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Acidic Properties of Kaolinite in Water and Acetonitrile¹

ABSTRACT

Potentiometric titration of weakly acidic sites on soil clays is limited in aqueous media by the acidic property of water which prevents sharp endpoints. Generally, it is not possible to obtain sharp endpoints in aqueous systems for acids with pK_a > 8. In this study, kaolinite with K, H, or Al as the saturating cation was successfully titrated in water and acetonitrile using the glass-calomel electrode system for potentiometric determination. Sharper endpoints and wider working potential ranges were obtained in acetonitrile than in the aqueous system. Titer of base needed to reach the final endpoint was greater in acetonitrile than in water; for example, K-saturated clay required a negligible titer in water (0.4 meq/100 g clay) in contrast to acetonitrile, in which a significant titer (7.4 meq/100 g clay) was required. The larger titer in acetonitrile was attributed to (i) pH-dependent sites for which a quantitative endpoint was not obtained in water due to the acid-base properties of this solvent, and (ii) an increased surface acidity in acetonitrile. A comparable difference in total titers at the final endpoint was observed for the titration of H- of Al-saturated kaolinite in water and acetonitrile.

¹ Contribution from the Soil Sci. Dep. of the Florida Agric. Exp. Stn., Gainesville, as Journal Ser. no. 6123.

² Graduate Assistant, presently Assistant Research Scientist, Univ. of Florida, Gainesville, FL 32611, Associate Professor of Agronomy, Virginia Polytechnic Institute and State Univ., Blacksburg, 24061, and Associate Professor of Soil Sci., Univ. of Florida, Gainesville, 32611, respectively.