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.

References

1. P. Delahay, Double Layer and Electrode Kinetics, Interscience, New York, 1965.
2. J. Barthelmes, G. Pofahl and W. Plieth, Ber. Bunsen-Ges. Phys. Chem., 1992, 96, 1055.
3. J. Barthelmes, G. Pofahl, M. Panagiotakis and W. Plieth, J. Raman Spectrosc., 1993, 24, 737.
4. J. Barthelmes and W. Plieth, Metalloberfläche, 1995, 49, 173.
5. J. Barthelmes and W. Plieth, Electrochim. Acta, 1995, 40, 2487.
6. J. Barthelmes, G. Pofahl and W. Plieth, J. Electroanal. Chem., 1993, 349, 223.
7. P. Green and L. Tong, J. Am. Chem. Soc., 1956, 78, 4896.
8. W. Howard Jr. and W. Nelson, J. Mol. Struct., 1979, 53, 65.
9. M. Moskovits and J. Suh, J. Phys. Chem., 1984, 88, 5526.
10. M. Moskovits and J. Suh, J. Phys. Chem., 1984, 88, 1293.
11. X. Gao, J. Davies and M. Weaver, J. Phys. Chem., 1990, 94, 6858.
12. G. Clark, T. Andersen, R. Vakentine and H. Eyring, J. Electrochem. Soc., 1974, 121, 618.
13. H. Hamelin, T. Vitanov, E. Sevastianov and A. Popov, J. Electroanal. Chem., 1983, 145, 225.
14. P. Schmidt and W. Plieth, Ber. Bungen-Ges. Phys. Chem., 1987, 91, 323.
15. W. Hehre, R. Ditchfield and J. Pople, J. Chem. Phys., 1972, 56, 2257.