Nickel Molybdates

The normal salt, NiMoO₄, is obtained in the anhydrous condition by fusion of nickel chloride with sodium molybdate and sodium chloride. In the hydrated condition it may be obtained by crystallising from mixed solutions of nickel chloride and sodium molybdate; in the cold, a green pentahydrate, NiMoO₄.5H₂O, separates; whilst from a hot solution, crystals of composition 3NiMoO₄.2H₂O are obtained. From mixed solutions of nickel chloride and sodium dimolybdate, yellow crystals of the dihydrate, NiMoO₄.2H₂O, are deposited; when sodium paramolybdate is employed, green crystals of the trimolybdate, NiO.3MoO₃.18H₂O, separate.

Complex molybdates, of composition 5(NH₄)₂O.3NiO.16MoO₃.16H₂O and 5K₂O.3NiO.16MoO₃.21H₂O, may be obtained by treating hot solutions of ammonium or potassium paramolybdate with nickel sulphate. The potassium salt is probably more correctly formulated as 2K₂O.NiO.6MoO₃.8H₂O, and appears to contain 3 molecules of water of constitution, so that it may be considered to be a complex salt of the type K₄H₆[Ni(MoO₄)₆].5H₂O. Corresponding salts of silver and barium have been obtained by double decomposition. The sodium salt, Na₂O.2NiO.6MoO₃.17H₂O, obtained by treating a hot solution of sodium paramolybdate with a nickel salt, may be formulated as a salt of the same type, thus: Na₂NiH₆[Ni(MoO₄)₆].14H₂O.

Complex salts containing tetravalent nickel of the type 3R^•₂O.NiO₂.9MoO₃.xH₂O, may be obtained by treating a mixed solution of potassium or ammonium paramolybdate and nickel sulphate with a persulphate. The potassium salt forms as a fine dark brown crystalline precipitate, whilst the ammonium salt yields purple crystals. A barium salt, 3BaO.NiO₂.9MoO₃.12H₂O, is formed as a brown precipitate on the addition of barium chloride to a solution of the ammonium salt.