${\mathrm{SO}}_4^{2-}+{\mathrm{I}}^{-}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{SO}}_3^{2-}+{\mathrm{I}}_2;$

In the balanced equation, the number of ${\mathrm{H}}_2\mathrm{O} \mathrm{and} {\mathrm{H}}^{+}$ ions respectively are

$\begin{array}{l}{\mathrm{SO}}_4^{2-}\to {\mathrm{SO}}_3^{2-} {\mathrm{I}}^{-}\to {\mathrm{I}}_2\\ {\mathrm{SO}}_4^{2-}\to {\mathrm{SO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O} 2{\mathrm{I}}^{-}\to {\mathrm{I}}_2\\ {\mathrm{SO}}_4^{2-}+2{\mathrm{H}}^{+}\to {\mathrm{SO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O} 2{\mathrm{I}}^{-}\to {\mathrm{I}}_2+2\mathrm{e}\\ {\mathrm{SO}}_4^{2-}+2{\mathrm{H}}^{+}+2\mathrm{e}\to {\mathrm{SO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Net} \mathrm{reaction} : {\mathrm{SO}}_4^{2-}+2{\mathrm{H}}^{+}+2{\mathrm{I}}^{-}\to {\mathrm{SO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{I}}_2\end{array}$