THE SOLUBILITY PRODUCT OF CRYSTALLINE FERRIC SELENITE HEXAHYDRATE AND THE COMPLEXATION CONSTANT OF FESEO3+

The aqueous solubility of Fe-2(SeO3)(3) . 6H(2)O(c) was studied in deionized water adjusted to a range in pH values from 0.77 to 5.1 and in Na2SeO3 solutions ranging in concentrations from 0.0002 To 0.02 mol-dm(-3). The studies were conducted from both the undersaturation and oversaturation directions, with equilibration periods ranging from 7 to 1725 days. Stoichiometric dissolution of the solid war observed in solutions with pH values up to nearly 4. in general, concentrations of both Se and Fe decreased as pH increased from 1 to 4. Analyses of the equilibrated suspensions confirmed the equilibrium solid to be Fe-2(SeO3)(3) . 6H(2)O(c) and the aqueous Se to be selenite. Fitter's ion-interaction model was used with selected ion pairs to interpret the solubility data. The logarithm of the solubility product of ferric selenite Fe-2(SeO3)(3) . 6H(2)O(c) reversible arrow 2Fe(3+) + 3SeO(3)(2-) + 6H(2)O was found to be -4158 +/- 0.11. This value is less than any reported in the literature for a ferric selenite by more than 10 orders of magnitude. The solubility data and calculations show an extremely strong interaction between aqueous Fe3+ and SeO32-; interpretation of these data requires the inclusion of FeSeO3+ i.e. Fe3+ + SeO32- reversible arrow FeSeO3+, log K = 11.15+/-0.11.