9.The Operational Definition of the Zero Point of Charge in Soils

Soil Science Society of America Journal

Garrison Sposito

The balance of surface charge in a soil is developed from the perspective of coordination chemistry and is employed to derive the conditions under which the zero point of charge is equal either to the crossover point of two proton titration curves or to the point of zero net charge (point of equality between the cation and anion exchange capacities). It is shown that the crossover point will determine the zero point of charge if the net proton charge can be determined accurately and if a 1:1 background electrolyte comprising two ions of similar hydrated radius is used, either by itself or mixed with a fixed concentration of an ion that can be adsorbed specifically. If the background electrolyte comprises only a salt of an ion that can adsorb specifically, the crossover point will not be unique and will not determine the zero point of charge. The point of zero net charge will determine the zero point of charge if (and only if) the net surface charge density of outer-sphere complexes in the soil vanishes at the point of zero net charge.