Electrical Conductivity and the Surface Characteristics of Kaolinitic Clays and Clay-Humic Acid Complexes

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Abstract
The conductivites of the Na and H ion-exchanged forms of kaolinitic clay rejects from sand-wash- ing operations, both purified and as found naturally as a complex with soil organic matter, were examined. The two Na-clays showed linear conductivity-concentration characteristics, each having two regions with different slopes intersecting at -3.3% by weight of clay, which probably reflect a structural change from sol to gel. In the gel region, only the Na-counterions conducted, the reduction in conductivity with concen- tration being due to a smaller proportion of these ions in the Gouy layer and/or a decrease in their mobilities. In the sol, an additional, concentration-dependent conductivity arose from the electrophoretic motion of clusters of clay particles which gradually broke down on dilution. The two acid clays showed curved con- ductivity-concentration characteristics consistent with a weak acid dissociation equilibrium; the PKa values of 6.37 to 6.56 are close to those determined independently from titration with alkali. A stronger acid species detected in the titrations was not seen in the conductivity. The MOH~+/MOH/MO - model of the clay- surface species and one set of the predicted concentrations of each species (see preceding paper) are con- sistent with the observed conductivities. The MOH2 + species probably bridged the edges and faces of clay particles so that the H atoms were identical, but became different when alkali was added.

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