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.
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.
