Three reactions involving ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$are given below?

(i) ${\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{H}}_2{\mathrm{PO}}_4^{-}$

(ii) ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HPO}}_4^{2-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

(iii)${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{OH}}^{-}\to {\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{O}}^{2-}$

In which of the above reactions does ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$act as an acid?