Potassium Paramolybdate, K₆Mo₇O₂₄.4H₂O

Potassium Paramolybdate, K₆Mo₇O₂₄.4H₂O, is readily obtained (1) by evaporating a solution of molybdenum trioxide in excess of aqueous potassium carbonate, redissolving the dry residue in the smallest possible quantity of hot water, and crystallising; or (2) by precipitating such a solution by means of concentrated nitric acid or sulphuric acid. The milky liquid obtained by adding sufficient acid to redissolve the precipitate at first formed slowly deposits crystals of the salt, which cannot be purified by recrystallisation owing to the ease with which they decompose, yielding the trimolybdate and normal molybdate.

Potassium paramolybdate crystallises in prisms isomorphous with the corresponding ammonium salt. The fused anhydrous salt is but slightly soluble in water.