Laboratory Preparation of Phosphorus Acid

Phosphorus Acid (H3PO3) is prepared by the action of cold water on phosphorus (III) oxide or phosphorus (III) chloride.

  • P2 O3 + 3H2O → 2H3PO3
  • PCl3 + 3H2O → H3PO3 + 3HCl


Physical properties

It is a white crystalline solid with garlic taste.

Chemical Properties

1. Acidic nature: It is a dibasic acid and gives salts of two types.

  • H3PO3 + NaOH → NaH2PO3 (Sodium dihydrogen Phosphite) + H2O
  • H3PO3 + 2NaOH → Na2HPO3 (Disodium hydrogen Phosphite) + 2H2O

2. When it is heated it undergoes auto-oxidation and reduction to form phosphoric acid and phosphine.

  • 4H3PO3 → 3H3PO4 + PH3

3. It is a powerful reducing agent because it has P-H bond. It reduces silver nitrate solution into silver.

  • 2AgNO3 + H3PO3 + H2O → 2Ag +H3PO4 +2HNO3

Electronic structure

a2Use: It is used as a reducing agent.