Acid Character of Dioctahedral Vermiculite: Permanent and pH- Dependent Charge Components of Cation Exchange Capacity

Permanent and pH-dependent charge components of cation exchange capacity (CEC) of dioctahedral vermiculite were determined by titrating the native as well as acid-clay prepared by the action of H-resin on vermiculite. The base-titratable acidity of the acid-clay, freshly prepared as well as aged, was found to be greater than the acidity extractable with IN NaCl; the difference was attributed to the non-exchangeable pH-dependent component of the CEC. In the freshly prepared as well as aged samples, H+ and/or Al3+ ions were the only exchangeable cations. Basic-Al ions which like H+ and Al3+ ions balanced the isomorphous charge and whose amount increased at the cost of H+ and Al3+ ions as ageing proceeded were, however, non exchangeable; thereby indicating that the acidity displaced by unbuffered neutral salt leaching can not in general be equated with the permanent isomorphous charge. Basic-Al ions as well as octahedral-Al and Sl-O-Al units exposed on the lateral surfaces of the clay crystals due to lattice terminations have been held responsible for the pH-dependent component of the CEC of vermiculite.