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 Mo₆O₂₂^8', formed by hydrolysis of a portion of the Mo₁₂O₄₁^10' 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.