Juhyoun Kwak and Fred C. Anson, "Monitoring the Ejection and Incorporation of Fe(CN)63- and Fe(CN)64- Counterions at Protonated Poly(4-VinylPyridine) Coatings on Electrodes with the Scanning Electrochemical Microscope," Anal. Chem., 64 (3), 250 - 256 (1992.02.01.)

Abstract
The precise positioning of microtip electrodes close to the surface of substrate electrodes, as practiced in scanning electrochemical microscopy, was exploited to monitor the concentrations of Fe(CN)63- and Fe(CN)64- anions at the surfaces of protonated poly(4-vinylpyridine) coatings on glassy-carbon electrodes. Positive feedback, which enhanced the magnitude of currents at the monitoring tip electrode, gave way to negative feedback when reactant concentrations were increased to the point that electron propagation through the polyelectrolyte coatings became the current-limiting step. The ejection of counterions when cathodic currents were passed through coatings which were saturated with Fe(CN)63- was readily detected, especially when current steps were applled to the coated substrate electrode. Delayed arrival of counterions at the monitoring tip could be associated with the time required for the lons to traverse the coatings before they were ejected. Reincorporation of multiply-charged counterions immediately following their ejection appeared to be favored over the incorporation of singly charged anions present at much higher concentrations.