H3PO3 - Phosphorous Acid

What is Phosphorous acid?

Phosphorous acid which is also called phosphonic acid is a colourless oxyacids of phosphorus. It is produced in the form of a white volatile powder by the slow combustion of phosphorus. Its salts are called phosphites. It is conveniently prepared by allowing phosphorous trichloride to react with water. In industrial synthesis PCl3 is sprayed into steam at 190oC the heat of reaction is used to distill off the hydrogen chloride and excess water vapour.

Other names – trihydroxy-Orthophosphorus acid


Phosphorous Acid


1.65 g/cm³

Molecular Weight/ Molar Mass

82 g/mol

Boiling Point

200 °C

Melting Point

73.6 °C

Chemical Formula


Phosphorous Acid Structure – H3PO3

Phosphorous Acid Structure

Physical Properties of Phosphorous Acid – H3PO3


Sour odour


White solid, deliquescent

Covalently-Bonded Unit


Hydrogen Bond Acceptor





Soluble in water

Chemical Properties of Phosphorous Acid – H3PO3

  • Phosphorous acid has strong reducing properties it tends to be converted to phosphoric acid. On being heated dry phosphorous acid disproportionates to give phosphine and phosphoric acid.
  • H3PO3 + 3H3PO3 → PH3 + 3H3PO4

  • Phosphorous acid reacts with a base like sodium hydroxide forms sodium phosphate and water.
  • H3PO3 + 3NaOH → Na3PO3 + 3H2O

Uses of Phosphorous Acid – H3PO3

  • Used in the production of basic lead phosphonate PVC stabilizer, aminomethylene phosphonic acid and hydroxyethane diphosphonic acid.
  • Used as a strong reducing agent.
  • Used in the production of raw materials of phosphorous acid, synthetic fibres and organophosphorus pesticides etc.
  • Used in the production of high efficient water treatment agent amino trimethylene phosphonic acid.

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