1. Ortho Phosphoric Acid (H3PO4) 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
- It is a deliquescent crystalline solid.
- It is soluble in water.
- It is a tribasic acid. It combines with alkalies like NaOH to form three series of salts.
- On heating it gives pyrophosphoric acid at 523 K and at 589 K gives metaphosphoric acid
H3PO4 →(523K)→ H4P2O7 →(589K)→ 2HPO3 + H2O
- On reaction with silver nitrate, it gives yellow precipitate of silver phosphate.
H3PO4 + 3AgNO3 → Ag3PO4 + 3HNO3
- It is used in the preparation of HBr and HI as a substitute for sulphuric acid.
- It is used as souring agent in the preparation of soft drinks.
- It is used in the preparation of phosphate salts of sodium, potassium and ammonium.
- It is used in the manufacture of phosphatic fertilizers.