Molybdenum Oxide (Blue), Mo₃O₈

Molybdenum Oxide (Blue), Mo₃O₈ (?) or Mo₅O₁₄ (?), if a suspension of molybdenum trioxide in water be heated on a water-bath with a large excess of powdered molybdenum, a blue solution is obtained, containing, it is supposed, unpolymerised molecules of the compound Mo₃O₈; but it is by no means certain either that this formula represents the actual composition of the substance, or that there is but one blue oxide of molybdenum. On addition of certain salts polymerisation is considered (Dumanski) to take place, and the oxide passes to the colloid form.

The blue oxide may be prepared by double decomposition of ammonium dimolybdate and molybdenum chloride, and washing the precipitate first with ammonium chloride solution and then with water; Berzelius accorded the substance the formula MoO₂.4MoO₃. Rammelsberg's blue oxide, obtained by interaction of solutions in hydrochloric acid of molybdenum dioxide and trioxide, was given the formula MoO₂.MoO₃.3H₂O, while Muthmann's formula was MoO₂.2MoO₃, i.e. Mo₃O₈. The blue oxide may also be obtained by reduction of ammonium molybdate by means of hydriodic acid, by heating in nitrogen the oxysulphate Mo₂O(SO₄)₂, or the oxyoxalate Mo₂O(C₂O₄)₂, or by electrolysis. Guichard considered the blue oxide to be a compound of the formula MoO₂.4MoO₃.6H₂O, while Junius' formula is Mo₇O₂₀.

The blue oxide is best obtained by allowing powdered molybdenum to remain for a long time at ordinary temperatures in contact with an aqueous suspension of the trioxide, filtering, and then digesting with a further quantity of molybdenum. The solution is evaporated in vacuo. Cold water used for washing the solid should previously be rendered air-free. Another good method of preparation consists in precipitating in the cold, by means of excess of a solution of hydrated molybdenum tetrachloride, a solution of ammonium molybdate in hydrochloric acid; the precipitate is washed with air-free water in an atmosphere of carbon dioxide, and is dried in vacuo.

It is evident, therefore, that the blue oxide may be obtained in a solid state either by precipitation or by evaporation of a solution.

The blue oxide of molybdenum, which can be regarded as a molybdate of molybdenum, is a dark blue substance, of density 3.6 at 18° C., which consists, when obtained by evaporation of a solution, of brilliant vitreous particles, giving the substance a crystalline appearance.

It is a colloid, and is extremely soluble in water, solution taking place slowly in the cold but rapidly at 50° C. A number of salts, e.g. sodium or ammonium chloride, without reacting with the blue oxide, considerably reduce its solubility; other salts, e.g. sodium or magnesium sulphate, do not exhibit this effect. It is readily dissociated by heat into a mixture of the dioxide and trioxide, and for this reason cannot be dried by heat even in vacuo. Oxidation to the trioxide takes place with the dry substance or in solutions slowly at ordinary temperatures but much more rapidly on heating. Its reactions with acids and alkalies point to its formula being probably MoO₂.4MoO₃; with hydrochloric acid the tetrachloride and trioxide are obtained, while with caustic alkali an alkali molybdate and molybdenum dioxide are produced.