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Ammonium Paramolybdate, (NH4)6Mo7O24.4H2O

Ammonium Paramolybdate (ordinary ammonium molybdate), (NH4)6Mo7O24.4H2O, is the salt which is obtained by evaporation and crystallisation of a solution of molybdic anhydride in ammonia. It forms large crystals, usually bluish in colour owing to the presence of a lower oxide. The substance is soluble in water; its aqueous solution is stated apparently to contain ions of Mo6O228', formed by hydrolysis of a portion of the Mo12O4110' ions. (It is here assumed that the formula of the solid salt is 5(NH4)2O.12MoO3.7H2O.) The solution is completely reduced by hydrogen in presence of colloidal palladium at ordinary temperatures to molybdenum tetrahydroxide; the reduction goes further if a slight pressure is used and the solution is gently heated, a precipitate of the trihydroxide being formed. The paramolybdate is stable in air; on heating it finally leaves a residue of molybdenum trioxide. From the mother-liquor from which the tetrahydrate forms, a salt, (NH4)6Mo7O24.12H2O, may be obtained. From the nitric acid solution of ammonium molybdate, which contains also acid ammonium molybdate and molybdic acid, there separate on standing hydrates of molybdic anhydride.
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