SOIL SURFACE-CHARGE EVALUATION BY BACK-TITRATION .1. THEORY AND METHOD DEVELOPMENT

Until recently, surface charge chemistry of soils has been measured using two different approaches: ion adsorption and potentiometric titration. The former provides a good measure of both anion-exchange capacity (AEC) and cation-exchange capacity (CEC) as a function of pH, although the procedure is time consuming. Potentiometric titration is more rapid, but the resulting curve overestimates the change in surface charge with pH. The error is largely due to consumption of H+ and OH- by dissolution and hydrolysis reactions with dissolved ions instead of being limited to protonation-deprotonation reactions of surface charge sites. Our objective was to develop the theoretical aspects of the determination of surface charge using a back-titration method. After suspension in Ca(NO3)(2), a subsample of the soil is titrated slowly to pH 3, and then back-titrated to pH 8 or 10. A second subsample is similarly titrated to pH 3; the soil and the supernatant solution are separated by centrifugation and filtering; only the supernatant solution is back-titrated. The difference between the two titration curves represents the consumption of OH- by surface reactions corrected for dissolution and other solution reactions. The proposed method provides a relatively rapid means of obtaining pH-dependent surface charge using potentiometric titration while giving results comparable to those obtained by ion adsorption.