Information for additional details), and was further assumed for the diffusivity for CO2 • -. Values of D for S2O8 2-, S2O8 3•-, SO4 2-, and SO4 •-in 0.1 M Na2SO4 were all assumed to be 1x10 -5 cm 2 /s based on the range of D values reported for S2O8 2-in aqueous solutions (~0.6-1.4x10 -5 cm 2 /s). An E 0 of -0.197 V vs Ag/AgCl for the Ru(NH3)6 3+/2+ redox couple and diffusion coefficients of 5.7 x 10 -6 and 8.8 x 10 -6 cm 2 /s, respectively, for Ru(NH3)6 3+ and Ru(NH3)6 2+ were used. The heterogeneous reduction of Ru(NH3)6 3+ , eq 17, is fast and kinetically reversible at the moderately low scan rates used in this study, Figure . We found that the FD simulations were insensitive to any value of the heterogeneous rate constant, k 0 , above 1 cm/s. A value of 17 cm/s was used throughout to ensure that Ru(NH3)6 3+ occurs at the diffusion-limit rate under all simulation conditions. All FD simulations were performed using the commercial software, Digisim ® (Version 3.0.3b, Bioanalytical Systems, Inc.). Additional details of simulations parameters are found in the Supporting Information.