Dissociation of phosphoric acid and its proton balance equation.

Phosphoric acid is a triprotic acid. It dissociates in solution in three steps as shown in the following formulas:

H₃PO₄ + H₂O ↔ H₂PO₄^- +H₃O⁺

H₂PO₄^-  + H₂O ↔ HPO₄^-2 +H₃O⁺

HPO₄^-2  + H₂O ↔ PO₄^-3 +H₃O⁺

A proton balance equation for phosphoric acid added to water can be produced as described in the following diagram:

The specie H₂PO₄^- is produced after donation of one proton. However, HPO₄^-2 and PO₄^-3 are produced after donation of 2 and 3 protons respectively. This is accounted for in the equation:

[H3O+] = [H2PO4- ] + 2× [HPO4-2 ] +  3×[PO4-3 ] + [OH-]

Related post:

Proton balance equation for weak monoprotic acids.