(Solid + solute) phase equilibria in aqueous solution. XIII. Thermodynamic properties of hydrozincite and predominance diagrams for (Zn2 + + H2O + CO2)

Abstract

The thermodynamic properties ofZn5(OH)6(CO3)2 , hydrozincite, have been determined by performing solubility and d.s.c. measurements. The solubility constant in aqueous NaClO4media has been measured at temperatures ranging from 288.15 K to 338.15 K at constant ionic strength (I= 1.00 mol · kg−1). Additionally, the dependence of the solubility constant on the ionic strength has been investigated up to I= 3.00 mol · kg−1NaClO4at T= 298.15 K. The standard molar heat capacity Cp, mofunction fromT= 318.15 K to T= 418.15 K, as well as the heat of decomposition of hydrozincite, have been obtained from d.s.c. measurements. All experimental results have been simultaneously evaluated by means of the optimization routine of ChemSage yielding an internally consistent set of thermodynamic data (T= 298.15 K): solubility constant log*Kps 00= (9.0 ± 0.1), standard molar Gibbs energy of formationΔfGmo {Zn5(OH)6(CO3)2 } = (−3164.6 ± 3.0)kJ · mol−1, standard molar enthalpy of formation ΔfHmo{Zn5(OH)6(CO3)2 } = (−3584 ± 15)kJ · mol−1, standard molar entropy Smo{Zn5(OH)6(CO3)2 } = (436 ± 50)J · mol−1· K−1and Cp,mo/(J · mol−1· K−1) = (119 ± 11) + (0.834 ± 0.033)T/K. A three-dimensional predominance diagram is introduced which allows a comprehensive thermodynamic interpretation of phase relations in(Zn2++ H2O + CO2) . The axes of this phase diagram correspond to the potential quantities: temperature, partial pressure of carbon dioxide and pH of the aqueous solution. Moreover, it is shown how the stoichiometric composition{n(CO3)/n(Zn)} of the solid compoundsZnCO3 and Zn5(OH)6(CO3)2can be checked by thermodynamically analysing the measured solubility data.