Solid-solute phase equilibria in aqueous solution. XII. Solubility and thermal decomposition of smithsonite

The solubility constant of ZnCO3, smithsonite, in aqueous NaClO4 solutions has been investigated as a function of temperature (288.15 less than or equ al to T/K less than or equal to 338.15) at constant ionic strength l = 1.00 mol-kg(-1). In addition, the solubility of zinc carbonate has been determi ned at 2.00 and 3.00 mol-kg(-1) NaClO4 (298.15 K). The solubility measureme nts have been evaluated by applying the Davies approximation, the specific ion-interaction theory, and the Pitzer model, respectively. The thermodynam ic interpretation leads to an internally consistent set of thermodynamic da ta for ZnCO3 (298.15 K): solubility constant log *K-ps0(0) = 7.25+/-0.10, s tandard Gibbs energy of formation Delta(f)G(circle minus) (ZnCO3) = (-737.3 +/-0.6) kJ-mol(-1), standard enthalpy of formation Delta(f)H(circle minus)( ZnCO3) = (- 820.3+/-3.0) kJ-mol(-1) and standard entropy S-circle minus(ZnC O3) = (77+/-10)J-mol(-1)-K-1. Furthermore, the DSC curve for the thermal de carbonation of zinc carbonate has been recorded in order to obtain the enth alpy of formation Delta(f)H(circle minus)(ZnCO3) = (-820.2+/-2.0) from the heat of decomposition. Finally, our results are also consistent within the experimental error limits with a recent determination of the standard entro py of smithsonite, leading to a recommended set of thermodynamic properties of ZnCO3: Delta(f)G(circle minus)(ZnCO3) = (-737.3+/-0.6) kJ-mol(-1) Delta(f)H circle minus(ZnCO3) = (-818.9+/-0.6) kJ-mol(-1) S-circle minus(ZnCO3) = (81.2+/-0.2) J-mol(-1)-K-1.