Molybdenum Trioxide, MoO₃

Molybdenum Trioxide, MoO₃, or Molybdic Anhydride, occurs naturally as molybdite or molybdenum ochre. It is prepared from molybdenite, MoS₂, by a process consisting essentially of roasting with or without admixture with sand, and extraction of the resulting mass with ammonia. Separation from copper is effected by the addition of ammonium sulphide, and ammonium molybdate is crystallised from the filtrate. From this compound the trioxide is obtained by heating in the air. It is preferable, however, to evaporate the solution of ammonium molybdate to dryness with excess of potassium carbonate, extract with water, evaporate again to dryness, and heat the residue with sulphur. After washing with hot water the residual molybdenum sulphide is again treated as already described, and sublimed in platinum vessels in a current of oxygen.

From wulfenite, molybdenum trioxide is prepared by digesting the mineral (previously washed with dilute hydrochloric acid) with concentrated hydrochloric acid; lead still remaining in solution after cooling and filtration is removed by the addition of sulphuric acid, and the filtrate is evaporated to dryness with the addition of a small quantity of nitric acid. The ammonia extract of this mass is then subjected to the method of purification previously described. Another method consists in decomposing the finely powdered mineral by means of concentrated sulphuric acid, diluting to precipitate lead sulphate, and evaporating the filtrate until precipitation of molybdic anhydride occurs. Molybdic anhydride is a white powder which, on heating, becomes yellow. It melts at 795° C. without decomposition, yielding a reddish-brown liquid, which solidifies on cooling to a yellowish-white fibrous crystalline mass, of density 4.696 at 26° C. It sublimes fairly readily in air, rhombic crystals, colourless and transparent, though small, being obtained. Like the dioxide, it is more strongly magnetic than the metal.

Reduction of the trioxide to the dioxide of molybdenum can be effected by means of hydrogen, nitric or nitrous oxides, or sulphur dioxide, while by continued heating in hydrogen a residue of the metal is obtained. The trioxide is slightly soluble in water, yielding a solution which is distinctly metallic to the taste, reddens litmus, and turns turmeric paper brown.

Molybdic anhydride, if unfused, is soluble in acids with the production of a number of complex acids; if previously fused it is, however, insoluble in acids though still soluble in alkalies, yielding the corresponding molvbdates.

Molybdenum trioxide is an acidic oxide which dissolves in water yielding a solution of molybdic acids, and combines with basic oxides yielding molybdates.

Two definite hydrates of molybdenum trioxide are known to exist, namely, MoO₃.H₂O and MoO₃.2H₂O.

The dihydrate is a canary-yellow substance, of density 3.124, which is deposited in a crystalline condition as monoclinic prisms, with

a:b:c = 1.0950:1:1.0664; β = 90° 41',

when to a 30 per cent, solution of nitric acid there is added with constant stirring an equal volume of a 15 per cent, solution of ammonium paramolybdate, ammonium nitrate added up to a concentration of 10 per cent., and the mixture sown with a few crystals of the dihydrate. The yellow crystalline body which separates from ammonium molybdate-nitric acid reagent is apparently the dihydrate. This hydrate is the most soluble in water of the molybdic acids. The solubility, i.e. number of grams of MoO₃ per 100 grams of solution, in water of the dihydrate is as follows:

Temperature, °C	20	30	40	50	60	70	75	79
Solubility	0.130	0.257	0.454	0.643	1.076	1.705	1.730	1.740

The solubility curve of the dihydrate cuts that of the monohydrate at 32° C. On heating, two distinct varieties of molybdic acid mono- hydrate are formed; on concentrating the solution of the dihydrate at 40° to 50° C., α-molybdic acid monohydrate is formed as asbestos-like white needles, retaining their water of crystallisation much more readily than β-molybdic acid monohydrate, which is obtained as small white needles, soluble in water, but not regenerating the dihydrate, by heating the dihydrate to 70° C. White α-molybdic acid is readily obtained by treating methyl molybdate with water. The solubility in water of the monohydrate is as follows:

Temperature, °C.	15	20	30	40	50	60	70	80
Solubility	0.112	0.237	0.293	0.34	0.40	0.47	0.42	0.52

At 60° C. the α-monohydrate apparently undergoes transformation into the β-monohydrate.

According to Rosenheim and Bertheim, cryoscopic measurements indicate the existence of octamolybdic acid H₂Mo₈O₂₅; according to Mylius, although there is no solid molybdic acid corresponding to telluric acid H₆TeO₆, colourless molybdic acid in aqueous solution corresponds to allotelluric acid.

Forsen describes a crystalline compound of composition H₄Mo₃O₁₁, which he regards as an anhydride of the fundamental hexabasic molybdic acid H₆Mo₃O₁₂.

A number of complexes of molybdic acid, particularly those with acetylacetone (MoO₂[CH(COMe)₂]₂), with salicylaldehyde, and with organic acids, have been described, and a number of measurements made of rotatory power.

When solutions of sodium molybdate (1 molecule) and hydrochloric acid (4 molecules) are warmed together, or when a solution of molybdic acid dihydrate is evaporated over sulphuric acid under diminished pressure at 20° C., colloidal molybdic acid is formed. From the solution of the hydrosol so obtained, molybdic acid is precipitated by electrolytes. Graham considered that, by dialysis of a solution of sodium molybdate in hydrochloric acid, he obtained colloidal molybdic acid.

Complex molybdic acids are dealt with in connection with the various elements which they contain; for example, for chlormolybdic acids.