Cobalt Tungstates

The anhydrous normal salt, CoWO₄, is obtained as greenish-black crystals by fusing together sodium tungstate, sodium chloride, and cobalt chloride. The hydrate, CoWO₄.2H₂O, results as a violet precipitate when a solution of alkali tungstate is treated with a solution of a cobalt salt.

Cobalt ditungstate, CoO.2WO₃.3H₂O, is obtained as a reddish-brown precipitate by double decomposition.

Cobalt paratungstate, 3CoO.7WO₃.25H₂O, is formed as a bright rose-coloured microcrystalline precipitate on mixing solutions of sodium paratungstate and a cobalt salt. It does not melt at a red heat, but on cooling it acquires a bluish tinge. The double salt, 3Na₂O.2CoO.12WO₃.30H₂O, is prepared in the same way as the corresponding double salt of calcium. It forms rose-coloured crystals which melt at red heat and solidify, on cooling, to a black lustrous mass. Double salts, of composition 2Na₂O.3CoO.12WO₃.41H₂O and Na₂O.4CoO.12WO₃.23H₂O, have been prepared by Rosenheim, who suggests for them the formulae Na₄Co₃H₁₀[H₂(WO₄)₆]₂.34H₂O and NaCo₂H₅[H₂(WO₄)₆].16H₂O, respectively.

The salt, 4CoO.10WO₃.35H₂O, is obtained as a pink granular powder by the method used for the corresponding barium salt.

Cobalt tritungstate, CoO.3WO₃.4H₂O, is described by Lefort.