THE INFLUENCE OF THE SALT CONCENTRATION AND THE DIFFUSION BOUNDARY-LAYERS ON THE BI-IONIC POTENTIAL

Measurements of bi-ionic potential (BIP) across three cation-exchange membranes (CEMs) have been carried out for KCl/CEM/NaCl, KCl/CEM/LiCl, and NaCl/CEM/LiCl systems, where CEM is a polystyrene and divinylbenz ene sulfonated membrane (CM2, from Tokuyama Soda), a perfluorosulfonic acid membrane (Nafion(R) 117, from Du Pont De Nemours) and an heterog enous membrane prepared by inclusion of cation-exchange resin in PVC ( CRP, from Phone Poulenc). The influence of the salt concentration and the diffusion boundary layers (DBLs) on the BIP values has been analys ed both theoretically and experimentally. The theoretical model is bas ed on the Nernst-Planck equations, and gives a good description for sa lt concentrations higher than 5 x 10(-4) M. For the CM2 membrane, the DBL thickness changes from 20-23 mu m in absence of stirring to 3-4 mu m for high stirring rates. Also, the ion diffusion coefficients in th is membrane have been estimated to be of the order of 10(-6) cm(2)/s. It has been observed that the counter-ion diffusion coefficients ratio (<(D)over bar (A)>/<(D)over bar (B)>) in the membrane increases signi ficantly when the membrane water content decreases, which suggests the possibility of achieving highly selective ion transport with low wate r content membranes.