From the following half-cell reactions and their potentials, what is the smallest possible standard e.m.f. for spontaneous reactions?
$\begin{array}{l}{\mathrm{PO}}_4^{3-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2{\mathrm{e}}^{-}⟶{\mathrm{HPO}}_3^{2-}+3{\mathrm{OH}}^{-}\left(\mathrm{aq}\right);{\mathrm{E}}^{\circ }=-1.05\mathrm{V}\\ {\mathrm{PbO}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2{\mathrm{e}}^{-}⟶\mathrm{PbO}\left(\mathrm{s}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right);{\mathrm{E}}^{\circ }=+0.28\mathrm{V}\\ {\mathrm{IO}}_3^{-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+4{\mathrm{e}}^{-}⟶{\mathrm{IO}}^{-}\left(\mathrm{aq}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right);{\mathrm{E}}^{\circ }=+0.56\mathrm{V}\end{array}$