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Alkali Metal Cation Exchange on Chambers Montmorillonite¹

ABSTRACT

Standard free energies of exchange, G°, were determined for the exchange of Na with Li, K, Rb, and Cs on a 0.1–0.5 micron fraction of Chambers montmorillonite having an exchange capacity of 123 ± 2 meq/100 g. The G° values ranged from +80 cal mole^-1 for NaLi exchange to -1886 cal mole^-1 for NaCs exchange and were approximately twice as large as those found previously for similar exchange reactions on Wyoming bentonite having an exchange capacity of 92 ± 2 meq/100 g. The G° values reflect cation selectivity differences of a factor of two to three between the two clays. In contrast, the standard heats of exchange, determined calorimetrically, were approximately the same for both clays and ranged from + 125 cal mole^-1 for NaLi exchange to -2575 cal mole^-1 for NaCs exchange. These results indicate that the differences in free energies of exchange between the two clays are due to entropy effects.

¹ This manuscript is published with the permission of the Dean of the University of Tennessee Agr. Exp. Sta., Knoxville, The research was completed at the Agr. Res. Lab. of the Univ. of Tennessee, Oak Ridge, which is operated by the Tennessee Agr. Exp. Sta. for the US Atomic Energy Commission under Contract no. AT-40-1-GEN-242.

² Present address: Dep. of Soil Science, University of Minnesota, St. Paul, Minn. 55101.

Received for publication January 7, 1971. Accepted for publication September 1, 1971.