SOLID-SOLUTE PHASE-EQUILIBRIA IN AQUEOUS-SOLUTION .5. THE SYSTEM CDCO3-CACO3-CO2-H2O

The system CdCO3-CaCO3-CO2-H2O was investigated potentiometrically at 25-degrees-C and 1 atm total pressure. Cd(1-x)CaxCO3 solid solutions were prepared over the entire compositional range. Aqueous solutions were kept at constant ionic strength I = 3 mol (Na)ClO4/kg H2O. Experimental results were depicted in Lippmann (x - x(aq) - log SIGMA *K(ps0)) diagrams. The solutus was investigated by pH and Cd2+ sensitive electrodes and it turned out that the solid solutions neither dissolved congruently nor attained stable equilibrium with aqueous solutions within one week of observation. Consequently, the excess Gibbs energy of mixing of otavite and calcite was determined by a method of successive approximations. Aqueous solutions were presaturated to the supposed equilibrium composition prior to equilibration with solid solutions, and constant pH values, corresponding to constant log SIGMA *K(ps0) values, indicated stable equilibrium. A recently proposed technique of sequential Bayesian excess parameter estimation was employed for data evaluation which led to the conclusion that the otavite-calcite solid solution series has ideal mixing properties.