Ammonium phosphomolybdate

Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula (NH4)3PMo12O40. The salt contains the phosphomolybdate anion, a well known heteropolymetalate of the Keggin structural class.

Synthesis
Ammonium phosphomolybdate can be made by heating ammonium orthomolybdate combined with phosphoric acid and nitric acid, yielding ammonium nitrate, water, and a yellow precipitate of ammonium phosphomolybdate is obtained.
 * 12 (NH4)6Mo7O24(H2O)4 + 7 Na2HPO4(H2O)  +  65 HNO3  → 7 (NH4)3Mo12PO40 +  51 NH4NO3 + 14 NaNO3  + 56 H2O

Normally, it often exists as a hexahydrate, a dark yellow fine crystal which is poorly soluble in water.

It is also obtained as a test result for phosphate ions.