Average Valence of the Gelatin Ion Determined by a Modified Theory of Membrane Equilibrium

Abstract

From Donnan's theory 1 we can calculate the depression of the osmotic pressure of a gelatin solution, by the addition of a salt like NaCl. According to this theory this depression should never exceed 50% of the pressure of the colloid electrolyte in complete absence of a diffusible electrolyte. Experimental observations show that this is far from being true. As shown by Loeb 2 and others, salt addition depresses the osmotic pressure of gelatin chloride frequently to less than one-tenth of its original size.

Manifestly such a deviation must be due to an insufficiency of the premises of the theory, probably to the too simple assumption of Donnan that the colloidal micellae carry single electrical charges. Calculations based on the assumption of the presence of multiple charges of the colloidal ion—to be published in extenso later—show that the fraction of osmotic pressure which remains after the maximal lowering by a salt excess amounts to 1/n+1 of the pressure in absence of a diffusible electrolyte—where n signifies the valence of the colloidal ion; hence the pressure is depressed to one-half for monovalent, 1/3 for bivalent, 1/11 for 10 valent colloidal ions, etc.

From Loeb's measurements 2 we can figure the osmotic pressure of gelatin chloride by adding to the observed values of osmotic pressure the counterpressure which is due to unequal distribution of the H⁺ ions on either side of the membrane. This latter magnitude can also be figured from Loeb's data. By the addition of an excess of HCl practically the entire gelatin is transformed into gelatin chloride. For a 1 % solution of gelatin chloride the osmotic pressure is thus figured as about 1520 mm. water column. By the addition of an excess of NaCl this is depressed to as low a value as nearly 23 mm.—all these data being taken from Loeb.