Phosphorous acid(H3PO3) is an oxyacid with phosphorus oxidation state +3. As H3PO3 is a dicarboxylic acid, various phosphites can be prepared, such as sodium phosphite, potassium phosphite, ammonium phosphite, manganese phosphite, calcium phosphite, and dicalcium phosphite. Among these, sodium phosphite and potassium phosphite are particularly important. Phosphorous acid(H3PO3) and its salts are mainly used as reducing agents in the plastics, pharmaceutical, and coating industries. They can be used to manufacture phosphites, phosphite esters, organophosphorus compounds, and organophosphorus pesticides. They can also be used as an antioxidant for nylon 1010, a stabilizer for synthetic fibers and polycarbonate, a heat stabilizer for polyethylene terephthalate, and a whitening agent for nylon.

I. Structure and Properties of H3PO3

1. Phosphorous acid(H3PO3) is an oxyacid, with the molecular formula H3PO3 and a relative molecular mass of 82. The molecular structure of H3PO3 contains a tetrahedral anion with one hydrogen atom directly bonded to a phosphorus atom, and the other two hydroxyl groups bonded to the phosphorus atom. Therefore, H3PO3 is a diprotic moderately strong acid and exhibits tautomerism, but the equilibrium is strongly biased to the right.

2. H3PO3 is a white crystalline solid with a garlic-like odor. Its density is 1.651 kg/L (21.2℃), and its melting point is 73.6℃. It is highly hygroscopic and deliquescent, and readily soluble in water and alcohol. At 20℃, 425 g of H3PO3 dissolves in 100 g of water. It slowly oxidizes in air to orthophosphoric acid, and decomposes into orthophosphoric acid and the harmful pH3 when heated to 180℃. In the presence of water, it forms orthophosphoric acid and releases hydrogen gas. Under certain conditions, heating to 110℃ causes dehydration to form phosphorous pyrophosphorous acid. H3PO3 is slightly stronger than phosphoric acid. It has strong reducing properties, readily reducing silver ions to metallic silver and sulfuric acid to sulfur dioxide. It is corrosive. Phosphorous acid can be oxidized to orthophosphoric acid by dilute nitric acid, halogens, and other oxidizing agents, but pure phosphorous acid cannot be oxidized by oxygen in the air unless the reaction occurs under the presence of iodine and light. Phosphorous acid (H3PO3) can react with HCHO and NH4Cl to produce acryloyloxytetramethylpiperidine, which is a good water treatment agent.

II. Preparation Principle

1. Hydrolysis of Phosphorus Trichloride Method

Industrially, phosphorous acid(H3PO3) is prepared by dissolving phosphorus trichloride in concentrated hydrochloric acid, followed by evaporation of the hydrochloric acid at low temperature:

PCl3 + 3H2O=H3PO3 + 3HCl

2. Electrolysis Method

Prepared by electrolysis of water using sodium phosphite as raw material; this can be considered as the reverse reaction of acid-base neutralization.