Potentiometric automatic titration (PAT) method to evaluate zero point of charge (ZPC) of variable charge soils

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Abstract
A potentiometric titration technique using an automatic titrator (PAT method) was evaluated for the determination of the ZPC of two different kinds of variable charge soils. The titration mode of PAT was proposed as follows: pre-equilibration for 2 min with continuous stirring, soil: solution ratio of 1 : 10, and cut off time (interval of titrant addition to the system) of 12 s for the 0.01 n system and 120 s for the 0.1 n system for volcanic ash soils and cut off time of 24 s for the 0.01 n system and 240 s for the 0.1 n system for strongly weathered soils. Here, 0.1 or 0.01 n system indicates that the concentration of both PDI and the salt solution is set at 0.1 or 0.01 n. ZPC value obtained by the PAT method showed a good agreement with the ZPC value obtained by the ordinary potentiometric titration method. Furthermore, HADS(%) (H+ adsorption percent) was defined as the ratio of H+ adsorption in the PAT method to that in the PT method, measured in the 0.1 n system, at a pH lower than the initial pH by a 0.75 pH unit. HADS(%) was found to be a good index of the reactivity of soil or clay surfaces with the potential-determining ions. All the volcanic ash soils examined exhibited a high surface reactivity, with HADS(%) being more than 70, whereas the strongly weathered Thai soils had a low in surface reactivity, with HADS(%) being less than 40.

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