Further decreasing L below 1.5 causes it to increase, a consequence of simple redox cycling of Ru(NH3)6 3+/2+ between the tip and the substrate electrode, Figure . FEM simulations were also used to determine the rate, k4, for the electron-transfer reaction between Ru(NH3)6 2+ and SO4 •-. Figure shows simulated approach curves with various k4 values, while the k2 and k3 were held constant at 1.0 × 10 6 M - s -1 and 1.0 × 10 10 s -1 , respectively. The simulated approach curves for it and is converged to the experimental data with increasing k4 values and remained sensitive to k4 for values approaching up to 1 × 10 9 M -1 s -1 , which is two orders of magnitude higher than our previous lower limit determined by CV using a glassy carbon macroelectrode. We conclude from the SECM data that k4 is greater than or equal to ~ 1 × 10 9 M - Finally, the approach curves of both the tip and the substrate electrode were simulated via FEM simulations using the rate constants determined by the above SECM measurements and compared to the experimental curves for 1.0 mM Ru(NH3)6 3+ and S2O8