Adsorbed ion activities and other thermodynamic parameters of ion-exchange defined by mole or equivalent fractions

The ‘Gaines and Thomas’ method for calculating the thermodynamic parameters of a cation exchange process has been criticized because equivalent rather than mole fractions were used in the definition of adsorbed ion activity coefficients. Although such coefficients are thus not theoretically correct, comparison with directly measured enthalpies of exchange shows that they indicate heterogeneity in the exchange process correctly. They also differ little from coefficients defined in terms of mole fractions. Such a re‐definition alters values of the thermodynamic equilibrium constant, and free energy and entropy of exchange by a constant factor, independent of cations and exchanger. Comparisons of series of cations, soils or clays using these parameters are therefore not affected.

RESP-08615