Solid-solute phase equilibria in aqueous solution. XVI. Thermodynamic properties of malachite and azurite-predominance diagrams for the system Cu~(2+)-H_2O-CO_2

摘要

The thermodynamic properties of the copper carbonates malachite, Cu_2(OH)_2CO_3, and azurite, Cu_3(OH)_2(CO_3)_2, have been obtained from solubility measurements as a function of temperatures 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~(-1) NaClO_4. Second, solubility experiments have been carried out from I = 1.00 mol·kg~(-1) to I = 3.00 mol·kg~(-1) NaClO_4 at constant temperature. T = 298.15 K. All experimental data have been evaluated by application of the optimization routeine of CheMSage, yielding an internally consistent set of thermodynamic properties for malachite (T = 298.15 K): log ~*K_pso~O(mal.) = (6.34 (+-) 0.10), DELTA_fG_m~o{Cu_2(OH)_2CO_3} = (-0903.3 (+-) 1.2) kJ·mol~(-1), DELTA_fH_m~o{Cu_2(OH)_2CO_3} = (-1067.1 (+-) 3.4) kJ ·mol~(-1), S_m~o{Cu_2(OH)_2CO_3} = (166.3 (+-) 2.5) J·mol~(-1)·K~(-1); and azurite (T = 298.15 K): log ~*K_pso~o(azu.) = (6.30 (+-) 0.10), DELTA_fG_m~o{Cu_2(OH)_2(CO_3)_2} = (-1434.2 (+-) 1.8) kJ·mol~(-1), DELTA_fH_m~o{Cu_3(OH)_2(CO_3)_2} = (-1675.1 (+-) 5.1 ) kJ·mol~(-1), S_M~o{Cu_3(OH)_2(CO_3)_2} = (254.4 (+-) 3.8) J·mol~(-1)·K~(-1). Wheras 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 Cu~(2+) - H_2O -CO_2 are discussed by the construction of two-dimensional as well as three-dimensional predominance diagrams.