Ferrous selenate, FeSeO₄

Iron dissolves in aqueous selenic acid, yielding ferrous selenate and a deposit of selenium, which collects upon the surface of the undissolved metal and shields it from further attack. The rate of solution is thereby so greatly retarded that as a method of preparing ferrous selenate it is not to be recommended. The net result of the action may be represented by the equation: -

3Fe + 4H₂SeO₄ = 3FeSeO₄ + Se + 4H₂O.

Ferrous carbonate is likewise but slowly attacked by selenic acid; a better method of preparing ferrous selenate consists in dissolving ferrous sulphide in a concentrated solution of the acid. The hydrogen sulphide evolved reduces a portion of the selenic acid in accordance with the equation: -

H₂SeO₄ + 3H₂S = Se + 3S + 4H₂O.

The solution is therefore filtered and when allowed to crystallise at 0°C. yields monoclinic crystals of the heptahydrate, FeSeO₄.7H₂O, isomorphous with ferrous sulphate, FeSO₄.7H₂O. The crystals are much less stable, however, rapidly becoming opaque. When crystallised from warm solutions the pentahydrate, FeSeO₄.5H₂O, is obtained, isomorphous with copper sulphate, CuSO₄.5H₂O, and ferrous sulphate, FeSO₄.5H₂O.