ABSTRACT
The ion-exchange reaction consists of the replacement of an electrostatically sorbed ion by a like-charged ion from solution. The exchangeable ions may be either positively or negatively charged; the ion exchanger may be solid or liquid; and the equilibrating solution may be aqueous or a mixture of solvents. This chapter concentrates on liquid-phase and solid-phase activity coefficients and the models which have been developed to estimate their values. It describes the chemical-thermodynamic treatment only, considering the alternative conventions and techniques that have been used for dealing with both solid and liquid phases. With the cation-exchange-reaction stoichiometry and the resultant exchange equilibrium constants explicitly defined, the standard states, reference states and reference functions for the reactants and products must be defined. In order to be consistent, equations for the value of any specific activity coefficient must tend to unity as the component approaches its reference state.
