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The Significance of Potentiometric Measurements Involving Liquid Junction in Clay and Soil Suspensions¹

ABSTRACT

A critical study has been made of the behavior of the liquid junction between the saturated KCl and the clay or soil suspension in attempt to evaluate the sign and the magnitude of the junction potential in measurements of membrane potentials and ion activities. In deflocculated suspensions of clay saturated with monovalent cations, the saturated KCl in the salt bridge short-circuits the potential drop across the diffuse double layer and thus exaggerates the magnitude of the membrane potential or the ion activity of the clay suspension predicted by the Gouy theory. In suspensions of Ca-clay and most soils, however, the effect of short-circuiting the small potential drop across the diffuse double layer is more than offset by the diffusion potential due to CaCl₂ that is formed ahead of the KCl diffusion front. The sign of this diffusion potential is such as to decrease the magnitude of the membrane potential or the ion activity of the clay suspension. Comparison of the observed osmotic pressure of clay suspensions with that calculated according to the Gouy and the Donnan theories fully confirmed the conclusions reached from the potentiometric data concerning the sign and magnitude of the liquid junction potential.

¹ Contribution from the Department of Agronomy, Cornell University, Ithaca, N. Y. The authors wish to express their appreciation to R. K. Schofield of the Rothamsted Experimental Station for his invaluable suggestions offered in connection with this problem. Presented in symposium before Division II, Soil Science Society of America, Cincinnati, Ohio, Nov. 19, 1952.

² Professor of soils, and graduate research assistants, respectively.

Received for publication November 28, 1952.