During the electrolysis of aq. $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4,{\mathrm{H}}_2$ is evolved at the cathode as ${\mathrm{E}}_{\mathrm{R}.\mathrm{P}.}$ of sodium (-2.71 V) is much lower than that of water (-0.83 V) while ${\mathrm{O}}_2$ is evolved at the anode as ${{\mathrm{E}}^{\mathrm{0}}}_{\mathrm{R}.\mathrm{P}.}$ potential of $\mathrm{S}{{\mathrm{O}}_4}^{2-}$ ion (-0.2 V) is much more than that of water (-1.23 V).