CRITICAL-EVALUATION OF THE EXTENSION OF ZERO-POINT OF CHARGE (ZPC) THEORY TO SOIL SYSTEMS

The points of zero salt effect (PZSE) of surface and subsurface horizons of soils ranging from an Hapludoll to a Paleudult were obtained from batch titrations in CaCl2 solutions with and without prior Ca saturation and in NaCl solutions following Na saturation. PZSE of Na-saturated samples equilibrated for 1 h were generally between pH 2 and 3, while those of Na- or Ca-saturated samples or unwashed samples equilibrated for 2 days were generally between 3 and 4. The effect of equilibration time was attributed to gradual sample decomposition accompanied by conversion of Na-saturated soil to an Al-Na-system. Points of zero net charge (PZNC) were evaluated by measurement of Ca and Cl adsorption at various pH levels. No net charge reversal from negative to positive occurred above pH 2.0. This discrepancy between PZSE and PZNC was attributed to specific adsorption of cations, H+ exchange with counterions associated with permanent charge sites and other reactions of H+ or OH- which did not result in a change in surface charge. From 30-90% of the H+ or OH- lost from solution during titration did not create an equivalent change in surface charge. Exchange reactions and reaction of H+ with Al and Fe in various forms are implicated in this regard. The use of potentiometric titrations for the estimation of electrochemical properties of soils should be regarded with considerable caution.