Three reactions involving $H_2 PO_4^{-}$ are given below :$(i)H_3PO_4+H_2O \to H_3O^{+} +H_2PO_4^{-} \qquad (ii) H_2PO_4^{-}+H_2O \to HPO_4^{2-}+H_3O^{+} \qquad (iii)H_2PO_4^{-}+OH^{-} \to H_3PO_4+O^{2-}$ <br>In which of the above does $H_2PO_4^{-}$ act as an acid ?
