Monitoring the Ejection and Incorporation of Ferricyanide Fe(CN)_6^(3-) and Ferrocyanide Fe(CN)_6^(4-) Counterions at Protonated Poly(4-vinylpyridine) Coatings on Electrodes with the Scanning Electrochemical Microscope

Abstract

The precise positioning of microtlp electrodes close to the surface of substrate electrodes, as practiced in scanning electrochemlcal microscopy, was exploited to monitor the concentrations of Fe(CN)_6^(-3) and Fe(CN)_6^(-4) 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 polyectrolyte coatings became the curent-limiting step. The election of counterions when cathodic currents were passed through coatings which were saturated with Fe(CN)_6^(3-) was readily detected, especially when current steps were applied to the coated substrate electrode. Delayed arrival of counterions at the monitoring tip could be associated with the time required for the ions to traverse the coatings before they were ejected. Reincorporation of multiply-charged counterions inmediately following their ejection appeared to be favored over the incorporation of singly charged anions present at much higher concentrations.