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

摘要

The thermodynamic properties of Zn-5(OH)(6)(CO3)(2), hydrozincite, have been determined by performing solubility and d.s.c. measurements. The solubility constant in aqueous NaClO4 media 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(-1) NaClO4 at T = 298.15 K. The standard molar heat capacity C-p,m(o) function from T = 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*K-ps0(0) = (9.0 +/- 0.1), standard molar Gibbs energy of formation Delta (f)G(m)(0){Zn-5(OH)(6)(CO3)(2)} = (-3164.6 +/- 3.0) kJ (.) mol(-1), standard molar enthalpy of formation Delta H-f(m)0{Zn-5(OH)(6)(CO3)(2)} = (-3584 +/- 15) kJ (.) mol(-1), standard molar entropy S-m(0){Zn-5(OH)(6)(CO3)(2)} = (436 +/- 50) J (.) mol(-1) (.) K-1 and C-p,m(o)/(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)(Zn)} of the solid compounds ZnCO3 and Zn-5(OH)(6)(CO3)(2) can be checked by thermodynamically analysing the measured solubility data. (C) 2001 Academic Press. [References: 33]