The SSVs in the presence of S2O8 2-, Figure , show several non-ideal behaviors: (i) a prewave at the foot of the main wave, (ii) a small hysteresis between the forward and reverse scan, and (iii) a sloping limiting current that becomes more apparent at high S2O8 2-concentrations. We believe that at least a partial explanation for these non-idealities is either the adsorption of S2O8 2-, the onset of direct S2O8 2-reduction at the C-UME electrode, and/or ion-pairing between Ru(NH3)6 3+ (and/or Ru(NH3)6 2+ ) with S2O8 2-. While we have not systematically investigated any of these possibilities, we note previous reports of ion-pairing of Ru(NH3)6 3+ with SO4 2-in aqueous solutions, and ion-pairing of Ru(bpy)3 2+ with S2O8 2-(where bpy = 2,2′-bipyridine) in mixed H2O/acetonitrile solutions. Additionally, the SSV response for the direct reduction of S2O8 2-at the C-UME, shown in the Supporting Information, indicates that S2O8 2-reduction, which is kinetically slow at C, commences at about -0.45 V vs Ag/AgCl. While these non-ideal behaviors limit our analysis of the SSV response, the increase in current upon addition of S2O8 The SSV responses of the C-UME in the presence and absence of S2O8 2-were simulated with the FEM method. Table and S2 in Supporting Information provides a complete list of thermodynamic and kinetic parameters used in the simulation. Values of the diffusion coefficients, D, of Ru(NH3)6 3+ and Ru(NH3)6 2+ , 8.43×10 -6 cm 2 /s and 1.19×10 -5 cm 2 /s, respectively, reported by Compton and coworkers were used. We used a value of D = 1×10 -5 cm 2 /s for S2O8 2-, consistent with the range of D values reported in the literature for this ion. In the absence of reported measured values, we also used D = 1 × 10 -5 cm 2 /s for S2O8 3•-and SO4 •-. The standard reduction potential for the reversible one-electron reduction of Ru(NH3)6 3+ (eq. 1) and one-electron reduction of SO4 •-(eq. S5) are -0.12 V and 2.21 V vs. Ag/AgCl (3.5 M KCl), respectively. For the simulated SSVs shown in Figure , a value of k3 = 1 × 10 10 s -1 was assigned to the dissociation rate constant of S2O8 3•-, consistent with theoretical estimates. However, we note that simulations were insensitive to the value of k3 for values greater than 10 6 s -1 . The homogeneous reaction between SO4 •-and Ru(NH3)6 2+ (eq. 4) was assumed to be diffusion-controlled; hence a rate constant of 1 × 10 10 M -1 s -1 was used for k4. The simulations used a Butler-Volmer formalism for the reductions of SO4 •-and Ru(NH3)6 2+ at the C-UME, but we assumed sufficiently large heterogeneous rate constants (k 0 = 17 cm/s for reduction of Ru(NH3)6 3+ , eq. 1, and 10 cm/s for reduction of SO4 •-, eq. 5) that the electron-transfer reactions can be assumed to be completely reversible on the time-scale of the simulations of the slow-scan rate SSVs (i.e., the SSV simulations do not depend on the precise values of k 0 ). The dimerization of SO4 •-to yield S2O8 2-was not included in the simulations; this assumption is justified by the very low concentrations of SO4 •-, a consequence of its rapid reaction with Ru(NH3)6 2+ and reduction at the C-UME (vide infra).