External reflection FT-IR techniques allowed for in situ examination of interfacial electrochemical events occurring in a thin-layer spectroelectrochemical cell. The formation of an adsorbed hexacyanoferrate (HCF) species on a platinum electrode during potential cycling in the ferri-/ferrocyanide redox couple was investigated. At neutral pH with KCl as electrolyte, a hexacyanoferrate complex forms during potential cycling in the ferri-/ ferrocyanide redox couple, but only if the potential limits are quite large. Characterization of the adsorbed complex indicates that it initially adsorbs as soluble Prussian Blue (KFe II [Fe III (CN) 6 ]) with no evidence of the presence of insoluble Prussian Blue [Fe 4 III (Fe 2 II (CN) 6 ]. This HCF complex can reversibly adsorb and grow on the electrode surface and desorb and decompose to reform solutionphase ferricyanide and ferrocyanide. Adsorption of the HCF complex decreases, but does not completely inhibit, electron transfer.

Fourier transform infrared (FT-IR) spectrometry adds complementary structural information to sensitive electroanalytical measurements. In this work, in situ, external reflection FT-IR techniques allowed for examination of interfacial electrochemical events through a thin-layer spectroelectrochemical cell in which a sinusoidally modulated potential was applied to the electrode. The infrared spectra of electrochemically modulated species were measured by a step-scan FT-IR spectrometer. The phasesensitive detection capability of this technique has been used to provide information about dynamic electrode processes and to enhance sensitivity to surface species. The cause of decreases in the heterogeneous rate constant, k°, of the ferricyanide/ferrocyanide redox process with time has been investigated. Previous authors offer a variety of opinions for the variation of k°for this reaction including the adsorption of polymeric hexacyanoferrate complexes, ferrocyanide, or ferricyanide on the electrode surface; other reports indicate that no species adsorb on the electrode during the electron-transfer process. Our results indicate that, with KCl as electrolyte, a hexacyanoferrate complex can form during potential cycling in the ferri-/ferrocyanide redox couple, but only under rather extreme experimental parameters. Measurements made by electrochemical modulation combined with step-scan FT-IR spectrometry show that, under other conditions, the change in rate constant of this redox process can be attributed to the adsorption of ferrocyanide ion.