USE OF MOLE OR EQUIVALENT FRACTIONS IN DETERMINING THERMODYNAMIC PARAMETERS FOR POTASSIUM EXCHANGE IN SOILS1

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Abstract
We investigated K-Ca exchange equilibria in two soils at three different temperatures. Thermodynamic parameters using the Argersinger and Gaines and Thomas conventions were compared. With both conventions, the selectivity coefficients (kv or kc) decreased and varied with increasing fractional potassium saturation (FK). This is indicative of nonideality of K-Ca exchange and the presence of heterogeneous sites for exchange. The difference between adsorbed-ion activity coefficients obtained by the two conventions was small at lower FK, but increased at higher FK. Inflexions, maxima, and minimum values for the activity coefficients occurred at virtually the same FK for both soils. Even though the selectivity and adsorbed-ion activity coefficients using the two conventions did not compare in magnitude, the curves were similar and showed an analogous trend. When these coefficients were reduced to the common thermodynamic parameters, such as the equilibrium constant (Keq) and the standard free energy (ΔG°) of exchange, the two conventions not only resulted in similar inferences of ion behavior, but also in the same magnitude of values at a given temperature.
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