Transient Convective Diffusion to a Rotating Disk Electrode as Monitored by Near-Normal Incidence Reflection Absorption Ultraviolet-Visible Spectroscopy

The time evolution of the integrated profile of a species generated at the surface of a rotating gold disk electrode by a potential step was monitored using UV-visible near-normal incidence reflection absorption spectroscopy. These experiments were conducted in aqueous K4[Fe(CN)₆] solutions using a potential step with a large enough magnitude to achieve, at steady state, diffusion-limiting current conditions for the oxidation of [Fe(CN)₈]⁴~with the monochromator set at 420 nm At this wavelength the ferric species displays an absorption maximum whereas the ferrous counterpart is essentially transparent. Excellent agreement was obtained between the results of these spectroscopic experiments and those calculated on the basis of the short-, and long-time solutions for transient convective diffusion to a rotating disk electrode reported in the literature without introducing any adjustable parameters. Simultaneous measurements of the current also yielded transient curves in harmony with those predicted by the theory. Possible modifications of this technique to acquire quantitative steady-state absorption spectra of the diffusion boundary layer as a function of the applied potential are discussed briefly.