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Standard Heats and Entropies of Exchange for Alkali Metal Cations on Wyoming Bentonite¹

ABSTRACT

Standard heats of exchange, H°, were determined for alkali metal cation exchange on Wyoming bentonite using a Calvet microcalorimeter. In addition, values of H° for Na-Cs, Na-Rb, and Cs-Rb exchange were evaluated from selectivity measurements conducted at 25 and 50C. Standard entropies of exchange, S°, were calculated by combining the standard heats with free energy values, G°, estimated from equilibrium selectivity coefficient determinations using the relationship G° = H° – TS°. Values of H° varied from +150 cal mole^-1 for NaLi exchange to –2,557 cal mole^-1 for NaCs exchange. Values of G° ranged from +48 to –1,081 cal mole^-1 for these reactions giving values of S° ranging from +0.5 to –4.9 cal mole^-1 deg.^-1. Both H° and S° were always negative for uptake of the preferred cation with the absolute values increasing with the degree of selectivity. While the entropy changes did contribute significantly to the standard free energies of exchange, they were only about 10% of the hydration entropy differences for Na-Li and Na-K exchange and 26 and 41% for Na-Rb and Na-Cs exchange, respectively.

¹ This manuscript is published with the permission of the Dean of the University of Tennessee Agr. Exp. Sta., Knoxville. Received March 20, 1969. Approved May 16, 1969. The Agricultural Research Laboratory of the University of Tennessee at Oak Ridge, Tenn., is operated by the Tennessee Agr. Exp. Sta. for the US Atomic Energy Commission under Contract no. AT-40-1-GEN-242.

² Agricultural Research Lab. of the University of Tennessee, Oak Ridge, Tenn.

³ Institut Agronomique, University of Louvain, Louvain, Belgium.

⁴ Professor of Chemistry, Fisk University, Nashville, Tenn.