Studies on ion-exchange resins. IV. Selectivity coefficients of various cation exchangers towards univalent cations

1. 1. The thermodynamic theory of ion-exchange processes predicts that the selectivity coefficient is determined by the ratio of the exchange ion activity coefficients in the resin phase, by differences in ionic volumes and by the thermodynamic osmotic pressure. 2. 2. The variation in selectivity coefficient with ionic composition of the resin phase appears to be, in the case of the alkali metal cations, primarily a pressure-volume effect. In the case of the quaternary ammonium ions, ion activity variations, resulting from adsorption of the organic ions on the resin matrix, appear to oppose the pressure-volume effects. 3. 3. The effects of temperature and concentration of the resin phase are consistent with the concept of ion hydration as it affects the pressure-volume term. The constancy of ionic activity ratios for the alkali metal cations is in accord with the Bjerrum, Harned, Stokes-Robinson theory. 4. 4. The selectivity coefficients are not a function of the ionic strength of the equilibrative solutions. In the absence of adsorption effects, the smaller ion is preferred. 5. 5. Selectivity coefficients for a pair of cations can be calculated from selectivity coefficient values for each of these ions with a third reference ion. This indicates that the thermodynamic osmotic pressure varies only slightly with different exchange cations for a given resin, and supports the view that the ionic activity coefficients are determined chiefly by the nature and concentration of the ion and not by interaction between the ions. 6. 6. Adsorption effects of the quaternary ammonium ions are more pronounced in the case of low capacity resins, due to the larger ratio of organic to polar groups.