The Thermodynamics of Ternary Cation Exchange Systems and the Subregular Model

Abstract

Abstract The thermodynamic theory of ternary exchange equilibria is developed. It is shown that the analogs of well‐known thermodynamic expressions for binary exchange systems can be derived for ternary systems as well. The subregular model of ternary cation exchange systems, which was applied recently to NH 4 ‐Ba‐La exchange on montmorillonite, is examined critically within the thermodynamic context. It is found that the subregular model for a ternary system cannot be expressed solely in terms of parameters that depend only on data obtained for binary systems, in contradiction with claims made recently in the literature. The subregular model is shown to contain one parameter that requires ternary exchange data for its evaluation. A calculation of this ternary constant for NH 4 ‐Ba‐La exchange on montmorillonite indicated that its value was zero within experimental precision. Nonetheless, in general it must be concluded that the behavior of ternary exchange systems cannot be predicted from data on binary exchange systems alone.