Sodium phosphite is a reducing agent, an alkaline chemical with a pH of 9-10. Phosphorus has a +3 oxidation state. Its molecular formula is Na2HPO3·5H2O. It is a colorless orthorhombic crystal, hygroscopic, and has a melting point of 53℃. Its crystal axial ratio is a:b:c = 0.6998:1:0.7813. Sodium phosphite has strong reducing properties and can precipitate metals from hot aqueous solutions of various metal salts. Dilute solutions are quite stable, but boiling it with caustic soda produces hydrogen gas: Na2HPO3+ NaOH→Na3PO4+ H2. It can be dehydrated into anhydrous salt by placing it in a vacuum, evaporating it with concentrated sulfuric acid, or heating it to 120℃. Above 200℃, it can be converted into sodium pyrophosphate. When heated with a hot air stream, a small amount of phosphine is released at 200-250℃. Its chemical properties differ significantly from sodium hypophosphite or sodium phosphate; it is a completely different inorganic salt.

There are two main methods for producing sodium phosphite: neutralization and metathesis. The principles of these two methods are as follows:

Neutralization: Phosphorous acid reacts with sodium hydroxide in a neutralization reaction:

H3PO3+ 2NaOH→Na2HPO3+ 2H2O

Metathesis: Calcium phosphite reacts with sodium sulfate (or sodium carbonate) in a metathesis reaction:

CaHPO3 + Na2SO4→ Na2HPO3+ CaSO₄

CaHPO3+ Na2CO3→Na2HPO3+ CaCO3

Currently, the first method is generally used in the industrial production of sodium phosphite. However, it should be noted that due to the diverse raw materials used in the preparation of phosphorous acid, the quality of the finished product varies greatly, resulting in significant differences in quality and price. For export or processes requiring higher precision, export-grade sodium phosphite should be selected.