Lithium orthophosphate, Li₃PO₄

The orthophosphate is precipitated by addition of a soluble phosphate to a boiling solution of a lithium salt, the formula of the product according to Mayer being 2Li₃PO₄,H₂O. The dihydrate, Li₃PO₄,2H₂O, is also known, and at 60° C. is slowly transformed into the semihydrate. Crystallization of the semihydrate from fused lithium chloride yields the anhydrous salt in crystals melting about 857° C., and with the density 2.41 at 15° C. The crystalline anhydrous salt is also obtained by fusing lithium carbonate with potassium phosphate. At 25° C. one litre of water dissolves 0.297 gram of Li₃PO₄.

Dilithium hydrogen phosphate has not been isolated, but Rammelsberg prepared lithium dihydrogen phosphate, LiH₂PO₄, by concentration of an acid solution containing lithium and phosphoric acid. The hygroscopic salt dissolves readily in water, and is converted at 200° to 250° C. into the glass-like metaphosphate, LiPO₃. On slow evaporation, phosphoric-acid solutions of the dihydrogen phosphate yield deliquescent crystals of the acid phosphate, LiH₂PO₄,H₃PO₄,H₂O.