Ditungsten Pentoxide

The Blue Oxide of Tungsten (Ditungsten Pentoxide) may be formed by oxidation of the dioxide, or by reduction of the trioxide. It is thus an intermediate oxide, and since it is itself prone to oxidation, it is difficult to isolate in a pure condition, and it is doubtful whether it has yet been so obtained. Our knowledge of its composition and properties is therefore limited. It is generally known as the " blue oxide" of tungsten, and there is evidence of pentavalent tungsten, but the blue product may also be regarded as essentially tungsten tungstate.

The formation of a blue compound by reduction of tungsten trioxide was first observed by Berzelius, and the compound was first prepared by Malaguti by heating the trioxide at 250° to 300° C. in a current of hydrogen. Carbon monoxide may be used instead of hydrogen. The blue oxide may also be obtained by cautiously heating ammonium para-tungstate in a closed crucible, by the electrolysis of fused sodium tungstate or of a boiling solution of a tungstate in hydrochloric acid, by decomposition of tungsten pentachloride or pentabromide by means of water, or by reduction of an acidified solution of a tungstate by means of zinc or stannous chloride. The product is a powder of which the colour varies from greenish blue to deep blue. It is readily converted to the trioxide by heating in oxygen or air. Its magnetic susceptibility is 0.755. When prepared in the wet way it is readily oxidised by nitric acid, but the product obtained in the dry way resists this reagent.

A similar blue compound was prepared by Allen and Gottschalk by heating on a water-bath a mixture of tungsten trioxide and stannous chloride in hydrochloric acid solution, or by heating the trioxide with concentrated hydriodic acid in a sealed tube at 200° C. The product was oxidised on standing in air, was insoluble in hydrochloric acid, but soluble in alkalies with evolution of hydrogen. Its composition appeared to be W₅O₁₄.H₂O. It reacted with ammonia, yielding some ammonium tungstate and a brownish-purple residue of composition W₃O₈.H₂O.

A colloidal solution of the blue oxide, known as tungsten blue, may be obtained by the electrolytic reduction of an acidified tungstate solution, or by first saturating a solution of metatungstic acid with hydrogen sulphide and then neutralising by means of ammonia or ammonium sulphide. The solution, when freshly prepared, is optically void, but, on standing, a luminous cone gradually appears. Under the influence of the electric current the colloid moves towards the anode. The composition of the dissolved oxide, according to Leiser, is W₄O₁₁. The solution may be used for dyeing silk, and also, but not so satisfactorily, cotton and wool.