Thermodynamics of Ion Exchange. An Alternative Methodology

Abstract

The thermodynamics of ion exchange is usually presented in terms of ion exchange equilibrium constants. This formalism is somewhat cumbersome, especially when more than two ionic species are involved in the exchange equilibria. It is shown that an alternative formulation using a notation developed previously by the author simplifies matters. Clausius−Clapeyron equations are derived for enthalpies of exchange under the practically convenient condition of constant solvent activity rather than constant solvent content of the exchanger phase. A definition is forwarded of thermodynamically ideal behavior in the exchanger phase which corresponds as closely as possible with the thermodynamic terms in the Nernst−Planck equations describing ideal ion transport behavior. When Kirkwood−Buff solution theory is applied to fixed charge exchangers, it turns out that all terms explicitly involving the fixed charge distribution are zero.