Double Phosphates of Magnesium

Microscopic prisms of NaMgPO₄.9H₂O were obtained by crystallising a solution of magnesium oxide in sodium dihydrogen phosphate. It becomes the monohydrate at 110° C. Rhombic plates of KMgPO₄.6H₂O are prepared analogously. It becomes the monohydrate at 110° C. and anhydrous on calcination. Water decomposes it.

Ouvrard obtained the double metaphosphate KMg(PO₃)₃ in prisms of density 2.4 at 20° C. and soluble in acids. Various other double phosphates with the alkali metals have been described – including RbMgPO₄.6H₂O, which was obtained by precipitating mixed solutions of magnesium and rubidium chlorides with disodium hydrogen phosphate. Unlike the corresponding potassium salt, it is stable to water.