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Chloride and Perchlorate Influence on Calcium-Potassium and Magnesium-Potassium Exchange

Dep. of Plants, Soils, and Biometeorology, Utah State Univ., Logan, UT 84322-4820

*Corresponding author.

ABSTRACT

There is some uncertainty regarding the effect of the background electrolyte on Ca-Na and Mg-Na exchange. We conducted a study to see if any of the effects attributed to the background electrolyte in heterovalent exchange with Na could be observed when K was the monovalent cation. The influence of the competing cations Ca²⁺ and Mg²⁺ on K⁺ exchange was evaluated by examining quantity/intensity parameters (Q/I), Gapon-convention selectivity coefficients, and adsorbed metal charge with Cl and ClO₄ as the background electrolyte. Exchange reactions were unaffected by the background electrolyte for four of the five soils studied. In those soils where exchange was unaffected by the background electrolyte, there was a significant difference between Ca-K and Mg-K Q/I curves and Gapon selectivity coefficients; Ca was preferred over Mg relative to K. The Q/I curves from Ca-K and Mg-K systems differed in the Cl media, but not in the ClO₄ media. Further, the Ca-K curve was affected by the media. Calcium-K and Mg-K selectivity coefficients differed in both Cl and ClO₄ media. The Ca-K selectivity coefficients determined in ClO₄ media were closer to Mg-K selectivity coefficients than those determined in Cl media. The adsorbed metal charge did not increase when the exchanger composition was changed from Mg-K to Ca-K in Cl media. The effect of the background electrolyte on Ca-K exchange was attributed to competition between K⁺ and CaCl⁺ for external exchange sites.

Received for publication December 18, 1990.

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