I.
THEORY OF SOLUTIONS
CHAIRMAN: HOMER W. SMITH, Sc.D.
"A Knowledge of the Laws of Solutions...'
By HoMER
W.
SMITH, Se.D.
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KNOWLEDGE of the laws of solutions,' it has been
compressioni of gases below their critical temperatures, and in the solid state, produced by
the removal of all solvent, [intermolecular
forces completely dominate].
"The hope may be expressed that the possibility of representing and studying all these
intermediate states, which cani be accomplished easily aild fully in the examiniation of
solutions, will be of considerable help in making easier the study of the laws of pure liquids." The foregoing is quoted, with slight
paraphrase, from the second edition of Wilhelm Ostwald 's Lehrbuch der allgemneinev
Chnerie of 1891.1*
Ostwald 's generalization stemmed largely
from 2 papers, then onlv a few years old, one
bv the Dutchman, Jacobus Heldricus van 't
Hoff (1852-1911), the other by the Swede,
Svaante August Arrhenius (1859-1927), which
contributed the major founLdations of our coIntemporary physical chemistry, of some of our
contemporary physiology, and, in no small
measure, of that sprightly if as yet ill-defined
contemporary discipline, the proponents of
"is inmportaut because almost all the chemical processes
which occur in nature. whether in animal or
vegetable organisms, or in the nonliving suirface of the earth, anid also those which are
carried out in the laboratory, take place between substances in solution. For examiiple, a
sound judgment regarding physiological processes
is impossible without this knowledge;
and this holds true for the greater numuber of
the scientifically and technicallv imeportant
reactions.
Solutions are mIlore important
thain gases, for the latter seldom react together at ordinary temperatures, whereas solutions present the best coliditioiis for the
occurrenee of all chemnieal processes.
"The discovery of the laws of solutiolns is
full of significance for the advanlce of phvsical chemistry.... The colligative laws whieh
apply to gases and dilute solutions always
maintain their character because the molecules of gases and of Fvery] dilute solutions
are so far removed fromn olie another that
nieither their m-utual interaction mior their
special nature, but only their iiunmbers, come
into play. But the individual characters of
the molecules become more importaant wheni
gases are compressed, or solutions are coneeintrated, and deviations froni the colligative
laws dominate more amid more, unltil at last,
in pure liquids produced bv the coiitinued
said,
.
*Ostwald 's Lehrbach had a rather checkered history.
The first edition in 2 volumes was published at Riga
in 1885-87, but rapid progress in physical chemistry
led to a second edition of 2 volumes, published at
Leipzig in 1891-93. A revised, second printing of this
second edition was published at Leipzig in 1896-190a,"
wx ith the second volume in 2 parts, followed by a fragment of a tlhird part. The first printing of the second
edition is Inot available to the writer, and he has followed the seconid priniting. Vol. 1, Book 4 (Solutions)
of the first priniting lhas beein tranislated by Muir.1
From the Departmenit of Physiology, Neew York
Ijaiversity College of AMedicine, New York, N. Y.
808
Circulation., Volume XXI, May 1960
LAWS OF SOLUTIONS
which choose to call themselves " biophysicists. ' Current studies of the nature of solutions are still in the stage of revising and
improving the pioneer ideas of van 't Hoff and
Arrhenius.
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Van 't Hoff had thought to be a chenmist, but
as an undergraduate student at the University
of Leyden he yielded to the lure of matheniatics, a subject wholly foreign to the prosaic
synthetic-analytic chemistry of the 1870's. In
Bonnl he worked with Friedrich Kekule, discoverer of the tetravalent nature of carbon
and the ring structure of benzene; and in
Paris with Charles Wiirtz, who proudly said
that chemistry was "a French science'"without fulminating either hiluself or his
mentors. Fromn Paris he went to Utrecht
where, in 1874, he won his Ph.D. with a conventional and wholly safe thesis on eyaiiacetic
and malonic acids 2 wisely refraining fromn
drawing the attentioni of his doctoral examiners to an heretical 11-page pamphlet which,
shortly before his examination, he had published privately. The Dutch title of this
pamphlet is beyond mny lingual ability, and
even in English it is too long to read.3*
Twenty-four years elapsed before this pamphlet reached the English language, whereupon the title was reduced to the simple
phrase, The Arrangement of Atoms in Space
(1898) .6 Thereafter this work won for van't
Hoff an international reputation as the
founder of stereochemistry, which deals with
the arrangements of atoms aiid molecules ini
3 dimensions instead of 2. Through the instigation of Johannes Wislicenus the paniphlet was translated into German ini 1877 7
whereupon it was deilounced by Hermann
Kolbe of Leipzig, one of the m-ost eminent organic chemists of the time but one who could
*The Dutch pamphlet was translated by van 't Hoff
into French in 1875,' a translation made necessary
by the fact that Joseph Achille Le Bel (1847-1930)
of Paris had expressed very similar views in November of 1874. Le Bel and van 't Hoff had come to
the theory of the asymmetry of the carbon atom independently, but because of van 't Hoff 's priority
and more thorough treatment of the problem of
optical activity he is generally given the major credit.
Circulation, Volume XXI, May 1960
809
not think in 3 dimensions, as "fanciful nonselnse [which] carefully avoids aniy basis of
fact, and is quite unintelligible to the calmn
investigator. . . ." As for Wislicenus, Kolbe
said that "he has gone over from the camp of
the true investigators to that of the speculative philosophers of ominous memory, who are
separated by olnly a thini medium from spiritnalisnz. " 8, pp. 86-87
Vani't Hoff 's capacity for abstractioni had
early led to an interest in chemnical kinetics.
and his thoughts now leaped from 3 to 4 dimensions. ''Good nmusie,'' he once said,
"makes it very pleasant to think of other
things"-other things, perhaps, being his second great work, his A'tudes de Dynamiqte
Chimique of 1884.9 This work was translated
ilnto Geriman'0 and English'1 12 years after
its publicatioil in French; it is frequently
mnentioned but rarely read because in alny language it is very searce. In the etudes van't
Hoff incorporated all that had hitherto been
kniown and added so imuch that was new that,
so it has beeni said, chemical dynamics was
seareely improved in the next 30 years.8, P. 343
Though the Table of Contents of the Etudes
suggests a veritable encyclopedia (which in a
sense it is), vani't Hoff 's writing is marked by
brevity and clarity, and by careful definitioin
of all mathematical terms; where a mathematical relation represelnts a broad generalization, this generalization is re-expressed in
words. He did not coneur in the view of Willard Gibbs, who once asserted that "matheimuaties is a language'" anid who generally
chose to speak no other.*
The Itutdes did not evoke the ridicule that
had greeted Atoms in Space; rather it was
received in silence because there seemed to be
11o one who could comprehend its dynamic
approach-until in a Norwegian review12 for
March, 1885, there appeared an exhaustive
critique giving it its proper valuation as one
*"It is said that, during his long membership
in the Yale faculty, Willard Gibbs made but one
speech, and that of the shortest. After a prolonged
discussion of the relative mnerits of language andl
mathematics as elementary disciplines, he rose to
remark, 'Mathematies is a language.'I )15, p. 26
810
SMITH
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of the classics of science. This critique was
written by a relatively unknown chemiiist at
the University of Upsala, one who was destined shortly to become as famous as van't
Hoff himself, Svante August Arrhenius.
It was in the last section of the Atutdes,
which deals with the topic of chemuieal affinity,
that van't Hoff first propounded his gaseous
theory of solutionl. He had been thinkinog
to determine the affinity of Glauber 's salt
(Na2SO4 1OH20) for its water of erystallizationl by measuring the vapor pressure of water
in equilibrium with the dry solid. Then it
camne about that one day in 1883, probably
just before the Etudes went to the printer, on
leaving his laboratory in Amsterdam van't
Hoff and his wife met the botanist Hugo de
Vries (1848-1935) (of mutationi" fame, and
who was Professor of Botany at the University), and de Vries talked to himi of the plasmolysis of plant cells, of isotonic solutions of
sucrose and salts, and of the remarkable quamititative studies in osmotic pressure carried out
by the botanist Wilhelmn Friedrich Philip
Pfeffer (1845-1920) and summarized in 1877
in Pfeffer 's monumental mnonograph, Osmo"
tische
Untersiuchungen."3
The discovery of osmotic phenomenia had
been made in 1748 by the Abbe Jean-Antoine
Nollet14 who, in seeking the source of the
bubbles that form in liquids when thev are
boiled, had covered a vial of boiled spirits
with a nmembrane-presunmably of pig's bladder-and submerged the covered vial under
water. He noticed that in a short time water
mnoved into the vial, causing the meimbrane
to bulge outward with great pressure. The
paradox, that water should move into a sealed
vial against pressure, he eorrectly explained,
after numerous well-controlled experiments,
by the supposition that the mnembrane is niore
permeable to water than to alcohol. (The
Abbe 's paper was published half a century before Dalton (1803) formulated the atomic
theory, a full century before Cannizzaro
(1858) resolved the long controversy between
those who went along with Avogadro and
those who went along with Berthollet.)
There followed a century anid more of unobtrusive experimeentation on osmosis, beginning with Parrot, who in 1815 studied the
mixing of liquids by diffusion. Some 13 years
later the physiologist Dutrochet introduced
the first, if crude, manometer into this problem and in 1827 Dutrochet16 coined the words
"endosmose" and "exosmose," meaning to
push or impel. He later showed that osmosis
can occur through a thini sheet of marble, and
that organic membranes are generally slightly
permeable to salts. The history of our problem in the next few decades bears mnany fainiliar names: Fischer, Gustav Magnus, Jerichau, Briicke, Matteuci and Cima, Liebig,
Jolly, Vierordt, Eckard, and-not the least
among physiologists-Carl LIudwig, who based
his filtration-reabsorption theory of renal
function on his own osmotic experiments anid
those of others. Important among chemists
was Thomas Graham, who in 1826 formulated
the basic laws relating the diffusion of gases
to their density, applied these laws to the diffusion of a solute through membranes in 1854,
and in 1861 divided solutes into crystalloids
and colloids. It was Graham who translated
Dutrochet 's osmose into the English "osmotic. "* The generalized law of diffusions
formulated in 1855 by A. Fick, is so well
known as to require its mention only.
Interest in osmosis spread from physiology
to botany when Pringsheim in 1854 and Naegeli in 1855 showed that the protoplasmic conitents of plant cells contract in strong sugar
or salt solutions, indicating that the surface
of the protoplasm constitutes a membrane
permeable to water but not to many solutes.
Moritz Traube is hard to classify: a pupil of
Liebig 's, he was primarily engaged in the
wine business, but worked on ehemical and
biological problems, fermentation, and cell
theory on the side. He had discovered in 1867
that osmosis could be studied by means of
membranes precipitated from colloidal or
other solutions over the open ends of glass
tubes; and, seeking a better membrane, he had
*J. Hogg used the nominative form "osmosis" in
1867, but the verbal ''osmose'' remained in use
until the end of the century.
Circulation, Volume XXI, May 1960
811
LAWS OF SOLUTIONS
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come upon the film of copper ferrocyanide.
Pfeffer, who, like Pringsheim and Naegeli,
had been studying plasmolysis (and who
seemed to have been the first to have spoken
of the bounding surface of the plant cell as
the "plasma membrane"), saw the potentialities of Traube's artificial membrane and in
1877 reported the most important single advance in the study of osmosis: he had precipitated the copper ferrocyanide film in the
porous wall of an unglazed earthen pot, thus
preparing the first truly rigid osmometer by
which osmotic pressure could be measured
accurately at widely different concentrations
and temperatures. It was about this date that
Hamburger began his studies on the hemolvsis
of red blood cells, and that de Vries himself
had begun his studies using (as he called it)
the "plasmolytic" method. In the ensuing 8
years de Vries had established the "isotonic"
concentrations (another word of his) for nuinerous electrolytes and nonelectrolytes, using
a variety of plant cells.
Hence, it seems that the stars of science
were in some sort of favorable conjunction
over Amsterdam when de Vries, the isotonically minded botanist who was well informed
on Pfeffer's plasma membrane and copper
ferrocyanide manoineter, met van't Hoff, the
disillusioned organic chemist who thought in
terms of 4 dimensions and whose E'tudes had
practically given birth to the science of chemical dynamics.
And so it was that in the last, hurriedly
written chapter of the Ytudes of 1884, van't
Hoff's attention turns from the chemical affinity of Glauber's salt for its water of crystallization, to the attraction which an aqueous
solution has for pure water, as revealed by
either de Vries' plasmolytic method or Pfeffer 's osmometer. Here van 't Hoff coins the
word "semipermeable" to describe Pfeffer 's
copper-ferroeyanided pot because it permits
the passage of water but niot of sucrose. Our
erstwhile chemist proceeds with prescient inspiration to show that the osmotic pressure of
Pfeffer's solutions is related to the (natural
logarithm of the) ratio of the vapor pressure
Circulation, Volume XXI, May 1960
of pure water divided by the vapor pressure
of the solution; and to this end he calculates
the needed vapor pressures from the relation,
established in 1870 by Guldberg (of mass-law
fame), between freezing-point lowering and
vapor-pressure reduction,'7 using data on the
freezing point of sucrose solutions published
by Raoult in 1883.'8
Then, it seems as though haste to get the
manuscript of the Etudes off to the printer
lands him in an anticlimax: using the recorded vapor pressure data, he calculates that
-if 2 molecules of water are removed from
CuSO4 5H20, an osmotic pressure of 1,300
atmospheres would be produced. He adds,
"The experiment could not of course be carried out in the usual way [i.e., in Pfeffer's
pot] since we are here dealinig with a solid
substanee."
* *d
That is all. Van 't Hoff had the rational approach to the study of solutions in his iktudes
of 1884, and he abandoned it. Within the year
he had turned one of those narrow corners
that make of life a tangled skein. On October
14, 1885, he purportedly "read" a "paper"
in Stockholm before the Royal Swedish Academy of Sciences, entitled "The lIaw of Chemical Equilibrium in the Dilute State, Gaseous
or Dissolved." This paper was published in 3
parts in the Transactions of the Academy of
1886.19*
Now, there is one thing of which I am certain, and that is that van't Hoff never "read"
this paper anywhere. As published, it runs to
some 57 large pages replete with tabular data,
diagranms, and differential equations. He undoubtedly said one thing and published another-but this is no sin because we all have
committed it. The spoken and written word
are different dialects within the mother
tongue. What he did say to the academicians
we will probably never know.
In the printed paper, however, he draws the
analogy between de Vries' plant cells and
Pfeffer's copper ferroeyanide osmometer; and,
under the principle of conservation of energy
*Part I of this papeir was republished, with
in Holland in 1886.20
chainges,
minor
812
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and the Carnot-Clausius prinieiple (that heat
does not pass spontaneously from one body to
another one which has a higher temiperature),
he applies a reversible pistoni-compression eyele to 2 ideal fluids separated by an ideal semipermeable membrane. Usinlg Pfeffer 's data on
the osmotic pressure of sucrose at various coneentrations and temperatures, de Vries' data
oIn the isotonie concentrations of various
solutes, anid Hamburger 's data onl the hemolvsis of red blood cells, he deduces the relationis
betweeni 3 of the 4 colligative properties of
solution-s: osmotic pressure, freezing-point
lowerincg, andvapor-pressure redletion.* And
hle shows that, for very dilute or ideal"' solutionis in whiclh inltermloleeular forces can be
nleglected the osmotic pressure conlformls with
the laws describing dilute or ideal gases, in
the senses that:
1. As stated by Boyl.e for gases, osimlotic
pressure is proportional to the coneenitration
of solute, the tem:lperature remlainiing constanit.
"Guldberg, wi-e Ihaxve aotel, lha(l dedluce(l the relation between freezinig-point lowverinig aind vaporpressure reductioni in 1870,'i a relation experimentallconfirmed by Raoult in 1878.-' Van 't Hoff now had
available to him Raoult 's demoonstration that: (1)
the ieduction of freezinig poinlt of equimiolecular
aqueous solutioils is indepenidenit of the niatur e of
the solute;"8 anid (2) the reduction of freezing poinlt
at equimolecular concentrations in various solveents
is constant (water being a notable exception).The 3 initerrelated properties of a solution osmotic
pressure, freezing-point loweriing, anid vapor -pressure
reduction were, on the suggestion of Wundts, calledI
' colligative'" by Ostwald in 1891.1 P. 30 Ostwvald
distiinguished additive, coinstitutive, aind colligative
properties: additive properties are those which are
the sums of the properties of the constituents (e.g.,
mass) ; constitutive properties depenid on the arrangenieit of the constituents of a pure substaniee (e.g.,
boiling poinit, optical activity, etc. ; colligative properties are those which have equal values for echemically
comparable (molar) quantities of the mnost different
substances (e.g., volume and pr essure relations of
gases, and the aforementioaed properties of solutiolns
in whielh the componen-t miolecules ar-e so Avidely separatedl from oine another that the mnutual interactions
of these molecules are reduced to a minimum).
It was not until 1889 that Beckmann" added boiling-point elevation to the other 3 colligative properties, the theoretical relationiships having been sup-
plied bly ''myv lhonorled friend'' Arrhenius.
SMITH
2. As stated by Gay-Lussac for gases, osmotic pressure is proportiolnal to the absolute
temperature, the cone entratioln renmainillg
conistamit.
3. As stated bv Avogadro for gases, other
conidition-s remaining constant, osmotic pressure is the same for anly 2 solutions containing
the same iiunuber of solute miolecules per unit
volumLe; aiid furthermnore, this pressure
ainounts to 22.4 atmospheres at O Celsius,
the figure 22.4 being all the imiore remearkable
wlihen one considers the differenee betweeni the
grain-nolecular weight of gaseous hydrogen.
which is 2, and that of dissolved sucrose,
wvhich is 342.
Aind so he applies to dilute or ideal solutionis the familiar ideal gas law equationi:
PY = nRT
where P is now osmnotic pressure in-stead of
gas pressure and a is the niumLber of mnolst dissolved in 1 liter of solution.
In short, lie savs that the osmotic pressure
of a dilute solutiomi is equal to the pressure
which tlie solute would exert if dispersed as
a gas at the saime temiperature amid in a volune
equal to that of the solution.
Inl van 7t Hoff 's interpretationi, osmotic
pressure depenids oni the inipact of the moleeules of the solute against the seniipermeable
membramie-for the all too obvious and equally
erronieous reasoni that "the ilmolecules of the
solvent, being presenit upoii both sides of the
miiembranie through which thev pass [freely!],
*He inieasis ceintigrade, of eourse, because Celsius'
thermiiiomiieter was the cenitigrade thermiionmieter turneid
upsi(le (lown.
i has heie beeen added to van 't Hoff 's gas lawiequation simnply to rouind out his thought. The "kilogr amii equivalent'e was apparently introduced into
the gas lanv equatioiu by Horstmann in 1881,[ and(l
convenience led rapidly to the sulbstitutioni of the
''gram equivalent' ' oi " gram-l miiolecule. " The latteie was first called a Mole by Ostwald in 1902,4
priniting 2,9.l12 whlen this writer also coined the phrase
Molenibrvch, oir viol fraction, to describe the mnolecular
,
first used by Raoult in
iixture
n1 + n
1888 inl his geineral law of vapor pressure reduction
of ethereal solutions :', 21
t1
P1-- 1'
formiiula,
P
lii -t
11'
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LAWS OF SOLUTIONS
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do lnot enter into considerationi. ' '@24, P. 15t He
recognized, however, that one could equally
well treat osmotic pressure in terms of the attraction of the solute moleeules for the solven-t
molecules-in dilute solutions the bombardment and attraction theories would be mathematically indistinguishable. The third interpretation, that osmotic pressure reflects the
difference in vapor pressure of the solvent in
the pure state and in the solution, that this
vapor-pressure difference is indeed the vis a
tergo of osmosis, was germinal in the 'titdes
of 1884; but he had turned the narrow corner
before he went to Stockholm and theneeforth
he follows the wrong road.
Why'?
The obvious fact of the 2-way traffic of the
solvent through the membrane can searcely
by itself explain van't Hoff's predilection for
the "bombardment" interpretation because
in the Ettudes he had clearly recognized that
the great affinity which Na2SO4- 1OH20 exhibits for its water of crystallization is reflected in a proportional reduction in the vapor pressure of water above the salt; and in
discussing Pfeffer's experiments he had uti*Ostwald echoes this argument in 1981 when he
writes "'There is no doubt that the cause of [osm-iotie]
pressure is to be sought for in the dissolved substance, for water cannot produce any enduring pressure, inasmuch as it passes through the separating
membrane without difficulty.'" How close he camiie
to the proper interpretation is revealed by the later
comment that "the [osmotic] cell behaves as if there
were a partial vacuum for water in its interior:
water flows in, and, if no opposing pressure is allouved
to develop, produces a continuous movement whichi
ceases only when the contents of the cell have beeome
the same as those of the space surrounding the cell,
i.e., have become pure water. A condition of [osmotic] equilibrium is possible only nhein the pressure
which prevails within differs sufficiently from that
which prevails without.
PP. 101 102 The belief that
only the solute (not the solvenit) in the osmometer is
under pressure was expressed as late as 1928 byBancroft and Davis.25
tThis volumiie contains an excerpt of referenice'
and all of referenceS22, 26, 27, 29, 32 and 37 translated into
English by Jones. Reference37 is reproduced in part
in Leicester, H. M. and Klickstein, H. S.: A Source
Book of Chemtstry: 14o00-900. New York, Toronto
and LIondon, McGraw-Hill Book Co., Inc., 1952, pp.
483-490.
Circulation, Volume XXI, May 1960
813
lized
reversible cycle consisting of a semaipermeable membrane and a vapor phase"The pressure," he said, "which causes the
flow of vapour fromn [one chamber to the
other] is therefore equal to the diminution in
the vapour pressure of water which is produced by dissolving the salt in it; . . . " Hence
one may suspect that he was also influenced
by that charm which is only skin deep (and
by which too many of us have at times been
swayed)-namely, the esthetic appeal of his
theory. The gaseous theory of solution, the
miraculous figure of 22.4, promised the answer
to an age-old mystery of why sugar dissolves
in water, an answer both beautiful and simple
-it turns into a gas. But nature is neither
beautiful nor simple; it is damnably puzzling
and complex, and no one yet has turned sucrose into a gas or even explained a semipermeable membrane.
a
However, to return to matters of record,
from the beginning vanl't Hoff recognized that
only dilute solutions conform with the gas
law equation, and then, only dilute solutions
of a few substances-sucrose is the classical
example. Most substances, such as strong
acids, strong bases, and most salts, which
Faraday long ago had called electrolytes* beeause their aqueous solutions conduct the voltaic current, behaved anomalously not only
with respect to osmotic pressure but also with
respect to vapor-pressure lowering and freezing-point reduction. For this anomalous behavior he had no explanation.
The Stockholmi paper of 1885 was reworked
in 1887 for publication in volumne 1 of the new
Ziitschrift fur physikalische Chemie, of which
Ostwald and van t Hoff were co-editors.
(Nothing elarifies our ideas so much as rewriting a paper once a year, perhaps for several years.) Van 't Hoff has nlow seeni Raoult 's
paper on " The General Law of the Vapor
Pressure of Solutions, ' '29 showing that for
equimolecular concentrationis of nonvolatile
*Faraday had introduced the words ' electrode, "
''electrolyte,'" "'electrolysis,'" and "'ion'" in 1839.
"Ion" he derived from the Greek ion meaning traveling or traveler.
814
solintes the vapor pressure of various solvents
is reduced by the same fractioni: i.e, solutions
that have the same osmotie pressure have the
same vapor pressure-but for van't Hoff, vapor pressure still remains merely another of
the "colligative" properties of a solution.
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Something new, however, is added in the
Zeitschrift paper, namely, ani explanation of
the anomalous behavior of strong acids, bases,
and salts.
Nearly 2 years before, on August 4, 1885,
van't Hoff had written Arrhenius, thanking
him for his favorable review of the k'tudes,
commentiiig on a mnemoir by Arrhenius concerning electrolytic conduction, and recapitulating some of the arguments in his Stockholm
lecture concerning chemical equilibrium. It
was not until March 30. 1887, however, that
Arrhenius replied, having waited until he
could read van't Hoff's paper in the Transactions of the Academny. Now he suggests that
the anomalous behavior of electrolytes inl
aqueous solution may be explained by their
dissociation into ions ;30, pP- 219f., 239f. in this letter he makes what is perhaps his most explicit
statement on the matter: "What I called in
mny paper, 'Sur la Couductibilite' [2nd part],
active molecules, are thus the same as dissociated molecules. One of the propositions
which I then put forward would now be written: -In all probability all electrolytes are
completely dissociated [into ions] at the most
extreme dilution. ''31, p. 1392; 30, p. 241
With this explanationi available to him,
van't Hoff writes, in this, his third paper on
osmotic pressure, whiceh appeared in the
October 21 number of the Zeitschrift,24' 32
"It may, theii, have appeared daring to give
Avogadro 's law for solutions [equal numbers, equal volumes, equal pressures] sueh
a prominent place [iii the theory of solution], and I should not have done so had
not Arrhenius pointed out to mne, by letter, the probability that salts anid analogous
substanees, when in solution break down into
ions." Daring it may have been, daring indeed it was, but he had already committed
himself to the gaseous theory, electrolytes not-
SMITH
withstanding, in the ftudes of 1884 and the
Stockholm lecture of 1885.
The question mlay be raised as to whieh
man, van 't Hoff or Arrhenius, stood in greater
debt to the other. Without the dissociation
theory the colligative properties of solutions
treated by van't Hoff could not be rationalized because "most" compounds did not conform with the gaseous theory; on the other
hand Arrhenius had not dared go all the
way on the dissociationi theory until this theory had found support in the observations on
osmotic pressure, etc., which had been colligated by van't Hoff.*
We may but briefly turn back to the history
of Arrhenius' idea. The study of electrolysis
aiid electrolytic conduction had had a long
history which cannot be encompassed here beyoond mentioning a few outstanding names,
such as Grotthus, Faraday, Clausius, Williamson, Hittorf and Kohlraush-the last-nalmed,
with his studelits, having perfected the use of
alternating current and the Wheatstone
bridge for the measurement of electrical conductivity of solutions. The necessary data
were available for reinterpretation by 1870,
but the ultimate idea of the complete dissociation of the solute had been effectively
blocked by the long-held conviction, basic to
the atomiic theory itself, that the 2 atoms of
NaCl, for example, are held together by powerful electrical forces and can- be separated
into "charged radieles" only bv the application of a substantial electrical current; and
also by the properties of pure metals such as
*To say that Arrhenius' theory "was a direct outcomiie of vani It Hoff 's osmotic pressure studies,' 8t P. 111
is to ignore the long history of electrolytic conduction
and other interpretations which had been advanced
before Arrhenius' time.
Vanlt Hoff had read Arrheniusi' 2 papers3 "4 of
1884, but the vague notion of "'active" and "inactive" particles in solution served only to supply himn
Mx itlh a new approach to the mass lawx, electromotive
force and related matters. The fact that the partial
dissociation of an electrolyte, such as NaCl, into 2electrically active particles would increase the nuimber of osmoticallv active particles wxas Arrheniiis '
contribution.
Circulation, Volume XXI, May 1960
LAWS OF SOLUTIONS
815
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sodium, and of pure gases such as chlorine,
which were irreconcilable with the properties
of a solution of NaCl.
Arrhenius had begun his studies on the electrical conductivity of solutions at Upsala under the organic chemist, Per Theodor Cleve,
but his ideas were never popular at Upsala
and his major work was carried out in Stockholm between 1881 and 1884 under Erik Edlung, Professor of Physics to the Swedish
Academy. Beyond his own observations, numerous physical-chemical data were available
to him by 1883, all of which called for some
unifying interpretation, but in his thesis submitted at Upsala in 188333 34 he had handled
the problem of electrical conductivity cautiously by referring (in Clausius' terms) to
"active" and "inactive" forms of the solute,
only the "active " form serving to conduct
the electric current. No explanation of "activity" is given nor is it stated why "activity"
increases with dilution, and the word "dissociation" is not mentioned. The older interpretations were reworked, quantified, and
given new applications, but without a complete break with tradition. Even so, the thesis
was heterodox enough to bring opprobrium
upon its author, who was granted his degree
(1884) non sine laude approbateur, the double
negative indicating in Sweden that he was
17th of May in the year 1883,* and I could
not sleep -that night until I had worked
through the whole problem. "8, p. 115
Only after he had read van 't Hoff's Stockholm lecture in 1886, however, did he venture
to publish 2 papers in Sweden35' 36 and to
write his definitive paper in German,37' 24 the
last appearing shortly after van 't Hoff's
paper in volume 1 of the new Zeitschrift. The
Zeitschrift elicited a mixed reaction on the
part of the scientific public-between van 't
Hoff and Arrhenius, so it seemed to some, it
had gotten off to a bad start. Though the gaseous theory of solution was soon accepted, the
theory of electrolytic dissociation was for
many years opposed by every seemingly rational, and many irrational arguments, except
by Ostwald, who championed the new theory
from the start. Once when Ostwald visited
Upsala, Cleve asked him ". . . and you are
also a believer in these little sodium atoms
swimming around'? "8, p. 117 It was unthinkable
that the bland action of water as a solvent
could cause a molecule to separate into its
electrically charged atoms-and one imagines
that there were those who hesitated to wash
their hands for fear that electric sparks might
jump up to their fingertips.
just "a little better than passing. ''31
Only after reading van't Hoff's paper,
It has been said that, to van't Hoff, his
osmotic theory meant chiefly a new and convenient way to determine molecular weights
of dissolved substances. I do not believe it.
Van't Hoff was less interested in molecular
weights than in chemical dynamics, chemical
affinity, and a rational theory of solution. In
retrospect, I regret that he abandoned the consideration of experimentally measured vapor
pressure for the circuitous reasoning of the
Clausius-Carnot cycle because thereby the
theory of solution lost many years. Yet van't
Hoff and Arrhenius-we will not attempt to
judge the relative merits of their contributions-between them started us along the road
and
noting the anomalous behavior of electrolytes
in respect to osmotic pressure, etc., did
Arrhenius break with tradition and state that
in infinitely dilute solutions electrolytes are
completely dissociated, and that van't Hoff's
law for osmotic pressure, and indeed all the
colligative laws, apply to all substances, nonelectrolytes and electrolytes, if partial or complete dissociation is taken into account.
The idea that some of the molecules of a
solute may, when in solution, undergo complete dissociation for a finite time into ions
which have virtually complete electrical and
chemical independence went considerably beyond the undefined term "active." It was (in
effect) this idea which, as Arrhenius later
said, occurred to him " on the night of the
Circulation, Volume XXI, May 1960
we travel now.
*His thesis was presented to the Academy on June
6 and must have been completed some time before.
SMITH
816
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Circulation, Volume XXI, May 1960
817
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Protoplasm
Protoplasmn is a systeim of exquisite sensitiveness. In order that it may survive it
must be protected from too great, or too rapid, or too irregular fluctuations in the
physical, physico-chemiiical, and chemical conditions of the environment. Stability may
sometimes be afforded by the natural environment, as in sea water. In other cases an
integument may sufficiently temper the external changes. But by far the most interesting
protection is afforded, as in man and higher animals, by the circulating liquids of the
organism, the blood plasma and lymph, or, as Claude Bernard called them, the milieu
interieur. In his opinion, which I see no reason to dispute, the existence and the constancy of the physico-chemical properties of these fluids is a necessary condition for
the evolution of free and independent life. This theory of the constancy of the milieu
interieur was an induction from relatively few facts, but the discoveries of the last
fifty years and the introduction of physico-ehemical methods into phvsiology have proved
that it is well founded.-L. J. Henderson. Blood. A Study in General Physiology. New
Haven, Yale University Press, 1928, p. 20.
Circulation, Volume XXI, May 1960
I. THEORY OF SOLUTIONS: "A Knowledge of the Laws of Solutions ..."
HOMER W. SMITH and HOMER W. SMITH
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Circulation. 1960;21:808-817
doi: 10.1161/01.CIR.21.5.808
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