Chemical elements
  Molybdenum
    Isotopes
    Energy
    Production
    Application
    Physical Properties
    Chemical Properties
    Alloys
    Compounds
      Molybdenum Hexafluoride
      Fluoroxypermolybdates
      Molybdenum Dichloride
      Molybdenum Trichloride
      Molybdenum Tetrachloride
      Molybdenum Pentachloride
      Molybdenum Oxychlorides
      Chlormolybdic Acids
      Molybdenum Dibromide
      Molybdenum Tribromide
      Molybdenum Tetrabromide
      Molybdenum Oxybromide
      Molybdenum Di-iodide
      Molybdenum Oxyiodide
      Iodomolybdic Acid
      Molybdenum Sesquioxide
      Molybdenum Dioxide
      Molybdenum Oxide Blue
      Molybdenum Trioxide
      Molybdates
      Aluminium Molybdates
      Ammonium Molybdate
      Ammonium Dimolybdate
      Ammonium Paramolybdate
      Ammonium Trimolybdate
      Ammonium Tetramolybdate
      Ammonium Octamolybdate
      Barium Molybdates
      Barium Paramolybdate
      Barium Trimolybdate
      Barium Tetramolybdate
      Barium Octamolybdate
      Barium Nonamolybdate
      Beryllium Molybdate
      Bismuth Molybdates
      Cadmium Molybdates
      Caesium Molybdates
      Calcium Molybdate
      Calcium Trimolybdate
      Calcium Tetramolybdate
      Calcium Octamolybdate
      Chromium Molybdates
      Cobalt Molybdates
      Cobalt Dimolybdate
      Cobalt Trimolybdate
      Copper Molybdates
      Ferrous Molybdate
      Ferric Molybdate
      Indium Molybdate
      Lead Molybdates
      Lithium Molybdate
      Lithium Dimolybdate
      Lithium Paramolybdate
      Lithium Trimolybdate
      Lithium Tetramolybdate
      Magnesium Molybdates
      Magnesium Paramolybdate
      Magnesium Trimolybdate
      Manganese Molybdate
      Mercurous Molybdates
      Nickel Molybdates
      Potassium Molybdate
      Potassium Dimolybdate
      Potassium Paramolybdate
      Potassium Trimolybdate
      Potassium Tetramolybdate
      Potassium Octamolybdate
      Potassium Decamolybdate
      Rhodium Molybdates
      Rubidium Molybdate
      Rubidium Dimolybdate
      Rubidium Paramolybdate
      Rubidium Trimolybdate
      Rubidium Tetramolybdates
      Silver Molybdates
      Normal Silver Molybdate
      Sodium Molybdate
      Sodium Dimolybdate
      Sodium Paramolybdate
      Sodium Trimolybdate
      Sodium Tetramolybdate
      Sodium Iodomolybdate
      Strontium Molybdate
      Thallous Molybdate
      Thallous Paramolybdate
      Thallous Tetramolybdate
      Thorium Molybdate
      Uranium Molybdates
      Uranyl Octamolybdate
      Zinc Molybdates
      Zinc Trimolybdate
      Zinc Tetramolybdate
      Zinc Octamolybdate
      Zirconium Molybdate
      Permolybdic Acid
      Molybdenum Sesquisulphide
      Molybdenum Disulphide
      Dimolybdenum Pentasulphide
      Molybdenum Trisulphide
      Molybdenum Tetrasulphide
      Thiomolybdates
      Ammonium Thiomolybdates
      Ammonium Molybdosulphites
      Potassium Thiomolybdate
      Potassium Thiodimolybdate
      Potassium Dithiodioxymolybdate
      Potassium Molybdosulphite
      Sodium Thiomolybdates
      Sodium Molybdosulphites
      Molybdenum Sulphates
      Molybdenum Selenide
      Complex Molybdoselenites
      Chromates of Molybdenum
      Molybdenum Phosphide
      Molybdic Metaphosphate
      Heteropoly-compounds with Phosphorus
      12-Molybdophosphoric Acid
      9-Molybdophosphoric Acid
      172-Molybdophosphoric Acid
      Molybdohypophosphates
      Molybdophosphites
      Molybdohypophosphites
      12-Molybdo-arsenates
      9-Molybdo-arsenates
      3-Molybdo-arsenates
      Molybdenum Carbides
      Molybdenum Carbonyl
      Reddish-violet Salts
      Yellow Salts
      Thiocyanates of Molybdenum
      Molybdenum Monosilicide
      Molybdenum Sesquisilicide
      Molybdenum Disilicide
      Molybdosilicic Acid and Molybdosilicates
      12-Molybdosilicic Acid
    PDB 1aa6-1qh8
    PDB 1r27-2jir
    PDB 2min-3unc
    PDB 3uni-4f6t

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.


Molybdenum ttrioxide Hydrates

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, °C2030405060707579
Solubility0.1300.2570.4540.6431.0761.7051.7301.740


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.1520304050607080
Solubility0.1120.2370.2930.340.400.470.420.52


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.

Colloidal Molybdic Acid

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.
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