Step-Scan FT-IR Spectroelectrochemical Analysis of Surface and Solution Species in the Ferricyanide/Ferrocyanide Redox Couple

Abstract

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 phase-sensitive 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.