Membrane potential of composite bipolar membrane in ethanol-water solutions: The role of the membrane interface

The membrane potential across a composite bipolar membrane (CBM) composed o f a cation-exchange membrane with an anion-exchange membrane is theoretical ly and experimentally analyzed for LiCl ethanol-water solutions. The theore tical approach is based on an extension of the Donnan equilibrium and the N ernst-Planck equation of monopolar charged membranes for the case of two io n-exchange layers by considering the effect of electrolyte ion pairing in t he external solution. The experimental results show that the effective memb rane charge densities of the two ion-exchange layers will become smaller th an those which are separately estimated for each layer. We have introduced a contact factor, zeta, into the theoretical approach to clarify this pheno menon in this study, and the theoretical predictions were in good agreement with the experimental data. The membrane potential measurements show that CBM has the characteristics of a bipolar membrane and can significantly con tribute to a better electrochemical characterization of the CBMs. (C) 1999 Academic Press.