Effect of a film layer on mono-divalent bi-ionic membrane potential

The bi-ionic potential generated across a standard cation exchange membrane was investigated in a mono-divalent bi-ionic system. Applying Henderson's theory to the film layer, the Nernst—Planck equation was solved in the film layer as well as in the membrane. The observed potentials were compared with the theoretically predicted values from parameters such as bulk concentrations, diffusion coefficients in the membrane and in solution, exchange capacity, corrected molar selectivity coefficient, and film layer thickness. The calculated potentials were not always in agreement with the experimental values, but reproduced properly the trend of potential change with concentration and stirring rate. The constituent potentials, i.e. the diffusion potentials in the film layer and in the membrane and the interfacial potentials, are also discussed. In the sodium—calcium bi-ionic system, the primary contribution to the total membrane potential was not the diffusion potential in the film layers and in the membrane, but the sum of two interfacial potentials. p]In the case of membrane-diffusion control, the bi-ionic potential was confirmed to follow the equation of Helfferich.