Oxoacids of Phosphorus

Phosphorus forms a large number of oxoacids.

All these acids are based on tetrahedral four coordinated phosphorus atom containing at least one P = O unit and one P – OH group. Condensed systems are formed by P – O – P linkage or P – P linkage.

The basicity depends on the H atoms bonded to O in the form of – OH.

Some of the common oxoacids of phosphorus are as follows:


Formula Oxidation state of P basicity structure


Hypophosphorus acid

H3PO2 +1 1 Hypophosphorus Acid

white P4 + alkali

Phosphorus acid

H3PO3 +3 2 Metaphosphoric Acid

P2O3 + 3H2O → 2H3PO3

Hypophosphoric acid

H4P2O6 +4 4 Hypophosphoric Acid

red P4 + alkali

Orthophosphoric acid

H3PO4 +5 3 Orthophosphoric Acid

P(red) + 5HNO3 → H2O + H3PO4 + 5NO2
P4O10 + 6H2O → 4H3PO4

Diphosphoric acid (Pyrophosphoric acid)

H4P2O7 +5 4 Diphosphoric Acid

heating phosphoric acid

2H3PO4 → H4P2O7 + H2O

Metaphosphoric acid

HPO3 +5 1 Phosphorus Acid

heating orthophosphoric acid to about 850 K

H3PO4 → HPO3 + H2O

Peroxophosphoric acid

H3PO5 +7 3 Peroxophosphoric Acid

P2O5 + 2H2O2 + H2O → 2H3PO5

Oxides of phosphorous:

1. Phosphorus pentoxide:

It is a chemical compound with molecular formula P4O10 (with its common name derived from its empirical formula, P2O5).

This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

P4O10 is prepared by burning elemental phosphorus with sufficient supply of air:

P4 + 5 O2 → P4O10

Phosphorus Pentoxide

2. Phosphorus trioxide:

It is the chemical compound with the molecular formula P4O6. Although it should properly be named tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound’s molecular structure, and its usage continues today.

It is a colorless solid. It is formally the anhydride of phosphorous acid, H3PO3, but cannot be obtained by the dehydration of the acid. It is a white, waxy, crystalline and highly toxic solid.

It is obtained by the combustion of phosphorus in a limited supply of air at low temperature.

P4(s) + 3 O2 (g) → P4O6(s)