Chemistry of the Ca–Se(IV)–H2O and Ca–Se(VI)–H2O systems at 25 °C

The Ca–Se(IV)–H2O and Ca–Se(VI)–H2O systems were studied by contacting either selenious acid or selenic acid solution with calcium oxide to attain equilibrium at 25 °C for one month. Analysis of the final solid phases and the associated solution, together with X-ray diffraction analysis and a study into the graphed relationships, showed the existence of three calcium selenites in the Ca–Se(IV)–H2O system — Ca2SeO3(OH)2·2H2O (Se(IV) = 4.8 × 10− 5–2.8 × 10− 4 M); CaSeO3·H2O (Se(IV) = 2.8 × 10− 4–0.86 M) and Ca(HSeO3)2·H2O (Se(IV) > 0.86 M). It also showed four calcium selenates in the Ca–Se(VI)–H2O system — Ca2SeO4(OH)2 (Se(VI) = 0.21–0.39 M); CaSeO4·2H2O (Se(VI) = 0.40–9.1 M); CaSeO4 (Se(VI) = 10.2 M) and CaSe2O7 (Se(VI) > 10.8 M). The X-ray diffraction analyses reported and SEM analyses indicate a high degree of crystallinity of all seven compounds. The stability and solubility regions for these compounds were defined versus pH, and the conventional solubility constants and conditional free energies of formation for the less soluble CaSeO3·H2O, Ca2SeO3(OH)2·2H2O, CaSeO4·2H2O and Ca2SeO4(OH)2 were calculated from solubility data obtained.