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)-CO 2 -H 2 O 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 CdCO 3 0 , about −1.5 for CdHCO 3 + , and −6.4 ± 0.1 for Cd(CO 3 ) 2 2− ( 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 KClO 4 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 Cd 2+ , measured P C 02 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 Cd 2+ and CO 3 2− , a new set of thermodynamic properties is recommended for otavite: ΔG f 0 = −674.7 ± 0.6 kJ / mol ; ΔH f 0 = −751.9 ± 10 kJ / mol ; S 0 = 106 ± 30 J/mol K; and ΔG r 0 for the reaction Cd 2+ + CO 3 2− ⇌ CdCO 3 ( s ) is −69.08 ± 0.57 kJ / mol.