Molybdic acid refers to hydrated forms of

molybdenum trioxide Molybdenum trioxide describes a family of inorganic compounds with the formula MoO3(H2O)n where n = 0, 1, 2. These compounds are produced on the largest scale of any molybdenum compound. The anhydrous oxide is a precursor to molybdenum metal, an ...

and related species. The

monohydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...

(MoO₃·H₂O) and the

dihydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

(MoO₃·2H₂O) are well characterized. They are yellow diamagnetic solids.

Structure of the solids

Solid forms of molybdic acid are coordination polymers. The monohydrate MoO₃·H₂O consists of layers of octahedrally coordinated MoO₅·(H₂O) units where 4 vertices are shared.Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications The dihydrate (image shown above) has the same layer structure with the "extra" H₂O molecule intercalated between the layers.

Structure of molybdic acid in solution

In acidified aqueous solutions of molybdic acid, the complex MoO₃(H₂O)₃ is observed. Once again, molybdenum adopts octahedral molecular geometry, probably with three

oxo ligand A transition metal oxo complex is a coordination complex containing an oxo ligand. Formally O2-, an oxo ligand can be bound to one or more metal centers, i.e. it can exist as a terminal or (most commonly) as bridging ligands (Fig. 1). Oxo ligan ...

s and three

aquo ligand In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perch ...

s.Solution structure of molybdic acid from Raman spectroscopy and DFT analysis, Oyerindea O.F., Week C.L., Anbarb A.D., Spiro T.G. Inorganica Chimica Acta, 361, 4, (2008), 1000-1007, The salts of molybdic acid are called

molybdate In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, althou ...

s. They arise by adding base to solutions of molybdic acid.

Applications

Many molybdenum oxides are used as

heterogeneous catalyst In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. P ...

s, e.g. for

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

s. Molybdic acid and its salts are used to make the

Froehde reagent The Froehde reagent is used as a simple spot-test to presumptively identify alkaloids, especially opioids, as well as other compounds. It is composed of a mixture of molybdic acid or a molybdate salt dissolved in hot, concentrated sulfuric acid, wh ...

for the presumptive identification of alkaloids.

References

Hydrogen compounds Acids Molybdates Coordination polymers {{inorganic-compound-stub