SOIL SURFACE-CHARGE EVALUATION BY BACK-TITRATIONS .2. APPLICATION

Measurement of net surface charge by potentiometric titration and ion adsorption methods often gives different estimates of pH-dependent exchange capacity. One of the major problems has been that potentiometric titration measures the H+ or OH- consumption due to surface reactions as well as those associated with dissolution of the soil and hydrolysis of ions in solution. In general, potentiometric titration has overestimated pH-dependent exchange capacity, compared with ion adsorption methods. The backtitration procedure presented here corrects for these errors by performing two titrations. After slowly titrating the soil sample, suspended in 0.1 M Ca(NO3)(2), to pH 3, the suspension is either titrated with 0.005 M Ca(OH)(2) or centrifuged and the supernatant alone titrated. By subtracting the amount of OH- required to titrate the supernatant from the amount required to titrate the soil-solution suspension, the net OH- consumption is calculated. If a value for pH-independent CEC is known, then the pH-dependent surface charge curve can be adjusted to give results that correspond closely to those obtained by ion adsorption. The proposed method is simpler and more rapid than the ion adsorption procedures. The inflection points on the titration curve can also be correlated with the chemical composition of the soil. Preliminary results suggest that the pH-dependent CEC curve can be predicted from knowledge of organic matter content and various forms of amorphous Fe and Al.