The solubility of potassium from soil illites. IV. Rates of reaction and exchange constants

Abstract

Potassium-sensitive cationic electrodes have been used to study the rates of release of potassium from five illitic soil clays. This release was complete within a few minutes for clay-size particles and was probably controlled by diffusion. The electrode response time of 15 sec to 0.001 pK unit was too slow to permit accurate measurement of the rate of release, as the process was more than 90% complete in this time. As expected, the chemical reaction was too fast to be measured independently of diffusion. The electrodes were also used to calculate stepwise changes in Gapon constants as potassium (or sodium) was adsorbed by the clay in competition with calcium ions. The Gapon constants for potassium decreased greatly as potassium was adsorbed, but did not reach the low values expected for external exchange sites until comparatively large amounts of potassium were taken up. Potassium ions were held much more tightly thm sodium ions until the external sites began to be filled. The continuous change of Gapon constant with potassium content precluded a sharp distinction between interlayer and external exchangeable potassium. Values of the constants corresponding to the equilibrium solubilities measured in earlier work showed that the clays had a strong preference for potassium over calcium at the potassium contents in these clay preparations, and this showed that the solubility equilibria refer to potassium in the interlayer regions of the clay minerals.