Joseph William Mellor.

A comprehensive treatise on inorganic and theoretical chemistry (Volume 1) online

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partition between two gases, a pressure will be exerted upon the partition. It is
easy to show that the particles of a dissolved substance exert a similar pressure
when a partition is placed between the solution and solvent so that the partition
offers no obstacle to the free circulation of the molecules of the solvent, but resists
the free passage of the molecules of the dissolved substance.

A piece of wet bladder is stretched and wired over the head of a wide thistle -headed
funnel with a stem about 10 cm. long. When nearly dry, the bladder is removed and the


hot funnel is smeared about the rim with marine glue. The bladder is immediately wired
securely in position. The thistle -headed funnel is nearly filled with a concentrated solution
of cane sugar and joined by means of pressure tubing or a rubber stopper with a piece of
capillary tubing of mm. bore bent S-shaped as indicated in Fig. 14. The funnel is immersed
in a jar of water. The level of the index of coloured water in the capillary tube is marked
with gummed paper, and the apparatus is allowed to stand over night. In the morning the
liquid in the capillary will have risen about 10 cm. Water has obviously passed from the
beaker through the membrane into the sugar solution.

The passage of water through a membrane in this manner is called osmosis
from the Greek woyxos, a push. If the osmosis be inwards, towards the solution,
H. Dutrochet's term endosmosis can be used ; if outwards, exosmosis. The mem-
brane permeable to the solvent, impermeable to the dissolved substance, is called
a semipermeable membrane. The extra hydrostatic pressure exerted upon the
membrane by the sugar solution was styled, by W. F. P. Pfeffer (1877), " the osmotic
pressure of the sugar solution." Solutions with the same osmotic pressure are said
to be iso-osmotic or isotonic.

Experiments on osmosis were made by Abbe Nollet (1748). l He showed that if
the opening of a glass vessel containing alcohol be tightly covered with a bladder and
inverted in water, the contents of the vessel increase so that the bladder sometimes
bursts. F. Parrot next studied the phenomenon in 1803, and N. W. Fischer in 1822.
F. Parrot saw the important bearing of this subject on phenomena
or processes which occur in the living organism. Then K. J. H.
Dutrochet took up the subject in 1826 and subsequent years. The
greatest interest centred about the changes of level which occurred
when two different liquids separated by an animal membrane were
kept in contact. G. Magnus (1827), E. B. Jerichau (1825), E.
Briicke (1842) tried to develop a theory of the process ; K. Vierordt
(1845-8), P. Jolly (1849), J. von Liebig (1848), C. Ludwig (1849),
A. Fick (1854), and T. Graham (1861) investigated the subject of
osmosis through animal membranes.

The action is curious. In the ordinary nature of things the
sugar would diffuse into the solvent until the whole system had one
uniform concentration. The membrane retards this. If the sugar 4 Illus

cannot get to the solvent, the solvent goes to the sugar a case of ^tion of Os"
Mahomet and the mountain. Molecules of sugar and molecules motic Pressure,
of water attempt to pass through the membrane ; the way is open
for the molecules of water, but not for the molecules of sugar. Water can pass
freely both ways. The extra pressure on the solution side of the membrane the
solution pressure is supposed to be due to the bombarding of the membrane by
the molecules of sugar. Equilibrium occurs when the number of molecules of
water passing downwards through the membrane is equal to the number passing
in the opposite direction. The resulting pressure is the solution pressure or the
osmotic pressure of the solution.

Let us be perfectly clear about this or we may be led into error. The/acZ observed
is that the osmotic pressure is the excess of the hydrostatic pressure on the solution
side of a semipermeable membrane over the pressure on the solvent side. The
hypothesis here suggested often styled J. H. van't Hofi's kinetic theory of solutions
(1886) is that this pressure is due to the bombarding of the semipermeable membrane
by the dissolved molecules trying to diffuse into the solvent and make solvent and
solution one uniform concentration. The hypothesis was developed in a very im-
portant memoir : The role of osmotic pressure in the analogy between solutions and
gases (1887). 2 The hypothesis has served as a stimulus to much valuable work ; there
are, however, other possible explanations of the phenomenon. The merits of rival
hypotheses cannot be settled by'symposia although discussion may bring fundamental
issues into relief. Harsh experience alone can shatter or establish this interesting
analogy for comparaison n'est pas raison.

Imagine the experiment arranged a little differently. Suppose the aqueous


solution of sugar in the lower part of a cylinder, Fig. 15, to be separated from the
pure solvent in the upper part of the cylinder by a semipermeable membrane A,
so fitted that it can slide freely up and down the cylinder. The upward osmotic
pressure of the solution will naturally force the piston upwards, and a weight, P,
equivalent to the osmotic pressure of the solution, will be required to keep the semi-
permeable membrane in one fixed position.

Many hypotheses have been suggested to explain the function of the membrane
in osmotic phenomenon, ranging between the purely physical conception which refers
the effect to the passage of the liquid through capillary pores, and the purely chemical
conception of a combination between the membrane and the liquid passing through.
M. Traube (1867), S. U. Pickering (1891), and W. Sutherland (1907) considered the
semipermeable membrane acted as a kind of sieve which allowed the passage of the
molecules of the solvent, but obstructed the passage of the supposed larger mole-
cules of the solute. This hypothesis is now abandoned, for no attempt to distinguish
between true pore diffusion occurring through capillary openings and the so-called
true endosmosis occurring through smaller molecular interstices, has proved success-
ful ; and even in the case where collodion membranes and porcelain plates serve
as partitions, S. L. Bigelow (1907) found that the same laws described the passage
of liquids through both ; there is no experimental evidence clearly distinguishing
between the passage of a liquid through capillaries and through molecular interstices.
According to the solution hypothesis, a substance will pass through a membrane
only if it is soluble therein. According to this hypothesis, if two miscible liquids,
A and B, are separated by a membrane, and the membrane has
the power to absorb or dissolve only one of them, say A, this
liquid will be dissolved on one side of the membrane and given
up on the other, and if the liquid B is in a closed cell, an hydro-
static pressure will be there developed. The magnitude of this
pressure will depend on the relative attractions or solubility of
A and B in the membrane. If A is soluble and B insoluble or
sparingly soluble, the membrane will be saturated with A on one
side and supersaturated on the other, and there will be a transfer
of solvent through the membrane until hydrostatic pressure is
developed sufficient to check the flow. Hints of this hypothesis
FIG. 15. Osmotic were given by T. Graham, but M. PHermite (1855) published
Pressure. the first clear statement of a possible development of osmotic

pressure by a selective action of the membrane, and he gave the
three-liquid experiment with chloroform, water, and ether with the express idea
of demonstrating that a substance which passes through the membrane dissolves
in that membrane. Accordingly, argued M. PHermite, there must be a relation
between solution and chemical union ; osmotic phenomena are not the result of a
special force, but rather the effect of forces of affinity similar to those acting in
solutions. L. Kahlenberg (1906) also has sought for evidence in support of the
solution theory of osmosis.

The following is A. C. Brown's modification of M. 1'Hermite's three-liquid layers to
illustrate the development of osmotic pressure by the solvent action of the membrane. A
concentrated solution of calcium nitrate is saturated with phenol and the mixture poured
into a tall narrow cylinder. The excess of phenol rises and floats upon the surface of the
calcium nitrate solution. The phenol should not be in larger excess than is required to give
a layer a few millimetres thick. Distilled water saturated with phenol is carefully poured
above the two layers of liquid in the cylinder. The water floats on the surface of the phenol.
The water on both sides of the phenol can traverse the partition of phenol, but the calcium
nitrate cannot pass through. Hence the layer of phenol is a semipermeable membrane.
Mark the level of the layer of phenol in the cylinder by means of a piece of gummed paper.
If the upward motion of the layer of phenol be marked from day to day, it will be found to
rise higher and higher, and finally surmount the rest of the liquid in the cylinder.

Osmotic phenomena can be obtained by continuous and by discontinuous or
porous films. With continuous films it is necessary for the solvent but not for the


solute to dissolve in the membrane ; with porous films it is necessary for the pure
solvent to be adsorbed by pores so small that only the solvent not the solute can
pass through. Benzene, toluene, and pyridine were found by L. Kahlenberg to pass
through a rubber membrane while water does not. Hence rubber probably acts as
a semipermeable membrane to the three first-named liquids, because these liquids
dissolve in the rubber.

W. Ramsay (1894) illustrates the production of an osmotic pressure in solutions
by the following analogy illustrating what has been termed the osmotic pressure of

A palladium vessel at 250 to 350 is filled, at atmospheric pressure, with nitrogen gas
or with some gas not absorbed by the warm palladium. This vessel is immersed in hydrogen
at a given pressure ; hydrogen gas diffuses through the metal membrane until the
increase of pressure inside the vessel is nearly equal to the outside pressure. In one
experiment, this increase was equivalent to 733 mm. of mercury, which is " regarded as the
osmqtic pressure of nitrogen dissolved in hydrogen." The excess pressure is independent
of the concentration of the hydrogen molecules, for the pressure of the hydrogen is the
same on both sides of the septum. The (osmotic) pressure of the nitrogen is produced by
the bombardment of the nitrogen molecules on the walls of the vessel, while the osmosis of
the solvent hydrogen is possible in virtue of its faculty of dissolving in the metal membrane
under conditions where the solute nitrogen is insoluble.

In ordinary or positive osmosis the direction of flow of the solvent, water, is from
the less towards the more concentrated solution ; in some cases the direction of
flow is from the more to the less concentrated solution ; the phenomenon is then
styled negative or reversed osmosis. H. Dutrochet first described osmosis with
inorganic membranes, and T. Graham attributed the phenomenon to chemical
interaction between the salt and the membrane. F. E. Bertel, P. Girard, and
H. Freundlich attribute the anomalous effect to the electrical endosmose ; the flow of
liquid is brought about by a difference in electrical potential, the two ends of the
capillary pores in the membrane becoming oppositely charged. Potential differences
of this kind were shown to exist in animal cells by M. Oker-Blom and W. Ostwald ;
in frog's muscle by A. Briinings ; in vegetable skins by M. Loeb and R. Beutner ; in lung
tissue by R. S. Lillie and P. Girard; in copper ferrocyanide membranes byR. Beutner ;
and in clay by A. Briinings. According to W. D. Bancroft, the sign of the electric
charge on the membrane is dependent on the absorption of anions or cations.
J. Perrin ascribed the polarization to contact electrification being dependent on the
preponderance of H*-ions or OH'-ions. F. E. Bartel also showed that the appearance
of negative osmosis is dependent on the pore diameter, for the phenomenon occurs
with solutions of magnesium chloride only when the pore diameters are less than
0'4jLt. J. Mathieu found negative adsorption occurs with a number of dilute solutions
when adsorbed by porous plates, membranes, or capillary tubes, such that the
liquid adsorbed by the capillary tubes from 2V-solutions was often only N ; and
he suggests that if the capillary were fine enough only pure water would be adsorbed.
Summing up the literature on the subject, W. D. Bancroft says : (1) Osmotic phe-
nomena may occur with a porous diaphragm provided we have very marked negative
adsorption and provided the diameter of the pores is so small that the adsorbed
films fill practically the whole of the pores. (2) A porous diaphragm will act as a
semipermeable membrane in case there is no measurable adsorption of the solute
and in case the adsorbed films fill the pores completely. (3) In the usual case of a
semipermeable diaphragm, we do not have a porous diaphragm and the semiper-
meability is due to the fact that the solvent dissolves in the diaphragm while the
solute does not to any appreciable extent under the conditions of the experiment.
(4) A liquid is not to be considered as a porous substance and solubility does not
depend on porosity. Again, A. M. C. Chanoz found that when the two sides of the
membrane differ, as with a skin, differences in the osmosis are obtained depending
on whether a given side of the membrane is in contact with solution A or solution B.
These differences disappear, of course, when the two sides of the membrane are


alike, as with parchment paper. It seems probable that the behaviour of the
membrane depends largely on its greater or less permeability.

Animal membranes are objectionable when exact measurements are required,
because to a certain extent the results depend upon the nature of the membrane,
which is not strong enough to withstand the great pressures developed by osmosis ;
and, most serious of all, the membrane is not quite semipermeable, so that an
appreciable amount of, say, sugar does actually pass through. It would therefore
be as profitable to measure the pressure of a gas in a leaking vessel as to try to measure
the osmotic pressure of a solution with a membrane which allows part of the dissolved
substance to pass through. We therefore fall back on artificially prepared mem-
branes. No artificial membrane has been so successful as a film of copper ferro-
cyanide deposited between the inner and outer walls of a porous earthenware pot
prepared by M. Traube, 3 and described in 1867 in his Experiments zur Theorie der
Zellenbildung und Endosmose. The film is made by steeping a clean porous pot in
an aqueous solution of potassium ferrocyanide, rinsing in water, and then 1 sub-
merging the pot in an aqueous solution of copper sulphate, and subsequently washing
out the soluble salts. The deposition of the copper is symbolized by the equation :
2CuS04+K 4 FeCy6=Cu2FeCy 6 +2K 2 S04. The porous pot with its semipermeable
membrane is fitted with a suitable manometer to indicate the pressure. In 1877,
W. F. P. Pfeffer made some measurements with cells prepared in this manner.
The apparatus was immersed in a large bath of water to maintain the temperature
constant during the experiment. Analogous experiments were made by H. de
Vries (1878), G. Tammann (1888), P. Walden (1892), etc. Earl of Berkeley and
E. G. J. Hartley (1904) placed a solution of sugar in a porous earthenware pot with
a semipermeable membrane of cupric ferrocyanide, and surrounded the pot with
water. The pressure on the solution was increased until it was just sufficient to
prevent the passage of water into or out of the cell through the septum of the ferro-
cyanide. H. N. Morse (1901-9) employed an apparatus similar to that of W. F. P.
Pfefler, but he improved the quality of the membrane by depositing the cupric
ferrocyanide in the pot electrolytically ; and also improved the joints between the
cell and the manometer ; and the manometer itself.


1 Abbe Nollet, Hist. Acad. Sciences, 101, 1748 ; Lemons de physique experimental, Amsterdam,
1754 ; R. J. H. Dutrochet, Ann. Chim. Pkys., (2), 35. 393, 1827 ; (2), 37. 191, 1828 ; (2), 49. 411,
1832 ; (2), 51. 159, 1832 ; Memoires pour servir a Vhistoire anat. et physiol. der vegetaux et des
animaux, Paris, 1837 ; L' agent immediat du mouvement vital, Paris, 1826 ; K. Vierordt, Pogg.
Ann., 73. 519, 1848 ; E. Briicke, ib., 58. 77, 1843 ; P. Jolly, ib., 78. 261, 1849 ; C. Ludwig, ib.,
78. 307, 1849 ; A. Fick, ib., 94. 59, 1855 : ; G. Magnus, ib., 10. 160, 1827 ; E. B. Jerichau, ib., 34. 613,
1835 ; J. Liebig, Ueber einige Ursachen der Saftbewegung in tierischen Organismus, Braunschweig,
1848 ; W. F. P. Pfeffer, Osmotische Untersuchungen, Leipzig, 1877 ; T. Graham, Phil. Trans., 151.
183. 1861.

2 H. Dutrochet, Ann. Chim. Phys., (2), 60. 337, 1835 ; G. Flusion, ib., (8), 13. 480, 1908;
T. Graham, Phil. Trans., 144. 177, 1854'; P. Girard, Compt. Rend., 146. 927, 1908; 148. 1047,
1186, 1909 ; 150. 1446, 1910 ; 153. 401, 1911 ; F. S. Bartel, Journ. Amer. Chem. Soc., 36. 646,
1914 ; 38. 1029, 1916 ; S. L. Bigelow, ib., 29. 1576, 1907 ; 31. 1194, 1909 ; H. Freundlich, Kolloid.
Zeit., 18. 11, 1916; M. Oker-Blom, Pfluger's Arch., 48. 191, 1901 ; A. Brimings, ib., 84. 241,
1903 ; 117. 409, 1907 ; R. S. Lillie, Amer. Journ. Physiol, 28. 194, 1911 ; P. Girard, Rev. Gen.
Science, 20. 694,1909 ; R. Beutner, Journ. Phys. Chem., 17. 344, 1913 ; S. L. Bigelow, ib., 15. 659,
1911; 16. 318, 1912; W. D. Bancroft, ib., 16. 312, 1912; 21. 441, 1917; J. Mathieu, Ann.
Physik f (4), 9. 340, 1902 ; F. Trouton, B. A. Rep., 84. 288, 1914 ; W. Ostwald, Zeit. phys. Chem.,
6. 71, 1890; M. Loeb and R. Beutner, Science, 34. 866,1906; J. Perrin, Journ. Chim. Phys.,
2. 601, 1904; W. Ramsay, Phil. Mag., (5), 38. 206, 1894 ; W. Sutherland, ib., (5), 44. 493, 1897 ;
S. U. Pickering, Ber., 24. 3629, 1891 ; M. Traube, Gesammelte Abhandlung, Berlin, 200, 213,
1899; M. 1'Hermite, Ann. Chim. Phys., (3), 43. 420, 1855; Compt. Rend., 39. 1177, 1854;
L. Kahlenberg, Journ. Phys. Chem., 10. 141, 1906 ; Trans. Faraday Soc., 3. 23, 1907 ; J. H. van't
Hoff, Arch. Neerl, 20. 239, 1886 ; Zeit. phys. Chem., 1. 481, 1887 ; Phil. Mag., (5), 26. 81, 1888;
Harper's Scientific Memoirs, 4. 11, 1899; F. Tinker, Nature, 97. 122, 1916; A. M. C. Chanoz,
Recherches experimentales sur les contacts liquides, Paris, ] 906.

8 M. Traube, Archiv. Anat. Physiol. Wiss. Medizin., 87, 129, 1867 ; Gesammelte Abhandlungen,
Berlin, 200, 213, 1899 ; W. F. P. Pfeffer, Osmotische Untersuchungen, Leipzig, 1877 ; H. N. Morse



and co-workers, Amer. Chem. Journ., 26. 80, 1901 ; 34. 1, 1905 ; 36. 39, 1906 ; 37. 324, 425,
588, 1907 ; 38. 175, 1907 ; 39. 667, 1908 ; 40. 194, 1908 ; 41. 257, 1909 ; Earl of Berkeley
and E. G. J. Hartley, Proc. Roy. Soc., 73. A, 436, 1904 ; Phil Trans., 206. A. 481, 1906 ; Earl of
Berkeley, E. G. J. Hartley, and C. V. Burton, ib., 209. A 177, 1908 ; 218. A, 295, 1919 ; H. de
Vries, Arch. Neerl, 13. 344, 1878 ; Zeit. phys. Chem., 2. 415, 1888 ; P. Walden, ib., 10. 619, 1892 ;
G. Tammann, Mem. Acad. St. Petersburg, 35. 169, 1887 ; Wied. Ann., 34. 299, 1888.


8. The Osmotic Pressure of Dilute Solutions and the Gas Laws

Every formula obtained by the application of thermodynamical considerations alone
to a mixture or solution remains the same, no matter what assumptions be made regarding
the molecular condition of the substances. Consequently, thermodynamics alone cannot
decide whether solution is attended by a chemical change in the molecular state of the dis-
solved substance or otherwise.- P. DUHEM (1894).

J. H. van't Hoff's kinetic theory of osmotic pressure (1887) * emphasizes the
analogy between the process of vaporization and the process
of solution. In a solution the dissolved substance is dis-
tributed throughout the whole bulk of the solvent, and the
solvent plays the part of so much space. The vapour pressure
of a liquid in space will thus be represented by the osmotic
pressure of a solution. In the words of A. Rosenstiehl, the
osmotic pressure is analogous to the elastic force of vapours.
Just as the closed space above a liquid becomes saturated
with vapour, so does a solvent in contact with the solute form a
saturated solution. An increase of temperature augments the
vapour pressure of a liquid, and also the osmotic pressure of a

I. The relation between osmotic pressure and the concentra-
tion of the solution Boyle's law. W. Pfeffer in his Osmotische
Untersuchungen (Leipzig, 1877) obtained some data with the
apparatus which J. H. van't Hoff (1887) utilized, with remarkable
cleverness, in developing what he called " the role of osmotic
pressure in the analogy between solutions and gases." The ex-
perimental data showed that the osmotic pressure is very nearly
proportional to the concentration of the solution; otherwise

expressed, the osmotic pressure appears to depend upon the j, , g Meas

degree of crowding of the molecules of the dissolved substance. me nt'of Osmotic
Instead of repeating Pfefler's measurements, a selection from Pressure,
some later determinations with solutions of glucose (sugar) by
H. N. Morse (1907) can be quoted (temperature nearly 0, rounding off the decimals
to the nearest tenth of a unit) :

Concentration . . .0-1 0-2 0'3 0'4 0'5 0'6 1-0
Osmotic pressure . . .2-4 4-7 7-0 9'3 11-7 14'1 23'7 atm.
Equivalent gas pressure . . 2-2 4'5 6'7 8'9 11-1 13'4 22'3 atm.

In dealing with the concentration of solutions, it will be well to adopt the same unit
of comparison as that employed in dealing with gases, i.e. the molecular weight of
the solute expressed in grams per 22 '3 litres of solution at normal temperature and
pressure. H. N. Morse found that his direct measurements of osmotic pressure
came out best when referred to a constant volume of the solvent, not to the volume
of the solution.

Assume that a gram-molecule of glucose (180) were it a gas would occupy 22 '3 litres.
Hence, 0*1 gram-molecule will occupy 2 4 23 litres. By choosing the concentration so that in
Boyle's relation, PF = constant, a solution containing a molecular weight expressed in grams,
per 22'3 litres, has a concentration of 22'3 units when P = l, we get from Boyle's law
P-r-C7 = 22'3. The concentration, it will be remembered, is inversely proportional to the
volume. Hence for a concentration O'l, we get P = 2'23, for C=0'2, p = 4'46, etc.



The " equivalent gas pressure " is here calculated on the assumption that a
" sugar gas " obeying Boyle's law really exists. The results are plotted in Fig. 17.
The deviation of the osmotic pressure curve from the dotted curve emphasizes the
fact that the deviations of the osmotic from the equivalent " gas pressures " grow
larger with increasing concentrations, and hence exact proportionality occurs
only when the solutions are very dilute. For dilute solutions, the osmotic
pressure is nearly proportional to the concentration, or, as W. Ostwald puts it,
" the osmotic pressure of a sugar solution has the same value as the pressure the
sugar would exert if it were contained, as a gas, in the volume occupied by the
solution of course assuming Avogadro's rule." This is another way of saying
that the relation between the osmotic pressure of a solution and its concentration
has the same form as Boyle's law for gases.

The analogy does not work out so well for concentrated solutions as with dilute
solutions possibly owing to the disturbing effects of overcrowding produced by :
(1) molecular attraction between the molecules of the dissolved substance ; (2) the
volumes of the molecules themselves. The two effects for gases were discussed
when dealing with Boyle's law for gases. J. D. van der Waals' corrections for the
gas equation pv=RT, involves the introduction of terms for the mutual attraction
of like molecules and for the space occupied by the molecules, and the corrected

equation takes the form (p-\-a/v z )(v b)=RT, and
by regarding r in the equation pv=RT as the
volume of the solvent not of the solution, H. N.
Morse really corrected the equation for the space
occupied by the molecules of the solute as J. D.
van der Waals' did for gases. And (3) the mutual
attraction between the molecules of the solute and
solvent. On account of the enormous number of

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