Permolybdic Acid

If a solution of a molybdate be treated in acid solution with hydrogen peroxide, oxidation to permolybdic acid takes place, the solution becoming yellow. The colour remains on warming, and cannot be extracted by ether. Moreover, when molybdic anhydride, MoO₃, is dissolved in hydrogen peroxide, a yellow insoluble compound is deposited from the stable, dark yellow solution.

Pechard regarded the compound which he obtained both by double decomposition of the barium salt with sulphuric acid, and by dissolving molybdenum or certain of its oxides in hydrogen peroxide, as HMoO₄.2H₂O. The compound so obtained, although reasonably stable, is an active oxidising agent; hydrogen chloride is, for instance, oxidised to chlorine, but other strong acids are not attacked. Pechard describes several permolybdates, and assigns to them the formulae KMoO₄.2H₂O, NH₄MoO₄.2H₂O, etc., thus indicating their analogy to the persulphates of the type R^•₂S₂O₈; it is not yet certain, however, that compounds of this type (R^•MoO₄) actually exist. In fact, although Cammerer's formula, 2MoO₃.H₂O₂.H₂O, is comparable with that of Pechard, Pissarjewsky has prepared, by Pechard's method, a compound to which he assigns the formula H₂MoO₅.2H₂O. By interaction of molybdenum trioxide with 25 per cent, hydrogen peroxide, another hydrate of the acid H₂MoO₅ - probably 2H₂MoO₅.3H₂O - has been prepared.

From a study of the heat evolution on dissolving molybdic anhydride in varying quantities of hydrogen peroxide, and of the distribution of hydrogen peroxide between ether and aqueous molybdic acid, it is indicated that at least two permolybdic acids exist in acid solutions: (a) H₂MoO₅ or MoO₃.H₂O₂, and (b) H₂MoO₆. The constitution of these two compounds is not established. The molybdenum appears to be in the hexavalent state, and the following alternative structural formulae have been suggested:

(a) or ,

(b) or .

As to the permolybdates, potassium, rubidium, caesium, ammonium, and barium salts have been prepared. The formulae ascribed are somewhat complex; for example, for the orange-red ammonium salt (obtained by the action of hydrogen peroxide on a solution of ordinary ammonium molybdate), 3(NH₄)₂O.7MoO₄.12H₂O, and for the lemon- yellow ammonium salt (obtained by concentration of the mother-liquor of the orange-red salt), 3(NH₄)₂O.5MoO₃.2MoO₄.6H₂O. The aqueous solution of these salts contains hydrogen peroxide, and is readily decomposed by small quantities of alkali. Another salt, K₂O₂.MoO₃.H₂O₂, or K₂MoO₆.H₂O, has been prepared as an unstable solid evolving oxygen on treatment with water, by interaction of cooled solutions of hydrogen peroxide, potassium hydroxide, and Pechard's potassium permolybdate, and precipitation at a low temperature with alcohol.

Fluoropermolybdates, and oxalo- and other organo- salts of the peroxide, have also been described.