Molybdenum Sesquioxide, Mo₂O₃

Molybdenum Sesquioxide, Mo₂O₃, is obtained by suitable reduction of the dioxide or trioxide, e.g. by magnesium, zinc, or, preferably, zinc coated with cadmium, and sulphuric or hydrochloric acid, and is apparently known only in the hydrated condition. The hydroxide Mo(OH)₃, which may also be obtained electrolytically, was first prepared by Berzelius by the action of zinc and a slight excess of hydrochloric acid upon an aqueous solution of an alkali molybdate; a series of colour changes takes place, the colourless liquid becoming blue, then brown, and finally black. On addition of ammonia the hydroxide separates as a brown powder, which may be freed from traces of zinc salts by washing with very dilute hydrochloric acid. Further, the hydrated sesquioxide is obtained by interaction of potash or soda, and molybdenum dichloride or dibromide, hydrogen being simultaneously evolved, whereas a salt of the same oxide is obtained upon reduction of a solution of molybdic acid in hydrochloric acid by means of copper. The reduction of ammonium molybdate by colloidal palladium, if carried out at ordinary temperatures, yields the tetra-hydroxide Mo(OH)₄; if, however, the reduction is effected at 50° to 60° C. under a slight pressure, the trihydroxide is obtained as a black sludge:

(NH₄)₆Mo₇O₂₄.4H₂O + 21H = 7Mo(OH)₃ + 6NH₃ + 7H₂O.

Molybdenum trihydroxide is an amorphous black substance, becoming reddish brown by slow oxidation in the air. On heating to redness the substance deflagrates, but does not yield the anhydrous oxide. It is insoluble in caustic potash or potassium carbonate solution, but dissolves in excess of ammonium carbonate; its solutions in acids are black in colour.

The salts corresponding to this oxide are dark grey or black, while their solutions have a dark purple or black colour; they are only slightly oxidised in air. The sulphide is an insoluble brown substance, precipitated by hydrogen sulphide or by ammonium sulphide; the chloride, iodide, fluoride, sulphate, and nitrate are soluble. A number of other salts have been prepared.