Determination of effective fixed-charge density in membranes

The fixed-charge density, X, of cation exchange membranes was determined by three different methods: (a) an analytical method, where a membrane in H⁺ form was equilibrated with KCl solution and the amount of exchanged ions determined, (b) the same procedure, but with electric current flow through the membrane, and (c) by measuring an electroosmotic flow through the membrane and calculating X. While methods (a) and (b) yielded essentially the same values, (c) always gave a substantially lower estimate of X. From irreversible thermodynamics, a maximum possible value of X can be calculated, which for four of five membranes tested is lower than the analytically determined charge density. The electroosmotically measured data were always within this limit.