ION EXCHANGE IN SOILS FROM THE ION PAIRS K–Ca, K–Rb, AND K–Na1

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Abstract
Summary: Isotopic exchange of 43K, 24Na, 86Rb, and 45Ca was used to measure the equilibria between some arable soils and mixed chloride solutions of the ion pairs, K–Ca, K–Rb, and K–Na; the results were interpreted by a thermodynamic treatment of the exchange isotherm (Gaines and Thomas, 1953). The cation exchange capacities of the soils were not constant for the three ion pairs and decreased appreciably with K‐saturation in the K–Ca systems because Ca ions were trapped inside the interlayer spaces. That soils preferred K to Na in the K–Na systems and Rb to K in the K–Rb systems, was explained in terms of the standard free‐energy changes of the exchange reactions.The activity coefficients of the adsorbed ions, f, changed with K‐saturation differently for the three ion pairs: fNa and fCa decreased continuously with increasing K‐saturation whereas fRb remained almost constant. However, with increasing K‐saturation, fK increased in the K–Na systems, remained unchanged in the K–Rb systems, and first increased and then decreased in the K–Ca systems. These changes are interpreted in terms of the effect of the various ions on the interlayer space of 2 : 1 type clay minerals and the possible distribution of adsorbed ions between the Gouy and Stern layers.

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