Comparison of techniques for measuring ion exchange selectivities in zeolites

Two methods for measuring the selectivities of ion exchange reactions in zeolites were compared. Three zeolites (NaA, mordenite and ZSM-5) of differing ion exchange capacity were studied for uni-univalent (Rb+/Na+) and for uni-divalent (Sr2+/2Na+) exchanges. The first method involved rigorous chemical analysis of exchanged ions (Na+ and Rb+ or Na+ and Sr2+) in the solid and solution phases at equilibrium. The second method was based on the use of 86Rb and 85Sr tracers, which were measured in the solution phase after equilibrium, and the remaining concentrations were calculated by difference. The corrected selectivity coefficient (K G A/B) was calculated from the equilibrium data. Both methods gave reasonable accuracy (10–15%) when the metal loading in the zeolite was low to medium. The radioactive tracer method, however, resulted errors larger than 100% at high metal ion loading (>80%).