Ferrous selenite

Iron Selenites.—Although metallic iron does not appear to be soluble in selenous acid, yet selenites of iron are readily obtained in a variety of ways. When sodium selenite is added to ferrous sulphate solution, a white precipitate of ferrous selenite, FeSe03, is obtained.4 This becomes darker on exposure to air in consequence of oxidation. If the white precipitate is dissolved in hydrochloric acid, a portion of the selenium separates out, whilst ferric chloride and selenous acid remain in solution. Thus —... 

The efficacy of these reactions in wastewater treatment system remains doubtful, however. The reduction of selenate by ferrous sulfide is deemed to be important in wastewater treatment systems and can result in the formation of highly insoluble ferrohydroxy selenite, the solubility product of which has been estimated to be 10 (Geering et ai, 1968) ... 

Selenites and selenates are quantitatively reduced to elementary selenium when their acid solutions are warmed with ferrous sulfate. In contrast, tellurite and tellurate solutions remain unaltered because the redox potential of iron is not sufhcient to accomplish the reaction Fe i + Teiv(vi) Fe -f Te°. However, the reducing power of Fe+ ions can be raised considerably if the Fe+ ions are removed as soon as they are produced. Phosphoric acid is excellent for this purpose it immediately converts the Fe+ ions into [Fe(P04)2] " ions. Fluorides act analogously through the formation of [FeF,]- ions. Consequently, when tellurites or tellurates are warmed with a ferrous sulfate-phosphoric acid mixture there is complete reduction to free tellurium. 

---