Nickel Selenite, NiSeO₃

Nickel Selenite, NiSeO₃, has been prepared in several stages of hydration. The semihydrate, 2NiSeO₃.H₂O, is obtained as insoluble rhombic prisms by heating a solution of nickel carbonate in selenious acid to 230° C. in sealed tubes. It is stable at 100° C. The monohydrate, NiSeO₃.H₂O, is precipitated on addition of an alkali selenite to a solution of a nickel salt. On drying over sulphuric acid it becomes a grey powder. The dihydrate, NiSeO₃.2H₂O, has also been obtained. Nickel Selenate, NiSeO₄.6H₂O, may be obtained as green octahedral crystals by allowing a solution of nickel carbonate in selenic acid to evaporate slowly at ordinary temperature.

Nickel selenate unites with the selenates of the alkali metals to yield double salts of the type M₂SeO₄.NiSeO₄.6H₂O. These salts crystallise in holohedral prisms belonging to the monoclinic system, and are isomorphous with the double sulphates of analogous composition, of which ferrous ammonium sulphate is the best known example. The following salts have been carefully studied by Tutton: potassium nickel selenate, K₂SeO₄.NiSeO₄.6H₂O (density at 20° C. 2.559, molecular volume 206.14); and the nickel selenates of rubidium (density 2.856, molecular volume 219.96), caesium (density 3.114, molecular volume 229.17) and ammonium (density 2.243, molecular volume 216.53).