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Molybdates

Normal molybdates of the type R2MoO4 exist in solution but are relatively unstable, and readily form acid salts or complex poly molybdates. Thus dimolybdates, R2Mo2O7, can be obtained by fusion of molybdenum trioxide with sodium or potassium nitrate; trimolvbdates, R2Mo3O10, and tetramolybdates, R2Mo4O13, by heating molybdenum trioxide with an aqueous solution of sodium or potassium carbonate. Even more highly acid salts - for example, octa- and deca-molybdates - can be obtained. Solutions of normal molybdates, when treated with hydrochloric acid or nitric acid, yield a precipitate of acid molybdate; this reaction does not, however, take place with sulphuric, acetic, oxalic, or tartaric acids.

Different views have been expressed concerning the relation between the various types of poly molybdates. According to Rosenheim and his collaborators, the tetramolybdates are entirely similar to the meta-tungstates, and are therefore 12-molybdic acid hydrates of the type

R6H4[H2(Mo2O7)6],
whilst the octamolybdates, which yield hydrogen ions in aqueous solutions, are regarded as hydrogen salts with the same complex anion,
R3H7[H2(Mo2O7)6].

The same authors state that two distinct series of decamolybdates exist, either isomeric or polymeric, one sparingly soluble, and the other readily soluble, in water. The paramolybdates, which have been described under various formulae, appear to correspond with either 3R2O.7MoO3.aq. or 5R2O.12MoO3.aq. It has been suggested that these salts are derived from the hypothetical orthomolybdic acid H6MoO6 by partial substitution of the group Mo2O7 for oxygen, and may be formulated
R6[Mo(Mo2O7)3O3].
Condensation formulae have also been suggested. For example, ammonium paramolybdate, which has been obtained in the crystalline form in the anhydrous state, may be expressed by the formula

(NH4O)3MoO(O.MoO2)5O.MoO(ONH4)3,

and this view is supported by the preparation of two other ammonium heptamolybdates, of composition

(NH4O)3MoO(O.MoO2)5O.MoO2.OH
and
NH4O.(OH)2MoO(O.MoO2)5O.MoO2.OH.H2O.

When increasing quantities of hydrochloric acid are added to a saturated solution of ammonium paramolybdate, a series of hexabasic polymolybdates, having the general formula

(NH4O)3MoO(O.MoO2)mO.MoO(ONH4)3 + nH2O,

are obtained. Of these, compounds containing 9, 11, 12, and 13 atoms of molybdenum to the molecule have been prepared. All these poly-molybdates, with excess of alkali, yield orthomolybdates, which may be regarded as hexabasic trimolybdates,

(RO)3MoO.O.MoO2.O.MoO(OR)3 + nH2O.

A series of tetrabasic polymolybdates, containing up to seven molybdic groups, arise by the decomposition of the tridecamolybdates, and compounds containing more than thirteen molybdic groups are not known. The condensation of molybdic acid thus reaches its limit by the formation of tridecamolybdic acid.

The classification of molybdates into ortho-, meta-, and para-salts is obviously unsatisfactory. There appear to be two main groups of molybdates: (1) the hexabasic poly molybdates, which include among others the ortho- and para-molybdates; and (2) the tetrabasic poly-molybdates, including the metamolybdates, which arise from the hexabasic salts by hydrolytic rupture of their chain.

Molybdates combine also with other acidic oxides, forming a series of complex molybdates, the most important of which are probably the phosphomolybdates. The formation of such compounds as dichlormolybdic acid or molybdenum hydroxychloride, MoOCl2(OH)2, and the sulphate MoO3.SO3, illustrate the behaviour of molybdenum trioxide as a basic oxide.

The absorption spectra of molybdates have been investigated.

Molybdates in aqueous solution are reduced by the common reducing agents, yielding solutions which are reddish, blue, green, or brown, according to the prevailing conditions. Hydrogen sulphide, for instance, produces at first a yellowish colour, changing to green and then blue; finally a brown precipitate of molybdenum sulphide is obtained. For further discussion upon the reactions of molybdates.
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