The concentration profiles of the SO4 •-(Figure ) and the CO2 •-(Figure ) were examined at different concentrations of C2O4 2-, again in the presence of 0.54 mM Ru(NH3)6 3+ and 1.0 mM S2O8 2-. In the absence of C2O4 2-, the concentration of SO4 •-reaches a maximum value of 7.4 nM at a distance of 52 µm from the electrode surface (Figure , black trace). Then, the maximum concentration of SO4 •-decreases to 2.4 nM in the presence of 1 mM C2O4 2-(Figure , blue trace) and to 0.08 nM in the presence of 220 mM C2O4 2-(Figure , red trace and inset).