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 was. When sodium selenite is added to ferrous sulphate solution, a white precipitate of ferrous selenite, FeSeO₃, is obtained. 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: -

4FeSeO₃ + 12HCl = Se + 4FeCl₃ + 3SeO₂ + 6H₂O.

Several ferric selenites have been described, and may be divided into three groups, namely basic, neutral, and acid salts.

The salt, 3Fe₂O₃.8SeO₂.28H₂O, is obtained as a yellow, voluminous precipitate on adding sodium selenite solution to one of ferric chloride at room temperature. This, when heated to 230°-250°C. for several hours with water in a sealed tube, yields insoluble yellow crystals of Fe₂O₃.2SeO₂. This latter salt may also be obtained by heating the decahydrated neutral salt in the same manner.

Neutral Salts, Fe₂O₃.3SeO₂.xH₂O. - The following hydrates have been described: decahydrate, nonahydrate, heptahydrate, trihydrate, monohydrate.

The following salts are known: Fe₂O₃.4SeO₂.8H₂O, Fe₂O₃.4SeO₂.9H₂O, Fe₂O₃.4SeO₂.H₂O, and Fe₂O₃.6SeO₂.2H₂O.