Heteropoly-compounds with Phosphorus

Heteropoly-compounds of Molybdenum and Phosphorus include those acids and salts which contain acid anhydrides of one or more elements combined with a hydrate or salt of the acid of another element. Compounds which contain the acid anhydride and the acid hydrate or salt of the same element - for example, polychromates, polytungstates, etc. - are distinguished as isopoly- compounds.

Perhaps the most characteristic group of heteropoly-compounds is that of the phosphomolybdic acids which have hitherto been considered as complex acids. Much early work was directed towards the action of phosphoric acid on molybdic acid and the alkali molybdates with the object of explaining the nature of the products obtained. It was shown that when molybdic anhydride dissolved in orthophosphoric acid a yellow solution was obtained, which on evaporation gave a viscous uncrystallisable liquid. On adding ammonia to the yellow solution a yellow precipitate was obtained in the presence of acid. The precipitation was impeded by the presence of certain organic salts, e.g. ammonium oxalate or citrate, and the meta- and pyro-phosphoric acids only gave the reaction after being converted to the ortho-acid. The yellow precipitate was shown to be the ammonium salt of a definite phosphomolybdic acid, which was isolated from it by Debray by gently heating with excess of aqua regia, when the ammonia was expelled and a yellow solution remained which by spontaneous evaporation gave crystals of the acid, to which he assigned the formula P₂O₅.20MoO₃.25H₂O. It forms yellow salts, those of potassium, ammonium, rubidium, caesium, and thallium being insoluble in water.

The yellow phosphomolybdates are decomposed by excess of alkali, giving neutral molybdates and other white phosphomolybdates, soluble in alkaline solution and richer in phosphoric acid. These white salts are reconverted to yellow phosphomolybdates with liberation of phosphoric acid by the action of acids.

The composition of phosphomolybdic acid given by Debray was contested by Rammelsberg, who found the ratio P₂O₅:MoO₃ to be 1:22, while other workers found 1:24. Numerous other combinations in which the proportion of the two oxides varied considerably have been described.

The study of these complex compounds was simplified by the application of the electrical conductivity as an indicator in the neutralisation of acids, and it was found that the phosphomolybdates and other heteropoly-compounds could be satisfactorily formulated by the application of a modification of Werner's co-ordination theory suggested by Miolati and extended by Rosenheim. According to this theory, the complex phosphomolybdic compounds may be divided into two groups, the members of the first group possessing a heptavalent complex anion with phosphorus as the central atom, surrounded by six divalent atoms or groups of atoms. The anion is derived from [PO₆]''', the oxygen atoms being replaced, wholly or in part, by the dimolybdate ion (Mo₂O₇)''. These complex anions are of a deep yellow colour, which is imparted to their salts in the absence of coloured cations. The members of the second group contain colourless anions derived from [PO₄]''', the oxygen being replaced by the ion (MoO₄)''. Compounds are said to be saturated when all the oxygen atoms of the parent anion are replaced by metallic acid anions, unsaturated compounds containing some replaceable oxygen.