In situ surface enhanced Raman spectroscopy (SERS) has been used to determine quantitatively coverages of hemin and/or its reduced counterpart irreversibly adsorbed on roughened silver electrodes as a function of the applied potential in aqueous electrolytes. Analyses of spectral data recorded at pH 3 over a wide voltage region made it possible to obtain potential dependent adsorption isotherms, from which the standard redox potential was found to be -0.12 V versus saturated calomel electrode (SCE), in excellent agreement with data reported in the literature for hemin adsorbed on other substrates. Similar measurements performed at pH 7 not only revealed significant deviations from ideal Nernstian behavior, but the inflection point of the isotherm, i.e. -0.22 V versus SCE, was found to be more positive than that believed to be associated with the μ-oxo derivative of hemin, expected to be the majority species at this pH. Both these effects could be accounted for by invoking a fast pre-equilibrium between the electrochemically active (monomer) and electrochemically inactive (μ-oxo dimer) forms of hemin in the potential region in question. The value of E°=-0.21 versus SCE at pH 7 estimated from fits to the SERS data based on this model was in line with that for the monomeric rather than the dimeric form of hemin.
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