During the electrolysis of dilute sulphuric acid using Platinum (Pt) electrodes, water is oxidised at the anode in preference to sulphate ions. The reactions possible at the anode are: $2\text{H}_2\text{O}(l) \rightarrow \text{O}_2(g) + 4\text{H}^+(aq) + 4e^-$ ; $E^\circ_{\text{cell}} = +1.23 \text{ V}$ $2\text{SO}_4^{2-}(aq) \rightarrow \text{S}_2\text{O}_8^{2-}(aq) + 2e^-$ ; $E^\circ_{\text{cell}} = +1.96 \text{ V}$ For dilute sulphuric acid, the oxidation of water (the reaction with the lower $E^\circ$ value) is preferred, resulting in the evolution of Oxygen gas at the anode . (At higher concentrations of $\text{H}_2\text{SO}_4$, the oxidation of sulphate ions is preferred).