However, we do not have an explanation for why k2 in the SSV experiments appears to increase as a function of S2O8 2-concentration. As shown below, with increasing concentration of S2O8 2-, a slightly larger k2 value is also needed to simulate SECM approach curves obtained in 0.1 M KCl solutions containing 1.0 mM Ru(NH3)6 3+ , consistent with the SSV findings.