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Ion Activities in Sodium-Clay Suspensions¹

ABSTRACT

A study has been made of dissociation and hydrolysis of Na-clay as a function of the degree of Na⁺ saturation of the clay and the concentration of NaCl in the clay suspension. The activities of Na⁺, H⁺, HCO₃^-, and Cl^- in the clay suspension were measured by the Donnan equilibrium method. From the Donnan distribution of ions between the two phases, it has been possible to apportion the observed Na⁺ activity in the clay suspension to Na⁺ dissociated from the clay and to diffusible sodium salts present in the clay suspension. The activity of Na⁺ dissociated from the clay and the activity of Na⁺ due to the presence of diffusible sodium salts in the clay suspension were both found to increase with the increasing degree of Na⁺ saturation of the clay and increasing concentration of NaCl or Na-clay in the clay suspension. The anomalous increase in the activity of Na⁺ dissociated from the clay with the increasing concentration of NaCl or Na-clay in the suspension may be attributed to the decrease in the thickness of the ionic atmosphere around the clay particle, resulting in greater concentration of osmotically active Na⁺ ions in the outer part of the diffuse double layer.

¹ Contribution from the Department of Agronomy, New York Agricultural Experiment Station at Cornell University, Ithaca, N. Y. Presented before Sections I and II, Soil Science Society of America, State College, Pa., August 28, 1951.

² Associate Professor of Soils, Colorado A. & M. College, Fort Collins, Colo. (on leave), and Professor of Soils, Cornell University, respectively.