Laboratory Preparation of Ortho Phosphoric Acid


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


Physical properties

  • It is a deliquescent crystalline solid.
  • It is soluble in water.

Chemical properties

  • 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.

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