The osmotic pressure of colloidal solutions and the influence of electrolytes and non-electrolytes on such pressure

Abstract

Determinations were made of the osmotic pressure of gelatin and egg albumin; the colloids were used (1) in approximately pure solution, and (2) after the addition of various electrolytes and non-electrolytes to the colloidal solution; in this case the substance used was added in the same concentration to the outer fluid of the osmometer so as to pervade the entire system on both sides of the membrane in uniform concentration. The osmotic effects observed under these conditions can be due only to the colloid and not to the added substance. The colloidal solution is found, however, after the addition of an acid, alkali, or neutral salt, to exhibit an altered osmotic pressure, the degree of alteration varying with the nature and concentration of the added electrolyte. Non-electrolytes are found to have no appreciable influence on the osmotic pressure of these colloids.

The osmometer employed is constructed as follows: The membrane is composed of a moderately thick film of nitro-cellulose (celloidin or gun cotton) and is of the form and capacity of a 50 c.c. round bottomed flask; it is made by coating the interior of such a flask with a thin film of a 10 per cent, solution of celloidin in equal parts of alcohol and ether, and then removing the solvent by evaporation and bathing in hot water. Such membranes are strong and inextensible, readily permeable to crystalloids and water, and (if of the proper thickness) almost impermeable to the above proteids. The manometer is a straight narrow glass tube passing through a rubber stopper which is bound by an elastic band into the neck of the flask-shaped membrane. The latter, after introduction of the colloidal solution, is immersed in a definite volume of the pure solvent (water, or water plus electrolyte used) contained in a battery jar; the jar is covered by a glass plate to prevent evaporation.