Phosphorous acid, with the molecular formula H3PO3, is a strong dicarboxylic acid, not a monocarboxylic acid. It is characterized by good water solubility, high reducing power, strong hygroscopicity, and easy oxidation. Phosphorous acid is a colorless, transparent orthorhombic crystal with a garlic-like odor. Cooling the molten material with seed crystals yields large biprismatic crystals. Its melting point is 74℃. Its five physicochemical properties are as follows:

1. Strong Reducing Power:

The phosphorus in H3PO3 has a +3 oxidation state, resulting in strong reducing power and relative stability in the solid state. It readily reduces metal ions such as Ag, Ni, and Cu to their metallic states and can also reduce concentrated sulfuric acid to sulfur dioxide.

2. Diverse Raw Materials and Processes:

Phosphorous acid with a purity of over 99% is generally prepared by the hydrolysis of PCl3, with the reaction formula: PCl3 + 3H2O → H3PO3 + 3HCl. For a purity of 98%, it can generally be prepared by recovering byproducts such as dimethyl ester residue, pentanoyl chloride, and propionyl chloride.

3. Highly Hygroscopic and Easily Soluble in Water:

Solubility of Phosphorous Acid in Water at Different Temperatures:

4. Relatively Strong Acidity:

Phosphorous acid typically exhibits dicarboxylic acid properties, slightly stronger than phosphoric acid but weaker than hypophosphorous acid. H3PO3 is a dicarboxylic acid with two ionizable hydroxyl hydrogen atoms, while H3PO4 is a tricarboxylic acid. However, the hydrogen atoms directly bonded to phosphoric acid (H3PO3) do not ionize, making it a stronger acid than phosphoric acid. This is attributed to the different intramolecular electronic effects. Only the two hydroxyl hydrogen atoms bonded to oxygen (O) are ionizable, making it a dicarboxylic acid. The P-H bond, due to its high bond energy and weak polarity, exhibits almost no acidity and cannot ionize like the hydrogen atoms in carboxylic acids.

5. Heat of Solution:

The heat of solution is 29.6 kJ/ml, the heat of formation is 94.05 kJ/mol, and the heat of formation for aqueous solutions is 952.8 kJ/mol. The heat of solution for phosphorous acid (H₃PO₃) with excess water is 975.6 kJ/mol, and for H3PO3·3H2O it is 636.4 kJ/mol.

In summary, phosphorous acid is a reducing, highly soluble, and strongly acidic dicarboxylic inorganic acid with a wide range of applications. It is a major raw material for the production of potassium phosphite, potassium dihydrogen phosphite, aluminum phosphite, and calcium hydrogen phosphite.