The last H+ attached to the PO4--- ion is held very tightly, it will not dissociate to form an acid. If the pH of the solution is neutral, the HPO4-- ion will take an H+ from the water (forming H2PO4-), leaving OH- ions behind. So it will form a base in water.
-
It is amphoteric (a buffer) since it is the conjugate base of a weak acid:
H3O+ + HPO4= ---------> H2PO4- + H2O
Since it still has hydrogen it can serve as an acid, proton donor:
HPO4= + OH- ------------> H2O + PO4^-3
Thus in aqueous solution it acts either way depending upon whether acid or base is added.
H3O+ + HPO4= ---------> H2PO4- + H2O
Since it still has hydrogen it can serve as an acid, proton donor:
HPO4= + OH- ------------> H2O + PO4^-3
Thus in aqueous solution it acts either way depending upon whether acid or base is added.