Phosphorous acid is a dibasic inorganic acid with reducing properties, no harmful residues, hygroscopicity, deliquescence, and corrosiveness. It has a wide range of applications in plastics, fibers, pesticides, and inorganic salt production, and can be formulated into various solid, liquid, and concentration formulations. Phosphorous acid is one of the important raw materials for the water treatment agent aminotrimethylphosphonic acid (ATMP). It has the following six characteristics:

1. It has strong hygroscopicity, deliquescence, and corrosiveness. Strict requirements apply to its packaging and storage. Avoid storing it in humid conditions and use corrosion-resistant packaging materials. H3PO3 is composed of countless tiny crystalline particles with a very large specific surface area. This large surface area means more "contact points" exposed to the air, providing numerous adsorption sites for water molecules. Another reason is that water molecules are polar molecules, with one end carrying a partial positive charge and the other a partial negative charge. 1. Phosphorous acid crystals exhibit strong attraction between the cations on their surface and the oxygen ends of water molecules, while the anions attract the hydrogen ends. This attraction is strong enough to firmly adsorb water molecules onto the crystal surface, forming a hydrated layer. If this effect continues, the hydrated layer thickens, causing the crystal to gradually deliquesce and eventually dissolve completely to form a solution.

2. Phosphorous acid can react with strong bases, such as sodium hydroxide and potassium hydroxide. Therefore, it should generally be stored separately from alkaline substances.

3. It leaves no harmful residue after use. Prolonged exposure to air will gradually oxidize it into orthophosphoric acid or orthophosphate, which is a harmless chemical.

4. It can be used to produce acid salts and normal salts. For example, potassium dihydrogen phosphite (KH₂PO₃) is an acid salt, while sodium phosphite (Na₂HPO₃) is a normal salt.

5. It is a diprotic inorganic acid. Although its molecular structure contains three hydrogen atoms (H), only two hydroxyl groups (-OH) can ionize to produce the beneficial hydrogen ion (+H), thus classifying it as a diprotic inorganic acid.

6. Reducing Properties

The +3 valent phosphorus content is as high as 37.8%, indicating strong reducing properties, although slightly weaker than hypophosphite. It can reduce silver ions to elemental silver and divalent mercury ions to monovalent mercury ions or mercury itself. The reducing power of phosphorous acid comes from the phosphorus atom (P-H bond) directly bonded to a hydrogen atom in its molecule. This hydrogen atom is highly reactive and easily removed, leading to oxidation.

The production process of phosphorous acid involves numerous raw materials, resulting in significant variations in product quality. Currently, phosphorous acid produced using the phosphorus trichloride process is of much higher quality than that produced using other byproduct processes.