Ammonium Magnesium Orthophosphate, NH₄MgPO₄

Fourcroy precipitated a solution of a magnesium salt with ammonium phosphate or with sodium phosphate in the presence of an ammonium salt and ammonia. Precipitation is incomplete without excess of the alkaline phosphate and of ammonia to neutralise the acid liberated during the reaction -

MgCl₂+NH₄Cl+Na₂HPO₄ = NH₄MgPO₄+2NaCl+HCl.

The salt is also less soluble in ammonia than in water: between 20.5° C. and 22.5° C., 1 part of the double phosphate dissolves in 13,497 parts of water and in 60,883 parts of ammonia solution of density 0.961. Precipitation is only complete on standing or after vigorous shaking.

The hexahydrate usually precipitates in the cold, but is transformed into the monohydrate at 47°-48° C. in contact with water, and the degree of hydration of the precipitate varies with the temperature and the composition of the liquid.

Rhombic crystals of the hexahydrate occur as struvite, with a hardness of 21 and a density of 1.65. It also occurs in guano, in urinary calculi, in gallstones, and crystallises from alkaline urine.

The hexahydrate crystallises in various forms of the cubic system, has an alkaline reaction and an insipid taste.

Cubical crystals of the monohydrate, stable at 100° C., have been obtained by the action of neutral ammonium phosphate on magnesium sulphate. The hexahydrate does not alter in air up to 30° C., and, though Graham said it dried to the monohydrate, does not seem to dry to definite composition at 100° C.

Ammonium magnesium phosphate ignites to the pyrophosphate of magnesium with the evolution of ammonia and water.