Cation exchange capacity methodology III: Correct exchangeable calcium determination of calcareous clays using a new silver–thiourea method

Cation exchange capacity (CEC) is a fundamental property of clays and soils. Determination of CEC for samples that contain Ca-carbonate minerals, such as calcite and dolomite, is problematic because Ca2+ released by dissolution of carbonates during CEC determinations interferes with the accuracy of CEC values. This paper describes a new method for the determination of correct exchangeable Ca2+ values of calcareous clays and soils. For the method, a silver–thiourea exchange solution is saturated with Ca2+ by treatment with fine-grained calcite prior to the start of the exchange procedure. Using this exchange solution, calcite in the sample can no longer be dissolved but exchangeable Ca2+ is desorbed quantitatively. The case for dolomite is similar, because dissolution of dolomite is minimised. However, the determination of reasonable exchangeable Ca2+ values for samples containing gypsum is not possible because gypsum is soluble in the exchange solution.