MAGNESIUM PHOSPHITE

Description

Magnesium phosphite can have the molecular formula of the hydrogen phosphite, MgHPO3 or the dihydrogen phosphite, Mg(H2PO3)2. It is the former that appears when a solution of phosphorous acid is reacted with the oxide, hydroxide or carbonate:
MgO +H3PO3 ? MgHPO3 +H2O
The product is a hexahydrate. It can also be prepared by the action of ammonium phosphite on magnesium chloride:
MgCl2 (aq) +NH4HPO3 (aq) ? MgHPO3·6H2O +NH4Cl2 (aq)
If a sodium phosphite is used in the preparation, then the dihydrogen phosphite, Mg(H2PO3)2, is produced.

Physical properties

The nature of the hydrates of these hydrogenphosphite salts is controlled, to some extent, by the solution temperature employed. The hexahydrate, MgHPO3·6H2O, is obtained at room temperature (15–25°C). At 30–50°C, the tri-hydrate occurs whereas at >85°C, the monohydrate is most stable. This is due in part to the solubilities of these three salts. The hexahydrate is rather soluble at 16.4 g/100 ml of water (20°C) whereas the trihydrate is most stable at 50°C. It has been shown that at reaction temperatures less than about 60°C a crystalline magnesium hydrogen phosphite trihydrate, MgHPO3·3H2O, whose solubility increased from 8.9 at 20°C to 11.2 g/100 ml at 50°C and then to 6.5 at 95°C temperature.