Selenates

Selenic acid resembles sulphuric acid in being dibasic and both normal and acid salts are known. The salts can be obtained from the acid by the generally recognised methods, or the corresponding selenites may be oxidised, e.g. by fusion with sodium peroxide or by electrolysis in aqueous solution using platinum foil as anode and platinum wire as cathode. Very little selenium is formed at the cathode and most of the solute is converted into selenate at the anode.

The normal selenates of potassium, sodium, calcium and magnesium are very similar in properties to the corresponding sulphates, except that the potassium salt is much more soluble in water than potassium sulphate. Sodium selenate gives a decahydrate which effloresces, and the transition-point between this and the anhydrous salt is 31.8° C., above which point the solubility falls as the temperature rises. Calcium selenate gives a hemihydrate resembling plaster of Paris, and also a hydrate of composition CaSeO₄.l.5H₂O.

The selenates give many other examples of isomorphism with the sulphates, and therefore also with the chromates and manganates, and mixed crystals are easily obtained. The selenates, however, frequently take and retain their water of crystallisation less readily than the corresponding sulphates.

With the exception of the calcium, strontium, barium and mercurous salts, the normal selenates are readily soluble in water. Barium chloride and mercurous nitrate are therefore convenient precipitation agents. Barium selenate is, however, more soluble than barium sulphate, and also differs from the latter salt in being slowly reduced to selenite by hydrochloric acid; for these reasons precipitation with barium chloride is not applicable to the quantitative determination of selenic acid. A concentrated solution of selenic acid which has been saturated with barium selenate deposits crystals of barium selenic acid, H₂[Ba(SeO₄)₂].

Copper selenate, like the corresponding sulphate, is best known in the form of the pentahydrate. When the latter is dehydrated at 102° C. it is converted into the monohydrate, dehydration being complete at 230° to 235° C. Copper selenate can also be obtained in the trihydrated form.

By the action of ammonia on copper selenate pentahydrate, small bluish-violet needle-like crystals of copper selenate tetrammoniate monohydrate, CuSeO₄.4NH₃.H₂O, are obtained. On exposure to the air these crystals slowly evolve ammonia and become dull in appearance. They are soluble in water, giving a deep blue solution, which on dilution becomes lighter in colour and deposits basic copper salts. When the crystals are exposed to the action of air for many hours, the preliminary evolution of ammonia ceases and the blue product has the composition CuSeO₄.3NH₃.H₂O. When the bluish-violet crystals of the tetrammoniate monohydrate are dried over lime in a desiccator under a low pressure for some days, they lose their water but not their ammonia, and light blue-violet crystals of CuSeO₄.4NH₃ are obtained. On exposure to the air these crystals evolve ammonia, but kept over lime they are quite stable.

Ferrous selenate can be prepared by the action of selenic acid on ferrous sulphide; selenic acid does not dissolve iron:

FeS + H₂SeO₄ = FeSeO₄ + H₂S.

The production of the hydrogen sulphide serves to prevent the selenate being oxidised to the ferric state. There may also be a slight secondary reaction resulting in the precipitation of both sulphur and selenium. From the solution unstable monoclinic crystals of hydrated ferrous selenate, FeSeO₄.7H₂O, may be deposited.

The stability of the product is greatly increased by mixing the solution with the equivalent amount of one of the alkali selenates, when crystals of a double selenate, of the type M₂SeO₄.FeSeO₄.6H₂O, are deposited.

In the ready formation of double salts, selenates show further resemblance to the sulphates, and even "alums," e.g. M₂SeO₄.Al₂(SeO₄)3. 24H₂O, M₂SeO₄.Cr₂(SeO₄)₃.24H₂O and M₂SeO₄.Fe₂(SeO₄)₃.24H₂O (where M represents one of the alkali metals), have been prepared and found to be of the orthodox crystalline form. Monoclinic double selenates of the copper, iron and cobalt groups with the alkali metals have also been prepared, as well as the following: K₂SeO₄.MgSeO₄.6H₂O, Na₂SeO₄.MgSeO₄.4H₂O, K₂SeO₄.CaSeO₄.2H₂O. The last is interesting in that the corresponding sulphate is a monohydrate. No double selenate corresponding with glaserite, 2K₂SO₄.Na₂SO₄, has been obtained.

The selenates are not so harmful as the selenites in their physiological action, and it is probable that what poisonous nature they possess is dependent on their previous reduction in the organism to selenite.