Hydrolysis constants and ion-interaction parameters for Cd(II) in zero to high concentrations of NaOH−KOH, and the solubility product of crystalline Cd(OH)2

The solubility of Cd(OH)2(c) was studied in 0.01M NaClO4 solutions, from both the over- and the undersaturation directions, with OH− ion concentration ranging from 10−6 to 1.0 mol-L−1, and the equilibration period ranging from 2 to 28 days. Equilibrium Cd concentrations were reached in less than 2 days. The Cd(OH)2(c) solubility showed an amphoteric behavior. In the entire range of OH−/H+ investigated, the only dominant aqueous Cd(II) species required to explain the solubility of Cd(OH)2(c) are Cd2+, Cd(OH) 2 0 , and Cd(OH) 4 2− . The logarithms of the thermodynamic equilibrium constants of the Cd(OH)2(c) solubility reactions involving these species, that is, the reactions $$\begin{gathered} {\text{ }}Cd(OH)_2 (c) \rightleftarrows Cd^{2 + } + 20H^ - ,{\text{ }}Cd(OH)_2 (c) \rightleftarrows Cd(OH)_2^0 , \hfill \\ and Cd(OH)_2 (c) + 20H^ - \rightleftarrows Cd(OH)_4^{2 - } \hfill \\ \end{gathered}$$ were found to be −14.14±0.21, −7.04±0.21, and −5.62±0.32, respectively. The ion-interaction parameters reported in the literature, in conjunction with the values for Cd(OH) 2 0 −Na+(−0.20), Cd(OH) 4 2− −Na+ (β0 = 0.41, β1 = 0.7), and Cd(OH) 4 2− −K+ (β0 = 0.44, β1 = 1.44) obtained in this study, show that our low-ionic strength solubility data are also consistent with Cd(OH)2(c) solubility data obtained in solutions as concentrated as 10M in NaOH or KOH and 7M in Na(OH, ClO4).