Compositional constraints to identifying ternary interactions in ion‐exchange equilibria

Abstract

Cation exchange is one of the most important classes of reactions controlling the soil solution composition for major ions. Modeling ion-exchange equilibria is complicated by ternary solid-phase interactions. The present study alleges that spurious ternary-interactions can be reported when dilution of the exchanging ions by other ions on the solid-phase is not accounted for in models. Ion-exchange equilibria data from the NH 4 + -Ca 2+ -K + -vermiculite system is used to demonstrate that previous reports of ternary interactions in this system are an artifact of how solid-phase mole fractions are defined; what mathematicians call "compositional incoherence." A graphical method to distinguish the effects of dilution from real chemical interactions is presented using mole fractions renormalizing in the ternary system so that they are defined the same way as they are in binary systems. Potassium was shown to not influence NH 4 + -Ca 2+ exchange on vermiculite when these corrections for solid-phase dilution were used.