The oxidation of bromide on platinum electrodes in aqueous acidic solutions: Electrochemical and in situ spectroscopic studies

Certain aspects of the oxidation of bromide in acidic aqueous electrolytes have been investigated via a combination of rotating disk electrode (RDE) and in situ reflection absorption UV-visible spectroscopic techniques. Koutecky-Levich plots of polarization curves recorded with a Pt RDE in 5 mM KBr in 0.1M HClO4 at rotation rates in the range 400 to 2000 rpm yielded for low overpotentials rates independent of the applied potential. This conclusion supports the reports of Conway et al. (J. Chem. Soc., Faraday Trans., 1995, 91, 283) which relied on the analysis of the results different type of experiments performed in more concentrated bromide solutions. Advantage was taken of the very high molar absorptivity of tribromide, i.e. ε(λ = 266 nm) = 4.09 × 10⁷ cm2 mol-1, a solution phase species generated via the reaction of bromine and bromide, to monitor quantitatively its presence within the diffusion boundary layer and thus gain insights into the reaction mechanism. Numerical simulations of the electrochemical-chemical (EC) mechanism that take into account the dynamics of formation and dissociation of tribromide, yielded good agreement with both the optical and electrochemical data collected in this work under strict diffusion control.