Tungsten Dioxide, WO₂

Tungsten Dioxide, WO₂, may be obtained as a brown amorphous powder by heating the trioxide to dull redness in a current of hydrogen. The residue remains in a stable condition, if allowed to cool in contact with hydrogen, for about twenty-four hours, but it is difficult to obtain the pure dioxide by this means, as reduction is liable to proceed further unless the temperature is carefully regulated. It may also be produced by heating tungsten di-iodide to 500° C. in a current of carbon dioxide, by acting on the tetrachloride with water, or by heating a mixture of ammonium chloride and normal potassium tungstate, the residue being well washed with water. In the wet way it is obtained as copper-red spangles by the addition of moderately strong hydrochloric acid and zinc to tungsten trioxide, to one of its hydrates, or to sodium met atungst ate.

The crystalline form has been obtained by heating lithium para-tungstate in a current of hydrogen at about the melting-point of glass for forty-five minutes. The product, after cooling, was treated successively with boiling water, concentrated hydrochloric acid, and lithium hydroxide solution, and then very carefully washed with warm water; it remains as an opaque, brown, micro-crystalline powder, with a metallic lustre.

The density of tungsten dioxide, according to Karsten, is 12.1109. It is stable in the air but is readily oxidised. Heated in oxygen, it yields the trioxide; at 500° C. in nitrous oxide or nitric oxide, the blue oxide, W₂O₅, results, while nitrogen peroxide yields the trioxide. The following thermal values have been determined:

W + O₂ = WO₂ + 127,900 calories.
4WO₂ + O₂ = 2W₂O₅ + 125,200 calories.
2W₂O₅ + O₂ = 4WO₃ + 140,400 calories.

Heated in chlorine, the yellow oxychloride, WO₂Cl₂, is obtained. Tungsten dioxide thus acts as a powerful reducer, and will convert mercuric and cupric salts to the mercurous and cuprous condition, and precipitate the metal from ammoniacal silver solutions. The amorphous variety is soluble in hydrochloric and sulphuric acids, yielding red solutions which on standing undergo partial oxidation with loss of colour; the crystalline dioxide is unacted upon, even by the hot concentrated acids. Nitric acid has a slow oxidising action. Concentrated alkali solutions dissolve the amorphous oxide, with formation of the tungstate and liberation of hydrogen, but have no action on the crystalline variety.

Tungsten dioxide is reduced to the metal by heating with carbon to about 1020° C.

The corresponding hydroxide, W(OH)₄, has been prepared by the electrolytic reduction of solutions of tungsten trioxide in hydrochloric or hydrofluoric acid. It is a brown powder, insoluble in sodium hydroxide, sulphuric acid, or acetic acid, but soluble in concentrated hydrochloric acid, yielding a greenish solution which rapidly becomes blue owing to oxidation of tetravalent tungsten to the pentavalent condition.