Note 9 tri — Chapter XXI · CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE
George Kamensky translation
When peroxide of hydrogen acts upon a solution of potassium molybdate well-formed yellow crystals belonging to the triclinic system separate out in the cold. When these crystals are heated in vacuo they first lose water and then decompose, leaving a residue composed of the salt originally taken. They are soluble in water but insoluble in alcohol. Their composition is represented by the formula K2Mo2O82H2O. An ammonium salt is obtained by evaporating peroxide of hydrogen with ammonium molybdate. The following salts have also been obtained by the action of peroxide of hydrogen upon the corresponding molybdates: Na2Mo2O66H2O—in yellow prismatic crystals; MgMo2O810H2O—stellar needles; BaMoO82H2O—in microscopic yellow octahedra. A corresponding sodium pertungstate has been obtained by Péchard by boiling sodium tungstate with a solution of peroxide of hydrogen for several minutes. The solution rapidly turns yellow, and no longer gives a precipitate of tungstic anhydride when treated with nitric acid. When evaporated in vacuo the solution leaves a thick syrupy liquid from which ray-like crystals separate out; these crystals are more soluble in water than the salt originally taken. When heated they also lose water and oxygen. Their composition answers to the formula M2W2O82H2O, where M = Na, NH4, &c. The permolybdates and pertungstates have similar properties. When treated with oxygen acids they give peroxide of hydrogen, and disengage chlorine and iodine from hydrochloric acid and potassium iodide.
Piccini (1891) showed that peroxide of hydrogen not only combines with the oxygen compounds of Mo and W, but also with their fluo-compounds, among which ammonium fluo-molybdate MoO2F22NH4 and others have long been known. (A few new salts of similar composition have been obtained by F. Moureu in 1893.) The action of peroxide of hydrogen upon these compounds gives salts containing a larger amount of oxygen; for instance, a solution of MoO2F22KFH2O with peroxide of hydrogen gives a yellow solution which after cooling separates out yellow crystalline flakes of MoO3F22KFH2O, resembling the salt originally taken in their external appearance. By employing a similar method Piccini also obtained: MoO3F22RbFH2O—yellow monoclinic crystals; MoO3F2,2CsFH2O,—yellow flakes, and the corresponding tungstic compounds. All these salts react like peroxide of hydrogen.
In speaking of these compounds I for my part think it may be well to call attention to the fact that, in the first place, the composition of Piccini's oxy-fluo compounds does not correspond to that of permolybdic and pertungstic acid. If the latter be expressed by formulæ with one equivalent of an element, they will be HMoO4 and HWO4, and the oxy-fluo form corresponding to them should have the composition MoO3F and WO3F while it contains MO3F2 and WO3F2, i.e. answers as it were to a higher degree of oxidation, MoH2O3 and W3HO3. But if permolybdic acid be regarded as 2MoO3 + H2O2, i.e. as containing the elements of peroxide of hydrogen, then Piccini's compound will also be found to contain the original salts + H2O; for example, from MoO2F22KFH2O there is obtained a compound MoO2F22KFH2O2, i.e. instead of H2O they contain H2O2. In the second place the capacity of the salts of molybdenum and tungsten to retain a further amount of oxygen or H2O2 probably bears some relation to their property of giving complex acids and of polymerising which has been considered in Note 8 bis. There is, however, a great chemical interest in the accumulation of data respecting these high peroxide compounds corresponding to molybdic and tungstic acids. With regard to the peroxide form of uranium, see Chapter XX., Note 66.