FRIDAY, JANUARY 28, 1921

CONTENTS

ELECTRIFICATION OF WATER AND OSMOTIC FLOW1

I

THE exchange of water and solutes between the cell and the surrounding fluid is one of the important factors in the mechanism of life, and a complete theory of the osmotic flow is therefore a postulate of biology. It was a marked advance when the experiments of Pfeffer and de Vries led van't Hoff to the formulation of the modern theory of osmotic pressure. According to this theory the molecules of the solute behave like the molecules of a gas in the same volume and at the same temperature, and the gas pressure of the solute measures the "attraction" of a watery solution for pure water through a strictly semipermeable membrane. Yet it is obvious to-day that in a liquid the electrical forces between solvent and solute must play a rôle and no adequate provision is made for these forces in van't Hoff's law. Traube rejected van't Hoff's theory altogether, suggesting instead that the osmotic flow was from the liquid with lower to the liquid with higher surface tension (and higher intrinsic pressure).

Tinker has shown that van't Hoff's theory for osmosis holds strictly only in the case of ideal solutions, i.e., when the process of solution occurs without heat of dilution and change in volume, but that in the case of non-ideal solutions Traube's ideas explain the deviations from the gas law which are bound to occur. When two different ideal solutions containing equal numbers of particles of solute in equal volume are separated by a strictly semipermeable membrane, equal numbers of molecules of water will diffuse simul

1 Presidential address prepared for the Chicago meeting of the American Society of Naturalists, December 30, 1920.

taneously in opposite directions through the membrane and no change in volume will occur. When, however, the same experiment is made with two non-ideal solutions containing equal numbers of molecules in equal volume, the result is different. As Tinker has demonstrated mathematically, in this case the flow of water must be from the solution having the lower intrinsic pressure and lower surface tension to the solution with higher intrinsic pressure and higher surface tension. This is what Traube claims, and his theory explains therefore, as Tinker points out, the deviations from the gas law in the case of non-ideal solutions, but it does not prove that the gas law of osmotic flow does not hold in the case of ideal solutions and Traube's theory can not therefore replace van't Hoff's theory.

II

There is a second group of forces not taken into consideration in van't Hoff's law, namely the influence of the chemical nature of the membrane on the solvent. These forces become noticeable when the membrane separating the solution from the pure solvent is not strictly semipermeable. When water is in contact with a membrane it undergoes as a rule an electrification and this electrification of the particles of water plays a great rôle in the rate of the osmotic flow when the solution into which the water diffuses is an electrolyte.

The assumption that water diffusing through a membrane is as a rule, electrified, is justified by a large number of observations. Quincke demonstrated that when water is pressed through capillary tubes it is found to be electrically charged (the sign of charge. being more frequently positive); while the tube has the opposite sign of charge, e.g., negative, when the water is positively charged. When two solutions of weak electrolytes are separated by a membrane (which may be considered as a system of irregular capillary tubes) an electric current causes water to migrate to one of the two poles, according to the sign of its charge. By this method of so-called electrical endosmose it can be shown

that water diffuses through collodion branes in the form of positively charge ticles. Collodion bags, cast in the sh Erlenmeyer flasks, are filled with a we neutral solution of an electrolyte M/256 Na,SO,, and dipped into a filled with the same solution of Na,SO. The opening of the collodio is closed with a rubber stopper perfora a glass tube serving as a manometer. a platinum wire, forming the negative trode of a constant current, is put th the glass tube into the collodion bag the other pole of the battery dips int outside solution, the liquid in the glass rises rapidly with the potential gradie tween the two electrodes. The water fore migrates through the collodion mem in the form of positively charged par The writer has made a number of e ments concerning the osmotic flow th collodion membranes, and it is the purp this address to give a brief survey o results.

III

When a collodion bag is filled with a tion of a crystalloid, e.g., sugar or salt dipped into a beaker containing pure the pure water will diffuse into the sol and the level of liquid in the capillary tube serving as a manometer will rise the same time particles of the solute diffuse out of the bag (except when the is a protein solution or a solution of other colloid). The concentration of a talloid solute inside the collodion bag therefore become constantly smaller finally the solution is identical on both of the membrane. Nevertheless the re force with which a given solution insi collodion bag "attracts" the pure wate which the bag is dipped can be measur the initial rise in the level of water i manometer, before the concentration ( solution has had time to diminish t great extent through diffusion. Since:

2 Loeb, J., J. Gen. Physiol., 1918-19, I. 1919–20, II., 87, 173, 273, 387, 563, 659, 67:

first minutes accidental irregularities are liable to interfere with the result, we measure the rise in the level of liquid in the manometer during the first 20 minutes.

If the initial rise of level of liquid in the solution is thus measured it is noticed that it occurs approximately in proportion with the concentration of the solution when the solute is a non-electrolyte. The rate of diffusion of pure water into a solution of cane sugar through a collodion membrane is therefore approximately a linear function of the concentration of the solute within the limits of O and 1 M. This is what we should expect on the basis of van't Hoff's theory of osmotic

pressure.

If, however, a watery solution of an electrolyte is separated from pure water by a collodion membrane, water diffuses into these solutions as if its particles were positively charged, and as if they were attracted by the anion of the electrolyte in solution and repelled by the cation with a force increasing with the valency of the ion (and another property of the ion to be discussed later).

Pure water diffuses into a M/128 solution of NaCl through a collodion membrane more rapidly than it diffuses into a M/64 solution of cane sugar; water diffuses into a M/192 solution of Na,SO, or Na, oxalate still more rapidly than into a M/128 solution of NaCl; and into a M/256 solution of Na, citrate water diffuses more rapidly than into a M/192 solution of Na,SO,, and into a M/320 solution of Na,Fe(CN), still more rapidly than into a M/256 solution of Na, citrate. Assuming complete electrolytic dissociation of the electrolytes in these cases, the influence of the five solutions mentioned should be identical according to van't Hoff's theory. We notice, instead, that the "attraction" of the solutions for water increases with the valency of the anion. This is true for all neutral solutions of salts contained in a collodion bag, regardless of the nature of the cation.

If a collodion bag containing a neutral solution of a salt with bivalent cation, e.g., M/192 CaCl, or MgCl2, or with a trivalent

cation, eg., M/256 LaCl,, is dipped into a beaker with pure water we notice no rise in the level of water in the manometer. In solutions with bivalent or trivalent cations the repulsion of the cation equals or exceeds therefore the attraction of the anion for the positively charged particles of water diffusing through the pores of the collodion membrane. Hence we conclude from these (and numerous similar) experiments that the particles of water diffuse through a collodion membrane as if they were positively charged and as if they were attracted by the anion of an electrolyte and repelled by the cation with a force increasing with the valency of the ion.

It seemed of interest to find that concentration of a cane sugar solution which just suffices to prevent the diffusion of water into a given solution of an electrolyte. Into each of a series of beakers, all containing the same neutral salt solution, e.g., M/192 Na2SO,, was dipped a collodion bag containing a cane sugar solution of different concentration, from M/128 to 1 M, and it was observed in which of these sugar solutions the level in the manometer rose during the first 10 minutes, in which it fell, and in which it remained constant. It was found that the cane sugar solution which was just able to balance the