Ion exchange involving several groups of homogeneous sites

Abstract

Crystalline exchangers such as zeolites may contain more than one group of different exchange sites. The theoretical basis of these multi-site exchangers has therefore been examined. Overall exchange isotherms and selectivities have been expressed in terms of the properties of j different kinds of co-existent site. The resulting plots of log k_c against equivalent cation fraction of the entering ion (Kielland plots) are complex, showing maxima, minima, inflection points and sigmoid shapes according to the properties of the several groups of sites. It is concluded that linear Kielland plots in such circumstances can at best only be approximations. Even where each group of sites behaves ideally the overall exchange isotherm can be strongly non-ideal. Where Kielland coefficients C₁ are not zero for the overall isotherm, this isotherm may show maxima and minima (and hence limited solid solubility of the end members of the exchange), the criterion for which is a positive value of C₁ which varies according to the valence of the exchanging ions. By an appropriate choice of quantities characterizing each of two groups of sites Kielland plots and isotherms were found, as an example, for chabazite which closely agreed with those determined experimentally.