Separation of the Diffusion Potential within a Membrane and the Donnan Potential on the Basis of a Measurement of Transient Membrane Potentials

Abstract

Abstract In the perfluoro carboxylate cation exchange membrane–aqueous electrolyte systems, two steps appeared in the time course of the membrane potential in response to a pH jump in the external solution. LiCl, NaCl, and KCl were used as the electrolytes. According to the experimentally observed curve of the potential transient, the membrane potential was divided in two portions: the potential difference at the membrane/solution interface (Donnan potential) and the intramembrane diffusion potential. It was found that the differences among the total membrane potentials for three kinds of alkali metal ion systems were mainly attributed to the generation process of the diffusion potential within the membrane. The pH dependencies of the Donnan potential and intramembrane diffusion potential were calculated on the basis of a theory starting at the Nernst–Planck equation for the ion diffusion. Calculations were carried out taking into account the association/dissociation reaction of hydrogen ions and ion exchange groups and the heterogeneous structure of the membrane. By fitting the theoretical equations to the experimental results, the best-fitting values of relative mobility of ion and the dissociation constant of COOH group in the membrane were obtained.