| Abstract: | Cation 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. |
