Three reactions involving $ H_2 PO^- _ 4 $ are given below.
(i) $ H_3 PO_4 + H_2O \rightleftharpoons H_3 O^+ + H_2 PO^- _4 $
(ii) $ H_2 PO^- _4 + H_2 O \rightleftharpoons HPO^{-2} _4 + H^+ _3 O$
(iii) $ H_2 PO^- _4 + OH^- \rightleftharpoons H_3 PO_4 + O^{2-} $
In which of the above does $H_2PO^- _ 4 $ act as an acid.
According to lowry-bronsted acid base theory in (ii) reaction $ H_2 PO_4 ^- donates H^+ ion to H_2 O$ it acts as an acid.