Field dependence of the protonation equilibrium of 2-pyridinecarboxylic acid on copper in sulfuric acid

Abstract

The adsorption of 2-pyridinecarboxylic acid (2-PCA, picolinic acid) on polycrystalline Cu-electrodes from a 0.1 M sulfate electrolyte is investigated through surface-enhanced Raman spectroscopy (SERS). The surface-active species and their geometries are determined. The picolinate anion and the 2-PCA zwitterion on the metal surface are coordinated through the carboxylate group, leaving the N-atom of the heteroaromatic ring free and directed to the bulk of the solution. The protonation equilibrium observed is potential dependent. The overall change of the pK _ad of the adsorption equilibrium in the vicinity of the electrode surface is calculated by analysing the changes in the SERS-intensity of adsorbate characteristic vibrations. The alteration of the pK _ad with potential (0.75 V ^-1 ) is, basically, due to the differences in the dipole moments. The pK _ad of the adsorption equilibrium at the potential of zero charge (p.z.c.) is estimated. The pK _ad of 1.6 is ca. three units smaller than the pK value in the homogeneous solution.