Ortho phosphoric Acid

Ortho phosphoric Acid, H3PO4:


1. It is made by dissolving phosphorus pentoxide in water and then boiling the solution.

P2O5 + 3H2O → 2H3PO4

2. Laboratory preparation:

In laboratory orthophosphoric acid can be made by boiling a mixture of red phosphorus with 50 percent nitric acid in a flask fitted with a reflux condenser on a water bath till no more oxides of nitrogen are released.

Iodine performs as a catalyst. The product is evaporated beneath 453 K and then cooled in a vacuum desiccator surrounded by freezing mixture whenever crystals of orthophosphoric acid are deposited.

P + 5HNO3 → H3PO4 + 5NO2 +H2O

Physical properties:

1. This is a deliquescent crystalline solid.
2. This is soluble in water.

Chemical properties:

1. It is a tribasic acid. It joins with alkalies such as NaOH to form three sequences of salts.

H3PO4 + NaOH → NaH2PO4 + H2O
                          Sodium Di hydrogen Phosphate

H3PO4 + 2NaOH → Na2HPO4 + 2H2O
                           Disodium hydrogen Phosphate

H3PO4 + 3NaOH → Na3PO4 + 3H2O
                            Sodium Phosphate

2. On heating, it provides pyrophosphoric acid at 523 K and at 589 K it provides metaphosphoric acid.


3. On reaction with silver nitrate, it provides yellow precipitate of silver phosphate.

H3PO4 + 3AgNO3 → Ag3PO4 +3HNO3


1. It is utilized in the preparation of HBr and HI as a replacement for sulphuric acid.
2. It is utilized as souring agent in the manufacture of soft drinks.
3. It is utilized in the manufacture of phosphate salts of potassium, sodium, and ammonium.
4. It is utilized in the preparation of phosphatic fertilizers.


Being a tribasic acid, the structure of phosphoric acid is symbolized as:

1392_h3po4 struc.jpg


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