Cation exchange membranes as solid solutions

Abstract

The ion exchange equilibrium between KCl and SrCl 2 solutions and the cation selective membrane CR61 AZL386 has been examined. Equilibrium isotherms are measured for membrane—electrolyte equilibria for two total concentrations of Cl −in the electrolyte solution and at three different temperatures. The results show that Sr 2+ is strongly preferred to K + in the membrane. Selectivity increases with temperature in the range 10°C to 40°C. It decreases with increasing total concentration from 0.01 N to 0.03 N. Electrolyte absorption in the membrane was not detected for the experimental conditions used. Contents of water in the membrane is constant for external salt concentrations between 3 x 10 −8 N and 3 x 10 −2 N. The difference in water content between the pure K form and the pure Sr form is approximately 5%. A thermodynamic treatment of ion exchange is presented. The thermodynamic equilibrium constant for the exchange reaction 2 KR + SrCl 2 = SrR 2+ 2 KCl has been determined by two different integration procedures, R being an ion exchange site in the membrane. The variations of the thermodynamic functions Δ G mix, Δ H mix and Δ S mix are calculated from experimental data for an assumed mixing process of membranes in the pure forms. Activity coefficients for the membrane components are calculated from experimental data. Results are compared to two lattice models. A model with a random distribution of K +, Sr 2+ and a vacant site seems preferred at low concentration of Sr 2+. At high concentrations, Sr 2+ and the vacant site associate.