Abstract
The dependence of the SECM feedback current on finite heterogeneous electron‐transfer (et) kinetics at the substrate electrode was examined by experimental studies of the reduction of Fe(III) in 
at a Pt tip over a biased glassy‐carbon substrate. At the extremes of very fast and very slow et rates, the feedback current was identical to the theoretical response for conducting and insulating substrates. At intermediate et rates, the feedback response depended on the et rate and the tip insulator dimensions. These results are presented as a three‐dimensional calibration surface of the feedback current as a function of rate constant and tip‐to‐substrate distance. At very close tip‐to‐substrate spacings, heterogeneous rate constants greater than 1 cm s−1 could be distinguished. A model of the behavior of an unbiased substrate is also presented and is used to interpret experimental feedback current results.