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

The thermodynamic properties of Zn-5(OH)(6)(CO3)(2), hydrozincite, have bee n determined by performing solubility and d.s.c. measurements. The solubili ty constant in aqueous NaClO4 media has been measured at temperatures rangi ng from 288.15 K to 338.15 K at constant ionic strength (I = 1.00 mol (.) k g(-1)). Additionally, the dependence of the solubility constant on the ioni c 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 hav e been simultaneously evaluated by means of the optimization routine of Che mSage yielding an internally consistent set of thermodynamic data (T = 298. 15 K): solubility constant log*K-ps0(0) = (9.0 +/- 0.1), standard molar Gib bs 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: tempera ture, partial pressure of carbon dioxide and pH of the aqueous solution. Mo reover, it is shown how the stoichiometric composition {n(CO3)/n(Zn)} of th e solid compounds ZnCO3 and Zn-5(OH)(6)(CO3)(2) can be checked by thermodyn amically analysing the measured solubility data. (C) 2001 Academic Press.