Application
Calcium phosphite, as a non-toxic, dual-function rust-preventive pigment, is widely used in the paint and coating industry.
Description
Calcium phosphite can have the molecular formula of
CaHPO3 or Ca(H2PO3)2. It is the former that appears
when a solution of phosphorous acid is reacted with
the oxide, hydroxide or carbonate:
CaO +H3PO3 ? CaHPO3·H2O +H2O
The product is a monohydrate. It can also be prepared
by the action of ammonium phosphite on calcium
chloride:
CaCl2 (aq) +NH4HPO3 (aq)
? CaHPO3·H2O +NH4Cl2 (aq)
A white, crystalline powder is obtained. The solubility
of CaHPO3·H2O has apparently not been determined
exactly. If heated to about 250°C, the anhydrate
results. This is stable when heated further up to about
320°C where it decomposes to form phosphine and
hydrogen:
3CaHPO3 + heat ? 2PH3 + 3Ca(PO3)2 +H2
Physical properties
Calcium Phosphite is a colorless,
monoclinic crystal which loses water at about 150°C
and is slightly soluble in water but insoluble in alcohol.
The anhydrate, calcium phosphite (CaHPO3), is stable in
air even when heated at 95°C for 2 months.
