THERMODYNAMICS OF K–Ca ION EXCHANGE IN SOILS1

Abstract

SummaryCation exchange characteristics of the K:Ca saturated forms of five soils were measured at 25°C and 50°C. The rates of isotopic exchange of 42K and 45Ca were too fast to be measured except that of 42K in the K:Ca Harwell soil at 25°C. The slower isotopic exchange of K in this soil was attributed to the presence of a zeolite, clinoptilolite. The intra‐particle diffusion coefficient, Di, of K in this soil increased with K‐saturation to a maximum at about 40 per cent K, probably because of the ‘blocking’ action of the larger hydrated Ca ions at small K‐saturations in clinoptilolite.The CEC, measured by isotopic exchange along the K:Ca adsorption isotherm, decreased with increasing temperature probably because some interlayer spaces collapsed.The standard free energy, enthalpy, and entropy changes were negative for the reaction Ca‐soil+2K+⇌ 2K‐soil+Ca++. These results seem to show that K is more strongly bound than Ca by the soil and that the Ca‐preference shown by the isotherm at small external electrolyte concentration is caused by entropy changes in solution.Calculated activity coefficients of the exchangeable ions changed with K‐saturation similarly at both temperatures but values at 50°C were smaller than at 25°C.