The measurement of Donnan potentials with cellulose and aqueous solutions

Abstract

When a membrane, such as a sheet or a strip of cellulose, separates one solution of an electrolyte from another of different concentration, an electrical potential difference is established across the membrane, and this potential difference can be accurately measured by suitable experimental means. When potassium chloride is used as the electrolyte, the diffusion potential within the membrane is very small, and the latter may be regarded not as an inert sieve but as a macromolecular structure, the electrochemical properties of which are due to the presence of non-diffusible ionogenic groups, such as carboxylic acid groups. The dissociation, in an aqueous medium, of a portion of these groups causes the membrane to acquire a negative potential with respect to each of the contiguous solutions. A method is described whereby the single potential difference between the membrane and a solution of potassium chloride in which it is immersed may be determined. The experimental results are in agreement with the view that the membrane may be regarded as an aqueous region of uniform potential, and that this potential is dependent on a Donnan distribution of the mobile ions. The degree of dissociation of the non-diffusible ionogenic groups depends on the concentration of neutral electrolyte solution in contact with the membrane, since, on account of the Donnan effect, the pH of the solution imbibed by the membrane differs from that of its external environment. Values for the dissociation constant of the carboxylic acid groups in Cellophane have been obtained from the present results. When hydrochloric acid is the electrolyte, and in salt solutions of low pH, the dissociation of the carboxylic acid groups is almost entirely repressed, and the Donnan potential vanishes. The results obtained with materials of artificially enhanced carboxyl content are also in agreement with expectation, and indicate that the Donnan potential is due solely to the presence of carboxylic acid groups. The variation of the potential with the water imbibition of the cellulose lends further support to the concept of the swollen membrane as an equipotential region.