Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of the acid has three acidic protons. In solution, the acid dissociates according to the 3 acid equilibria given below. Some of the species are acids or conjugate acids, some are bases or conjugate bases, and some are conjugate bases in one equilibrium equation and then acids in another! These species are called "amphoteric." Look at each species in the equilibria expressions and see if a species appears in 1) more than one equilibrium and 2) is a base in one and an acid in the other. Recall how water can be both an acid or a base and is thus amphoteric.

H3PO4(aq)↽−−⇀H+(aq)+H2PO−4(aq)

H2PO−4(aq)↽−−⇀HPO2−4(aq)+H+(aq)

HPO2−4(aq)↽−−⇀H+(aq)+PO3−4(aq)