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

Use: It is used as a reducing agent.