Diffusion boundary layer of a rotating disk electrode as a thin-layer spectroelectrochemical cell.

A UV-visible rapid scan spectrophotometer (RSS) was coupled to a Au rotating disk electrode (RDE) for monitoring at near-normal incidence the reflection-absorption spectrum of the diffusion boundary layer in [Fe(CN)(6)](4)(-) aqueous solutions over a potential region in which [Fe(CN)(6)](4-) oxidizes, generating highly absorbing [Fe(CN)(6)](3-) (lambda(max) = 420 nm). Measurements were performed under steady-state conditions at rotation rates, omega, in the range 300 <or= omega <or= 2500 rpm, yielding well-defined spectra displaying characteristic features of [Fe(CN)(6)](3-). In agreement with theoretical predictions, the absorbance A at lambda(max), using as a reference A(lambda(max)) for the spectrum recorded at a potential negative to the onset of [Fe(CN)(6)](4-) oxidation, was found to be proportional both to the current and also to omega(1/2) under conditions in which E was positive enough for the reaction to proceed under diffusion-limited control.