 1925,
37, 522; Hofgartner, Monatash, 1911, 32, 523.
21. Newbery, Jour. Chem. Soc, 1914, 2427 ; Wien,
Wied. Ann., 1927, iv, 83, 327.
22. Dixon and Baker, Trans. Roy. Soc., 1884, 175,
617 ; Jour. Chem. Soc., 1886, 49, 94 ; 384 ; 1894,
603 ; Baker, ibid., 1922, 121, 568 } Chem. News, 1894,
69, 270 j Baker and Baker, Jour. Cliem. Soc., 1912, 101,
2339 ; Dixon, Ber., 1905, 38, 2419.
23. Baker ; Jour. Chem. Soc,, 1923, 123, 1223.
24. Smits, Proc. k. Akad. Weteusch. Amsterdam.
1923, 26. 266 ; Jour. Chem. Soc. f Abst. ii, 1923, 547 j
Jour. Chem. Soc., 1924, 125, 1068, 2554, 2573 ; 1926,
26G6.
25. Jorgensen, Jour. Prak. Chem., 1870, [2]., 16.
26. Bodenstein, Zeit. Phys. Chem., 1897, 221.
27. Tansley, Jour. Chem. Soc., 1923, 3164.
28. Va.n Laar, Zeit. phys. chem., 1898, 25, 79.
29. Armstrong and Worley, Proc. Roy. Soc., 1912,
A. 87, 604.
30. Drude and Nernst, Zeit. Phys. Chem., 1894, 15,
79 ; Polowzow, ibid., 1911, 75, 513 j Kohlrausch, Wied.
Ann., 1895, 56, 185 ; Lamb and Lee, Jour. Amer. Chem.
Soc., 1913, 35, 1667 ; Baxter and Wallace, ibid., 1916.
38, 91 ; Arrhenius, Theories of solutions, 1923, 184, 187 ;
Campbell, Jour. Chem. Soc , 1928, 653.
INDEX OF AUTHORS
Abbe Nollett, 105.
Armstrong, 67, 68, 283.
Arrlienius, 3, 110, 111, 112,
113, 198, 262, 281.
Avogadro, 261.
Babo, von, 177, 131, 198.
Baker, 260, 275, 277.
Bailey, 237.
Bancroft, 118,
Bates, 112, 113
Baxter, 33.
Beckmann, 164, 168, 173,
181, 183.
Bender, 22, 2-3, 35.
Bergen-Thun, 70.
Berkeley, 108.
Berth elot, 4.
Berthollet, 279.
Biot, 218.
Blagden, 160, 161, 162, 163.
Bodenstein, 280.
Boeseken, 223.
Bois-Beymond, 247.
Boyle, 261.
Buchkmeiner, 210.
Butrochet, 105.
Castell Evans, 154.
Caven, 179.
Cbassagny, 270.
Cheneveau, 210.
Clausius, 257, 258, 262.
Comey, 68.
Convert, 223.
Cooper, 32.
Coppet, 162, 163.
Crum Browu, 218.
Dale, 205.
Dalton, 7, 31, 32, 111.
Daniell, 264.
Davy, 10, 266.
Ditmar, 45.
Dixon, 275.
Dobbie, 232, 235, 236,
238.
Drude, 209, 230.
Dufour, 161.
Emden, 178.
Estovos, 82.
Euller, 31.
Fairweather, 273.
Faraday, 177, 225. 253, 255,
257, 263.
Farrow, 11.
Favre, 34.
Fawsitt, 45.
Ferguson, 179.
288
ASSOCIATION THEORY OF SOLUTION
Findlay, 118.
Fischer, 225.
Forty, 19.
Frazer, 108.
Frit Ephram, 13.
Gassend, 31.
Gervey, 222.
Gay-Lussac, 177, 261.
Ghosh, 75.
Gibson 273.
Gillers, Pen de St., 279.
Gladstone, 205, 211.
Gordon, 273.
Graham, 106.
Grosham, 24.
Guldberg, 279.
Guthrie, 193.
Guye, 217, 218.
Hagenback, 88.
Hantsch, 237.
Hartley, 108, 130, 232, 233,
234.
Hehner, 46.
Henkel, 208.
Henry, 7.
Herzog, 70.
Hess, 125, 209.
Hewitt, 241.
Hibbert, 211.
Hittorf, 257, 858, 264,
Hoff, van't, 3, 14 106, 108,
109, 112, 115, 164, 174,
175, 176, 197, 198, 199,,
260, 261, 262.
Holker, 31.
Jablazynaski, 72,
John, 271
Johnston, 32.
Jones, 70.
Joule, 31, 274.
Julius, 230.
Kaye, 26.
Kehrrnann, 233.
Kendall, 73, 74, 75, 76, 77.
Kirchhoff, 198.
Kohlrausch, 3, 250, 263.
Krafft, 32.
Kramers, 82.
Kundt, 231.
Laar van, 283.
Laby, 26.
Landolt, 119, 150, 154, 168,.
206.
Lander, 232, 235.
Laplace, 205
Lecat, 195.
Lentz, 46.
Leonard, 233.
Lorentz, 205.
Lowry, 225, 234.
Lumsden, 5.
Lunge, 47,
Mac Jones, 19.
Magdin, 176.
INDKX
289
Marignag, 31.
Maxim, 173.
Mazottoo, 193.
Mendelejeff. 4.
Michel, 32.
Millner, 75, 232.
Morse, 107, 108,
Murray, 273.
Myrick, 108.
Nernst, 3, 28, 116, 148, 150,
250, 271.
Neuberg, 152.
Nicol, 214.
Nollet, 81, 105.
Ohms, 253, 263, 273, 274.
Ostwald, 3, 20, 61, 89, 112,
281, 282, 283.
Paalzow, 247.
Parrot, 105.
Patterson, 118.
Pauchou, 177.
Pen de St. Gillers, 279.
Perkin, 226, 229.
Pfeffer, 3, 107, 108, 109, 261.
Planck, 189.
Playfair, 31.
Pinsep, 177.
Pulfrich, 206.
Ramsay, 35.
Raoult, 70, 163, 164, 181.
Ray, 47, 90, 91.
Richard, 33,
19
Riiber, 225.
Rilliet, 282.
Robertson, 273.
Roger, 89.
Roozeboom, 198.
Roth, 208.
Riidorff, 161, 162.
Schay, 112.
Scheuer, 227.
Schmeider, 220.
Shield, 85.
Sidgwick, 179.
Smiles, 90, 91, 205, 228.
Smits, 277.
Soret, 232.
Squibb, 46.
Stokes, 239.
Sugden, 71, 72.
Tammann, 34, 147, 148, 150;
177, 178, 179, 181.
Tansley, 282.
Thomson, 123, 124, 129, 130,
131, 132, 185, 188, 13,
140, 142, 143, 144, 145,
146, 147, 149, 150, 154,
159.
Thorpe, 89.
Traube, 35, 36, 106, 152.
Valson, 21, 26, 28, 34,
35.
Van der Willigam, 210.
Van Laar, 283.
290 ASSOCIATION THEORY OF SOLUTION
Vollie, 270. Wenzel, 279.
Waage, 279. Wien, 273.
Wagner, 209. Willigen, van der, 210.
Walden, 117, 212, 218, 219. Winkelmann, 189.
Walker, 25, 178, 273. Worleby, 283.
Wallace, 33. Worm-Muller, 247.
Wanklyn, 32. Wullner, 177, 181.
Washburn, 19. Young. 19.
Watson, 31. Zawidzki, 206.
INDEX OF SUBJECTS
Absorption, curve, 232, 233.
of light, 203, 230-38,
239.
spectra, 280, 233, 237.
and chemical cons-
stitu-tion, 232.
Absorptive power, 233.
Acids and bases, 76.
Active compounds, 73, 74.
Allotropic forms, 262.
Allotropy, 277.
theory of, 277.
Analyser, 214.
Anisotropic substance, 147
Angle of rotation, 213, 215.
Anions, 255, 265.
Anode, 278.
potential, 273.
Association, degree of. 85.
in solution, 69.
theory, 4, 5, 14, 20, 29,
75, 86, 103, 109, 115,
116, 117, 120, 123, 130,
138. 142, 148, 147, 149,
152, 178, 174, 175, 176,
199, 205, 223, 225, 226,
228, 229, 238, 241, 252,
266, 267, 268, 271, 272,
273, 274, 280, 281, 288.
Asymmetry, 217.
Atom, arrangement of, in .a
molecule, 215.
Atomic, a^nity, 229.
combination, 229.
relationship, 229.
volumes, 36.
Avogadro's law, 261.
Benzenoid band, 283, 234.
Binary mixtures, 73, 1C6,
176, 193, 194, 196, 205.
.Binnendruck. 34.
Binding force, 179, 200.
Blow holes, 8.
Boiling point, 194, 199, 275,
278.
: depression of, 7CX,
197, 198, 262. ;
elevation of, 3. 70,
181-188, 197, 19.
of maximum 19 6.
of minimum, 196.
of molecular, 1 82.
of solutions,
177-200.
water, 177.
Boyle's law, 261.
Catalysis, 224.
Cathode, 25$, 256, 267.
Cations, 25&, 265 f
Centre of gravity, 21^7.
3Q2
ASSOCIATION THEORY OF SOLUTION
Change in volume, 31, 206,
213.
* of rotation, 223.
Chemical affinities, 281.
changes, 124| 148, 255.
constitution and absorp-
tion spectra, 232, 233.
force, 254
nature, 183.
process, 147.
reaction, 138, 145, 147,
223, 237, 251, 253, 254,
259, 260, 275, 278, 279,
281.
velocity, 223,
224, 258, 259.
Chemically active, 278.
Compressibility of solutions, 8.
Concentrated solutions, 58,
183, 197, 198, 220, 240,
266.
Tammann's
theory of, 148, 151.
Concentration cell, 248, 251.
Conception of volume, 36.
Conductivity, electric, 223,
250, 254, 263, 264, 267,
270, 282.
molecular, 263, 282.
Conductors, 257.
Conservation of energy, law
of, 194.
Constant boiling point mix-
tures, 193, 194.
freezing point mixtures,
195.
solubility product,
250,
Contraction, 11, 14, 31, 82,
150.
in solution, 31-79, 82,
151, 213, 284.
Corpuscular theory, 267, 268.
Co-volume, 213.
Critical solution tempera-
tures, 9, 13.
Cryohydrates, 193.
Cryoscopic constants, 164,
165, 168.
methods, 176.
Current density, electric, 256,
273.
Deliquescence, 69.
Density, 18. -
and specific heat of
solution, 159.
Dextrorotatary, 215.
Diffusion potential, 249.
Dilatometer, 14, 61, 63.
Dilute solution, 8, 151, 183,
197, 198, 199, 210, 211,
237, 240, 266.
Dilution, degree of and chemi-
cal reaction, 145.
INDEX
293
Dilution, law, 282, 288.
molecular, 282.
Dispersion, 210.
Dissociation, 3; 112, 174, 262,
266, 275,
degree of, 75, 283.
in solution, 3, 264,
theory, 14, 29, 76, 101,
102, 111, 113, 114, 119,
123, 149,174,211,228,
249, 260, 263, 268, 271,
278, 278,280,281,284.
inadequacy of, 103,
109.
Drying, 275.
agent ; 275,
intensive, 277.
Efflorescence, 69.
Electric current, conveyance
of, 247, 252, 273,
density of, 256.
generation of, 247
""""" """" vu,
field, 267.
force, 254
Electrical conductivity, 11,
76, 113, 117, 118, 147, 223,
228, 237.
equilibrium, 247, 248,
251.
JSleetro- chemical equivalent,
253.
Electrode, 264, 265, 269, 272,
273.
potential, 249.
Electrolysis, 253, 255, 256,
264, 265, 267, 268, 270,
271, 272, 273.
laws of, 253, 278.
theory of, 268, 269.
Electrolytes, 3, 69, 75, 109,
112, 198, 211, 228, 252,
255, 257, 258, 259, 262,
264, 265, 268, 271, 273,
280.
Electrolytic dissociation, 28,
76, 110, 113, 123, 212,
228.
Electromotive force, 247, 248,
249, 253.
Electron theory, 267, 268,
279.
Electrostriction, 284.
Emergent ray, 214.
Equilibrium, chemical, 280.
conditions of, 137. 280.
in solution, 137, 280
Equivalent conductivity,
263, 264.
Eutetic mixture, 193.
Expansion, 11, 228.
curves, 119.
Explosion, 278.
Faraday's 1ft*, 253, 255.
294
ASSOCIATION THEORY OF SOLUTION
Fluorescence, 203, 231, 238-
241, 267, 253, 255.
Force, 174.
binding, 135.
Freezing poin't, 73, 74, 75,
118, 123, 160-176, 193,
199.
depression of, 3, 69,
76, 113, 161, 198, 262.
_ molecular,
162, 168.
Galvenic element, 247.
Gas laws, 260.
pressure, 261.
Gay-Lussac's law, 261
Heat of absorption of gas,
129,
chemical reaction,
146.
_ dilution, 123, 124,
130, 138, 139, 140,141.
formation, 136, 145.
. - fusion, 143, 144.
hydration, 123,
124, 131, 132, 185, 136.
neutralisation, 180.
reaction, 259.
solution, 123, 124 153.
o f insolubles,
180.
_ . _ . sparingly
solubles, 180.
Hydrated salts, 138, 179.
Hydrates in solution, 142.
solid, 58.
Hydration, determination, 67,,
71.
- ratio of, 70.
of ions, 34, 71.
i molecules, 34,
113, 225.
salts, 8, 113, 162,
Hydrolysis, 67, 280, 281. 283.
Hygroscopic property, 67.
Iceland spar, 214.
Ideal gas, 109.
Indicators, 280.
Internal friction, 88.
pressure, 8, 149.
Intra atomic adjustment,,
183.
molecular adjustment,
183
change, 67, 227,
234, 236.
vibration, 234.
Inversion of cane sugar, 67.
Ionic theory, 251.
lonisation, 75, 112, 113, 147,
212, 213, 228, 240, 260, 266,,
278.
Ions, 76, 109, 110, 112, 116,
255, 257, 258, 259, 260, 264>
265, 266, 271, 274, 282^283.
INDEX
295
Ions, dimensions of, 72
hydrations of, 71.
in solid crystals, 267.
IsQ-dynamic changes, 225.
Isomeric modifications, 236
Isomerisra, 234, 236, 287,
position, 232.
Isoropic band, 233,
Isotonic coefficients, 118.
Iso-tropic condition, 147.
Kinetic theory, 37, 100, 257,
269.
movements, 72, 73, 109,
115, 2 7 2, 279, 284.
Latent heat of vapourisation,
182, 183.
Lavorotatary power, 215, 218.
Lines of force, 226, 269, 272.
Magnet, 269.
Magnetic field, 225, 227, 267.
rotation of polarised
light, 225 229 235.
Mass action, law of, 279,
280, 281.
Mechanical mixture, 6, 138,
278.
Melting points, 198. 228,
276, 278.
Membranes, 106, 110.
semipermiable, 106,
Migration of ions, 71.
velocity, 265, 266.
Miscibility, 6.
Mobility of atoms, 241.
Moduli, 21, 23, 26.
Molar rotation, 216.
volume, 37.
Molecular conductivity, 117,
263.
contractions, 36, 39, 48,
44,45, 48, 58, 66, 72,
150, 151.
dilution, 231.
dispersion, 113.
motion, 130
rotation, 216-223.
structure, 235, 236, 237.
vibration, 144, 151, 226,
volume, 20, 24, 34, 36,
37, 38, 213.
Molecules, active, 2 77.
inactive, 277.
Muta-rotation, 223, 224, 225.
Mutual solubility of liquids, 10
Nicol's prism, 214.
Non-conductors, 257.
Non-electrolytes, 69, 86, 110,
198, 252.
Non-mechanical mixture, 6.
Ohm's law, 253, 273, 274.
Optical activity, 215, 217, 218.
Optical rotatory power, see
rotation.
Osmosis, 248, 252.
296
ASSOCIATION THEORY OF SOLUTION
Osmotic pressure, 3, 75, 108-
122, 174, 176,227,261.
abnormality of, 113.
work, 176.
Partial pressure, 7.
Partition coefficient, 71.
Pfeffer's law, 261.
Physical mixture, 67.
Polarimeter, 215.
Polarisation, 203, 214.
Polarizer, 214
Potential difference, ,11, 247,
267, 272.
203.
Potentioinetric measurements,
251.
Reactions, chemical, velocity
of, 223, 224, 225, 237.
Refraction, 203-214.
Refractiye index of liquid
mixtures, 77, 203-214.
Refractive power, 205, 231,
235.
molecular, 205.
of mixtures, 205,
207, 208.
salt solutions, 205,
207, 228.
Reversible reactions, 280.
Rotations, 217-228.
in solution, maximum
and minimum, 223.
Rotation in solution, inter-
ference by a third sub-
stance, 222.
_- - - magnetic, 285-
229.
Rotatary power, 148, 228, 229,
influence of sol-
vents, 117.
sign of, 217.
Salting out, 10.
Saturated solution, 61.
Semi-permiable, membranes,
112, 114, 176.
partitions, 4, 106.
Solar spectrum, 204, 210.
Solids, hydrated salts, 179.
Solubility, M7, 67, 68, 69,,
248, 249, 250, 279.
and pressure, 11.
temperature, 11.
curve, 13. 14.
break in, 61, 62.
determination of, 7.
lowering of, 71.
maximum, 13.
mutual influence of, 12.
of pas, 6, 7, 69.
liquid, 9, 69.
solid, 10, 11, 69.
Solute, 6.
Solution force, 174, 194.
mechanism of formation<
INDBX
297
and state of existence,
197, 199, 279.
pressure, 251.
tension, 251.
theory of, 24, 194, 199,
262, 278.
Solutions, concentrated, (see
concentrated solutions).
freezing of, 161.
ideal and non-ideal 118,
satur&ted, 61, 62.
super-saturated, 162,
Solvent, 6.
immiscible, 71.
Specific conductivity, 263.
gravities, 18-30, 210.
curve, 20.
in the manufacture
of, 19.
heat, 148, 153-160.
of solutions, 123.
refraction, 210.
resistance, 263.
rotation, 117, 215-222.
Spectrographic studies, 236.
Stokes's law, 289. .
Strong electrolytes, dissoci-
ation of, 75.
Sugars, modifications in solu-
tion, 2514
Surface tension,' 81-87.
Symmetry, 217.
Tautomerism, 241.
Ternary mixture, 197.
Thermal effects, 74.
Thermo-neutrality, 123.
Transition, 223.
temperature, 14, 62, 68.
Transport number, 265, 266.
of solvent, 71.
Ultra-violet spectra, 233.
Valson's law of moduli, 21,26.
Vapour density, 276.
pressure, 113, 123, 178-
181, 194, 199.
lowering of, 177-181,
262.
Velocity of anion, 264.
cation, 264.
chemical reaction,
67, 223, 224, 225, 237, 259.
light, 203.
Vibration, 269, 273.
Viscosity, 8, 73, 88-104, 117,
148, 228.
effect of temperature, 99.
Volume, changes on mixing,
20, 150, 213, 284.
*- conception of, 36, 284.
variation, 150.
Wave length, 215.
theory, 267.
Work done, 176, 199, 203.
X-ray, 267.
ERRATA.
Page Line
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41 2* .For "103070" raw* "1-03070."
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