Bi‐ionic Potentials with Sulfonic and Phosphonic Acid Cation‐Exchange Membranes

The use of bi‐ionic potential measurements for the study of the selective behavior of synthetic sulfonic and phosphonic acid membranes toward sodium and potassium ions is described, and the effects of membrane thickness and solution flow rate on the results are discussed. A procedure for obtaining the thickness of the diffusion layer in the solution at the membrane surface from experiments on the interdiffusion of sodium and hydrogen ions is described. The bi‐ionic potentials obtained with the system and the sulfonic and phosphonic acid membranes (over the pH range 2.0–13.0) are correlated with the relative mobilities of the ions in the membranes and their selectivity coefficients and the significance of the results discussed.