Non-stationary radiotracer method for diffusion coefficients of Cs⁺, Ba²⁺, Eu³⁺ tracers in Nafion-117 membrane

The ion-exchange kinetics of A^{z +} _sol ⇆ Na⁺ _mem (A^z+ = Cs⁺/Ba²⁺/Eu³⁺) in Nafion-117 have been measured using non-stationary radiotracer technique for trace concentrations of A^{z +} ions in external solution. A method based on non-steady state Nernst–Planck approach has been developed and used to fit the experimental ion-exchange profiles to obtain the diffusion coefficients (DCs) of these ions. The DCs, thus obtained, have been found to be appreciably lower than the literature-reported self-diffusion coefficients, indicating slower rate of ion-exchange. The results show that membrane DCs of these ions are modified by bulk electrolyte solution.