Application of the Stefan-Maxwell Equations to Diffusion in Ion Exchangers. 2. Experimental Results

The Stefan-Maxwell relations were used to analyze the ion exchange of Na⁺and Cs⁺in Dowex 50-X8. Tracer diffusion coefficients for both ions were measured as a function of composition. This allowed certain combinations of the Stefan-Maxwell interaction coefficients to be determined which in turn allowed limiting single ion tracer coefficients (Na diffusing in Cs form resin only and Cs diffusing in Na form resin only) to be estimated. Results show that use of these limiting tracer coefficients in the Nernst-Planck equations gives much improved prediction of ion exchange than does the use of usual self-diffusion coefficients. The improved results are attributed to the fact that the Stefan-Maxwell relations take into account important ion-ion interactions.