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Abstract

The fixed charge concentration of asymmetric cellulose acetate membranes caused by a small amount of carboxyl groups of the cellulose acetate was determined by titrating the corresponding H⁺ ions using the method of Heymann and Rabinov. The fixed charge concentration, C^o_X-, was thus estimated to be 3.4 • 10^-3 mequiv/w wet membrane. This value is compared with the effective fixed charge concentration determined by means of streaming potential measurements using solutions and asymmetric cellulose acetate membranes annealed at 82.5°C. Furthermore, the effect of the pH of the outside solution on the effective fixed charge concentration of the asymmetric cellulose acetate membrane is theoretically discussed assuming a dissociation equilibrium for the carboxyl groups. Thus, it is shown that the effective fixed charge concentration, C_X-, of weak ion exchangers depends strongly on the pH as well as on the electrolyte concentration of the outside solution, and on the valency of the counterions. Since the theoretical treatment of the influence of pH on the effective fixed charge concentration is based on approximations, only a semiquantitative agreement between experimental findings and theoretical results is obtained