Ion-exchange Selectivity and Electrolyte Concentration

Abstract

It is shown how, from a given exchange isotherm at a given total solution normality, one may calculate isotherms and exchange selectivity coefficients at other total normalities. When the exchange ions have different valences the isotherms depend strongly upon the total normality, and the selectivity of the exchanger, for the ion of higher valence increases without limit with increasing dilution. Also when the ions have different valences isotherms without inflexion points can become sigmoid with increasing dilution and sigmoid isotherms can loose their inflexions. In these circumstances sigmoid isotherms, for example in zeolites, cannot provide evidence that there is more than one kind of exchange site in the exchanger. However, the changes in isotherm shape or position with dilution are very small when the ions have the same valence. A way of diagrammatic representation of isotherms has been described which is independent of electrolyte concentration.