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Ionic Equilibria between Clay and Dilute Salt Solutions¹

ABSTRACT

Theoretical considerations are presented for the evaluation of the free energy change that accompanies an exchange of cations between a clay and a solution. Also, a means is proposed for characterizing the effective concentration of the ionic atmosphere of a substance that exhibits cationic exchange. Results accompanying an exchange of potassium for calcium in Putnam clay suggested a change of about –3,000 calories per chemical equivalent when the cations were present in the ratios characteristic of productive soils. The ionic atmosphere of the Putnam clay functioned as a salt solution whose hypothetical concentration was 0.039 normal when evaluated in terms of cationic exchange between potassium and calcium. Data for synthetic resins presented by other investigators were interpreted in the light of the results for this investigation. In order to serve as nutrient media for plants, such resins would require very small percentages of potassium to establish the same free energy relationships between potassium and calcium that were found for productive Putnam soils.

¹ Contribution from the Department of Soils, Missouri Agr. Exp. Sta., Columbia, Mo. Journal Series 1397. Presented before Div. II, Soil Science Society of America, Dallas, Tex., Nov. 18, 1953.

² Associate Professor in Soils.

Received for publication May 6, 1954.