Nickel Chromate, NiCrO₄

Nickel Chromate, NiCrO₄, is prepared in an analogous manner to the cobalt salt, the mixture being heated to 260° C. It yields lustrous black crystals, insoluble in water and only difficultly soluble in acids. The salt is not obtained by double decomposition of a nickel salt with an alkali chromate, this reaction producing double salts of nickel chromate, the stability of which increases in the order, potassium, rubidium, ammonium, caesium, and of which the general formula is M₂Ni(CrO₄)₂.6H₂O, where M is the alkali metal. The potassium salt yields mixed crystals with the analogous sulphate, K₂Ni(SO₄)₂.6H₂O. The dihydrate, K₂Ni(CrO₄)₂.2H₂O, forms yellow crystalline needles.