Solid-solute phase equilibria in aqueous solution.: XVI.: Thermodynamic properties of malachite and azurite-predominance diagrams for the system Cu2+-H2O-CO2

The thermodynamic properties of the copper carbonates malachite, Cu-2(OH)(2)CO3, and azurite, Cu-3(OH)(2)(CO3)(2), have been obtained from solubility measurements as a function of temperature as well as ionic strength of aqueous perchlorate media. First, solubility constants have been determined from T = 288.15 K to T = 338.15 K at constant ionic strength, I = 1.00 mol (.) kg(-l) NaClO4. Second, solubility experiments have been carried out from I = 1.00 mol (.) kg(-1) to I = 3.00 mol (.) kg(-1) NaClO4 at constant temperature, T = 298.15 K. All experimental data have been evaluated by application of the optimization routine of ChemSage, yielding an internally consistent set of thermodynamic properties for malachite (T = 298.15 K): log*K-ps0(0)(mal.) = (6.34 +/- 0.10), DeltafG(m)(o){Cu-2(OH)(2)CO3} = (-903.3 +/- 1.2) kJ (.) mol(-1) DeltafH(m)(o){Cu-2(OH)(2)CO3} in (-1067.1 +/- 3.4) kJ (.) mol(-1), S-m(o) {Cu-2(OH)(2)CO3} = (166.3 +/- 2.5) J (.) mol(-1) (.) K-1; and azurite (T = 298.15 K): log *K-ps0(0)(azu.) = (6.30 +/- 0.10), DeltafG(m)(o) {Cu-3(OH)(2)(CO3)(2)} (-1434.2 +/- 1.8) kJ (.) mol(-1), DeltafH(m)(o) {Cu-3(OH)(2)(CO3)(2)} = (-1675.1 +/- 5.1) kJ (.) mol(-1), S-m(o){Cu-3(OH)(2)(CO3)(2)} = (254.4 +/- 3.8) J (.) mol(-1) (.) K-1. Whereas these results confirm the literature data for the standard molar entropies, new values for the standard molar enthalpy of formation of malachite and azurite are recommended. Moreover, the phase relations in the ternary system Cu2+-H2O-CO2 are discussed by the construction of twodimensional as well as three-dimensional predominance diagrams. (C) 2002 Elsevier Science Ltd. All rights reserved.