Example 6

Solution To make an amphiprotic species behave as an acid requires a fairly good proton acceptor. Conversely, to make it behave as a base requires a proton donor.

a)
Acid:       H₂PO₄^– + OH^– ? HPO₄^– + H₂O

Base:      H₂PO₄^– + H₃O⁺ ? H₃PO₄ + H₂O

b)
Acid:       H₂O + S^2– ? OH^– + HS^–

Base:      H₂O + H₂SO₄ ? H₃O⁺ + HSO₄^–