Question

Why does orthophosphoric acid form three types of salts? Explain its ionisation behaviour.

Answer 1

Orthophosphoric acid (H₃PO₄) forms three types of salts because it is a tribasic acid containing three ionisable \[\ce{-OH}\] groups attached to phosphorus.

It ionises stepwise, releasing three H⁺ ions:

First ionisation: \[\ce{H3PO4 <=> H+ + H2PO^-_4}\]

Second ionisation: \[\ce{H2PO^-_4 <=> H+ + HPO^{2-}_4}\]

Third ionisation: \[\ce{HPO^{2-}_4 <=> H+ + PO^{3-}_4}\]

This stepwise ionisation results in three series of salts: acidic, normal, and basic phosphates.