EVALUATION OF ION-TRANSPORT PARAMETERS IN A NAFION MEMBRANE FROM ION-EXCHANGE MEASUREMENTS

Coupled diffusion of two counterions in a fixed-site, permselective, i on-exchange membrane is treated by the method of moments. Theoretical expressions are given for the evaluation of ion diffusion coefficients from the ion-exchange measurements. They are used to analyze the tran sport properties of a Nafion membrane in the presence of proton and a univalent (Li+ Na+, K+, Rb+, or Cs+) or divalent (Ru(2,2'-bipyridine)( 3)(2+)) cation. Diffusion coefficients of alkali metal cations, as cal culated from ion-exchange rates when the hydrogen ion is initially pre sent in the membrane (the forward process) or in the solution (the rev erse process), differ significantly. Moreover, only the latter paramet ers are comparable with the corresponding self-diffusion coefficients. These effects can be attributed to a different water content in Nafio n prior to forward or reverse exchange measurements and to the cross e ffects arising from the simultaneous transport of the two counterions. While the rates of Ru(2,2'-bipyridine)(3)(2+) exchange into the as-re ceived Nafion and the Nafion cast films are about the same, the presen t analysis yields a much lower value of the diffusion coefficient.