Acid Magnesium Phosphates, MgHPO₄

MgHPO₄.14H₂O separates from cold, dilute mixed solutions of magnesium sulphate and sodium phosphate in insoluble, tasteless, silky needles. The disodium salt precipitates it from magnesium acetate solution; it also precipitates an acid phosphate from magnesium sulphate,. which has been said to be the pentahydrate. Crystalline MgHPO₄.9H₂O has been obtained by digesting phosphoric acid in the cold with magnesium carbonate.

MgHPO₄.14H₂O becomes the hexahydrate at 100° C., and, at ordinary temperatures, dries gradually into a mixture of the hexa- and tri-hydrate, The trihydrate is said to be the stable salt in equilibrium with relatively dilute solutions of phosphoric acid.

A dihydrate has been said to result when ammonium magnesium phosphate is heated with alcohol -

NH₄MgPO₄.6H₂O ⇔ MgHPO₄.2(H₂O)+4H₂O+NH₄.

A monohydrate was obtained by heating a solution of magnesium carbonate in excess of phosphoric acid at 225° C. in sealed tubes. Its microscopic, hexagonal, lamellated crystals dissolve in dilute acids and do not change at 100° C.

Hot water decomposes the hexahydrate into insoluble Mg₃(PO₄)₂.5H₂O and soluble Mg(H₂PO₄). The dihydrate of the latter is obtained by heating magnesia on the water-bath with concentrated phosphoric acid, and cooling. It dissolves readily in water without decomposition.