Solubility-product constant and thermodynamic properties for synthetic otavite, CdCO3(s), and aqueous association constants for the Cd(II)-CO2-H2O system

Considerable disparity exists in the published thermodynamic data for selected species in the Cd(II)-CO₂-H₂O system near 25°C and 1 atm pressure. Evaluation of published experimental and estimated data for aqueous cadmium-carbonate species suggests an association constant, pK, of −3.0 ± 0.4 for CdCO₃⁰, about −1.5 for CdHCO₃⁺, and −6.4 ± 0.1 for Cd(CO₃)₂²⁻ (T = 298.15 K; P = 1 atm; I = 0). Examination of all available data for cadmium-hydrolysis species and β-Cd(OH)_2(s)) confirms that the consistent set of constants presented by Baes and Mesmer (Hydrolysis of Cations, 1976) is the best available. The solubility of synthetic otavite, CdCO_3(s), has been measured in KClO₄ solutions where I ≤ 0.1 M. We calculated pK_sp = 12.1 ± 0.1 (T = 25.0°C; P = 1 atm; I = 0) from measured concentrations of Cd²⁺, measured P_C02 and pH, our selected set of equilibrium constants, and activity corrections estimated using the Davies equation. Values at 5 and 50°C were 12.4 ± 0.1 and 12.2 ± 0.1, respectively. Based on the new solubility data and the CODATA key values for Cd²⁺ and CO₃²⁻, a new set of thermodynamic properties is recommended for otavite: ΔG_f⁰ = −674.7 ± 0.6 kJ/mol; ΔH_f⁰ = −751.9 ± 10 kJ/mol; S⁰ = 106 ± 30 J/mol K; and ΔG_r⁰ for the reaction Cd²⁺ + CO₃²⁻ ⇌ CdCO₃(s) is −69.08 ± 0.57 kJ/mol.