Oxy-Acids of Phosphorus
I. Phosphorus acid - H3PO3
It is prepared by the action of cold water on phosphorus (III) oxide or phosphorus (III) chloride.
P2O3 + 3H2O ® 2H3PO3
PCl3 + 3H2O ® H3PO3 + 3HCl
It is a white crystalline solid with garlic taste.
1. Acidic nature: It is a dibasic acid and gives salts of two types.
H3PO3 + NaOH ® NaH2PO3 + H2O
Sodium dihydrogen Phosphite
H3PO3 + 2NaOH ® Na2HPO3 + 2H2O
Disodium hydrogen Phosphite
2. When it is heated it undergoes auto-oxidation and reduction to form
phosphoric acid and phosphine.
4H3 PO3 ® 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
Use: It is used as a reducing agent
II. Ortho phosphoric Acid, H3PO4
1. It is prepared by dissolving phosphorus pentoxide in water and boiling the solution.
P2O5 + 3H2O ® 2H3PO4
2. Laboratory preparation: In the laboratory orthophosphoric acid can be
prepared by boiling a mixture of red phosphorus with 50% nitric acid in a flask fitted with a reflux condenser on a water bath till no more oxides of nitrogen are liberated.
Iodine acts as a catalyst. The product is evaporated below 453 K and then
cooled in a vaccum desiccator surrounded by freezing mixture when crystals of orthophosphoric acid are deposited.
P+5HNO3 ® H3PO4 +5NO2 +H2O
1. It is a deliquescent crystalline solid.
2. It is soluble in water.
1. It is a tribasic acid. It combines with alkalies like NaOH to form three series
H3PO4 +NaOH ® NaH2PO4 + H2O
Sodium Di hydrogen Phosphate
H3PO4 +2NaOH ® Na2HPO4 + 2H2O
Disodium hydrogen Phosphate
H3PO4 + 3NaOH ® Na3PO4 + 3H2O
2. On heating it gives pyrophosphoric acid at 523 K and at 589 K gives
H3PO4 H4 P2O7 2HPO3 + H2O
3. On reaction with silver nitrate, it gives yellow precipitate of silver phosphate.
H3PO4 + 3AgNO3 ® Ag3PO4+3HNO3
1. It is used in the preparation of HBr and HI as a substitute for sulphuric acid.
2. It is used as souring agent in the preparation of soft drinks.
3. It is used in the preparation of phosphate salts of sodium, potassium and ammonium.
4. It is used in the manufacture of phosphatic fertilisers.
Being a tribasic acid, the structure of phosphoric acid is represented as
III. B. Pyrophosphoric acid, H4 P2 O7
Preparation: Pyrophosphoric acid is prepared by heating orthophosphoric
acid to 523 K - 533 K.
2H3PO4 ® H4P2O7 + H2O
It is a colourless crystalline solid.
1. It is reconverted to orthophosphoric acid on boiling with water
H4P2O7 + H2O ® 2H3PO4
2. When heated strongly, it yields metaphosphoric acid
H4P2O7 2HPO3 + H2O
Structure: The Structure of pyrophosphoric acid is represented as: