Phosphorous acid is an inorganic oxyacid of phosphorus with the empirical formula H3PO3. It is commonly referred to as phosphorus(III) acid to emphasize the +3 oxidation state of phosphorus in the molecule. Unlike phosphoric acid (H3PO4), phosphorous acid contains a phosphorus–hydrogen bond and therefore behaves differently in acid–base and redox chemistry. For a concise overview of data and properties see chemical properties.

Structure and acidity

In condensed form the compound exists predominantly in the tautomeric form HPO(OH)2, which is often written as H3PO3 for simplicity. This arrangement gives two hydroxyl (–OH) groups and one P–H bond. Because only the protons on the hydroxyl groups are acidic, phosphorous acid is effectively dibasic (it can donate two protons) and forms salts known collectively as phosphites. For more on related anions and salts consult phosphites and derivatives.

Preparation and reactions

Industrial and laboratory preparation commonly involves the controlled hydrolysis of phosphorus trichloride (PCl3) in water, yielding phosphorous acid and hydrogen chloride. The substance is a mild reducing agent and is readily oxidized to phosphoric acid (P(V)) by strong oxidants. It reacts with bases to give phosphite salts and can be converted into organophosphorus compounds in synthetic chemistry. Typical synthesis and reaction pathways are summarized at preparation methods.

Uses and importance

  • Precursor to phosphonates and other organophosphorus compounds used in agriculture and materials chemistry.
  • Source of phosphite anions in the manufacture of corrosion inhibitors and metal treatment additives.
  • Applied in crop protection formulations (as salts or derivatives) where reduced phosphorus species are desired.

Its role as a reducing agent and as a building block for specialty chemicals makes phosphorous acid valuable in both industrial and research contexts. For safety and handling guidance see safety information.

Hazards and handling: Phosphorous acid is corrosive and can be harmful if inhaled, swallowed, or in contact with skin. It reacts with strong oxidizers and should be handled with appropriate personal protective equipment and storage precautions.