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

The aqueous solubility of Fe-2(SeO3)(3) . 6H(2)O(c) was studied in dei onized 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 ove rsaturation directions, with equilibration periods ranging from 7 to 1 725 days. Stoichiometric dissolution of the solid war observed in solu tions with pH values up to nearly 4. in general, concentrations of bot h Se and Fe decreased as pH increased from 1 to 4. Analyses of the equ ilibrated 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-intera ction model was used with selected ion pairs to interpret the solubili ty 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 mag nitude. The solubility data and calculations show an extremely strong interaction between aqueous Fe3+ and SeO32-; interpretation of these d ata requires the inclusion of FeSeO3+ i.e. Fe3+ + SeO32- reversible ar row FeSeO3+, log K = 11.15+/-0.11.