The conjugate base of ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ is:

$\underset{\text{ acid }}{{\mathrm{H}}_2{\mathrm{PO}}_4^{-}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\underset{\text{ conjugated base }}{{\mathrm{HPO}}_4^{2-}}$

${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ gives ${\mathrm{HPO}}_4^{2-}$ (conjugated base) in aqueous solutions. It acts as proton donor.