Application of carbon electrodes modified with a mercury layer of a different thickness for studies of the adsorption and kinetics of phase transients of cytidine

Electrochemical characterization of a pyrolytic graphite electrode with basal (PGEb) and edge orientation (PGEe), a glassy carbon electrode (GCE), and the same electrodes modified with mercury (MFE) was performed by cyclic voltammetry (CV) and capacitance measurements (C-E curves and electrochemical impedance spectroscopy, EIS). The kinetics of phase transients of cytidine adsorbed on PGEb, PGEe, and GCE modified with mercury was studied by chronoamperometry (j-t curves) and capacitance measurements (C-E curves). In acidic (pH 5) solution the two-dimensional (21)) condensation of adsorbed cytidine molecules and formation of a physisorbed compact layer on the MFE takes place only in one potential region (region II). In alkaline solution (pH 8.3) cytidine forms two different 2D physisorbed condensed layers on the MFE. The first of these (region II) is located at more negative potential; the centre of this adlayer is situated around -0.8 V. The second 2D physisorbed film (region III) is formed at more positive potentials; the centre of the region III is around -0.2 V. The kinetics of phase transients of the cytidine films taking place by a potential jump from the dilute adsorption region (state la) to the 2D physisorbed film (region II) in acidic and alkaline solution at the PGEb modified with mercury (Hgm-PGEb) were studied. The phase transients of Ia --> II are about three times faster in acid solution than in alkaline solution for the same interval of the overpotential on the Hgm-PGEb. As long as the thickness of the mercury layer was greater than 2 mum, pronounced current maxima characteristic for the nucleation process, during phase transients for Ia --> II on the Hgm-PGEb surface are detected. With PGEe and GCE modified by mercury (Hgm-PGEe and GCE) such current maxima were not observed and only the adsorption process took place at these electrodes. (C) 2002 Published by Elsevier Science B.V.