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Thermodynamics of Potassium-Calcium and Magnesium-Calcium Exchange Reactions on a Kaolinitic Soil Clay¹

ABSTRACT

Thermodynamic parameters derived from experimental results were determined for the potassium-calcium and magnesium-calcium exchange reactions on a kaolinitic soil clay separated from the B horizon of a strongly weathered soil profile. The clay samples were equilibrated at 10° and 30°C with a mixed solution of KCl and CaCl₂ for the K-Ca system or MgCl₂ and CaCl₂ for the Mg-Ca system. For the K-Ca system, the selectivity coefficients were high indicating a greater affinity of the clay for K than for Ca. The high equilibrium constant and the negative values of standard free energy and enthalpy changes for the exchange of Ca for K also revealed a preference of the clay for K relative to Ca ions.

In the Mg-Ca system, the exchange isotherms and the low selectivity coefficients indicated a preference of Ca ions for the clay phase. This exchange of Ca for Mg was accompanied by an increase in the free energy and enthalpy changes also pointing to a more stable Ca-clay than Mg-clay.

¹ Contribution from the Dept. of Agronomy, Univ. of Ibadan, Nigeria.

² Senior Lecturer in Soil Chemistry.

Received for publication August 5, 1976. Accepted for publication March 8, 1978.