The intensity of monochromatic light (λ = 633 nm) reflected off the surface of a polycrystalline Pt electrode, R, has been measured in situ as a function of potential, E, during continuous voltammetric cycles in the range -0.25 < E < 1.05 V vs. SCE, both in 0.5 M HClO4 and 0.5 M H2SO4 aqueous solutions. Plots of the normalized differential reflectance, Δ R/R = [R(Esam) - R(Eref)]/R(Eref) vs. Qn, where Esam and Eref refer to the sampling and reference potentials, respectively, and Qn is the coulometrically determined relative oxide charge (in terms of e/Pt surface atom), were found to deviate from linearity in the PtOH formation region. Improved proportionality between Δ R/R and Qn was achieved up to Qn = 1, upon correcting Δ R/R for double layer effects using the method originally reported by Conway et al. . In the case of 0.5 M H2SO4, however, remaining discrepancies could only be resolved by accounting for the charge due to bisulfate desorption induced by PtOH formation on the surface. On the basis of these results, it has been concluded that for both types of electrolytes, Δ R/R is proportional to the PtOH coverage, θ, and independent of the applied potential for Qn ≤ 1, opening new prospects for monitoring θ quantitatively during the course of faradaic reactions.
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