MONOBASIC SALT

Description

This is prepared by dissolving the oxide, the hydroxide or the carbonate in excess phosphoric acid. The best acid to cation ratio appears to be at least 2.5 to 1.0. If the pH is allowed to climb more than about 3.5, the insoluble dibasic salt begins to appear. Addition of acid causes it to redissolve. All alkaline earth monobasic salts are water soluble so that the solution must be evaporated to obtain the salt. The result is the monobasic salt, Mg(H2PO4)2·3H2O. Its molecular weight is 175.3025 g/mol while that of the anhydrate is 121.2563 g/mol. Its CAS number is 13092-66-5. It is also known as: magnesium biphosphate; primary magnesium phosphate; acid magnesium phosphate; magnesium tetrahydrogen phosphate.
Monobasic magnesium phosphate decomposes upon heating and loses waters of hydration at 145, 168 and 173°C, respectively. The formation of the anhydrate is complete by 200°C. The conversion to the acid pyrophosphate, MgH2P2O7 is complete by 450°C. This compound then forms a metaphosphate glass whose composition depends upon the cycle of heating.