Determination of Cation Exchange Properties of Calcareous Soil and Bentonite Using a 50% Ethanol [Co(NH3)6]Cl3 Solution

Abstract

Accurate determination of cation exchange capacity (CEC) and exchangeable (exch.) cations in calcareous soils and clays has long been a challenge in the field. The hexamminecobalt trichloride ([Co(NH₃)₆]Cl₃) method is currently the most convenient approach for the simultaneous measurement of CEC and exch. cations in a single extraction process. Dissolution of CaCO₃ during the extraction process was inhibited by preparing a calcite-saturated [Co(NH₃)₆]Cl₃ solution. Preparation of this solution required 30 min of ultrasonic treatment, 30 min of stirring, and standing overnight. This procedure significantly reduced testing efficiency and required improvement. This study adopted effective measures to address this issue. Four standard calcareous soils and one standard calcareous bentonite were treated with varying amounts of quartz sand and CaCO₃, and a 50% ethanol [Co(NH₃)₆]Cl₃ solution was chosen as the extraction agent to inhibit CaCO₃ dissolution. With magnetic stirring, the entire extraction process was completed within 5 min. This eliminated the need for 1 h of shaking and for preparing the calcite-saturated [Co(NH₃)₆]Cl₃ solution, greatly improving efficiency. The results showed that the CEC and exch. cation values for the four soils obtained using this method were all within the standard ranges. The determination of exch. cations in the bentonite also fell within the range of the standard values. Ethanol concentration was a major factor inhibiting the dissolution of CaCO₃.