Potential-induced acid-base chemistry of adsorbed species

The pKa of bicarbonate ion adsorbed on the surface of Pt(111), HCO3−(ads), in CO2 -saturated KClO4/HClO4 aqueous solutions has been determined by judicious application of a theoretical model originally proposed by Smith and White (Langmuir1993,9 (1), 1–3) to explain voltammetric features found for non-redox active alkyl chain monolayers bearing carboxylic moieties irreversibly bound to gold surfaces. The analysis herein presented relied on HCO3-(ads) and CO32-(ads) coverages derived from in situ Fourier transform infrared spectroscopy measurements as a function of potential and pH reported by Martinez-Hincapie et al. (J. Phys. Chem. C2016,120 (29), 16191–16199), as input parameters, yielding a pKa for HCO3−(ads) of ca. 2.6 ± 0.2. This value is significantly smaller than that of HCO3−(aq) in bulk solutions, a phenomenon associated with the bonding of the species to the electrode surface, a factor that markedly modifies its acid base characteristics.