Surface voltammogram

The study by cyclic voltammetry confirms the spontaneous accumulation of timolol on the hanging mercury drop electrode after the solution was stirred for 60 seconds. Fig.l shows the cyclic voltammogram of a solution of 5x10 M of timolol at pH = 4.0. A big, well-defined cathodic peak can be seen at —1.03 V, which in succesive scans decreases rapidly assuring a fast desorption from the electrode surface. Voltammogram B shows the same pattern, but without accumulation. The resulting peak is much smaller than that obtained by accumulation. 

The left-hand side (Ihs) of the case diagram (regions C1-C3) locates surface voltammograms for any degree of kinetic reversibility, whereas the situation corresponding to semiinfinite diffusion is located on the rhs of the case diagram (A1-A3). For > 10 the kinetic classification depends only on the kinetic parameter A, whereas when (o < 0.2, the case assignment is determined by the coupled parameter In the... 

Figure C2.10.2. Cyclic voltammogram of Cu(l 11)/10 mM HCl and in situ measured STM micrographs revealing tire bare Cu(l 1 l)surface (-1.05 V, left) and tire (V3 x A/3)R30°-Cladsorbate superstmcture (-0.6 V, right) (from [39]).

The following two pictures (Figure 6.2-8a and b) were acquired at h-500 mV and at -I-450 mV vs. Cu/Cu and show that at h-450 mV vs. Cu/Cu monolayer high Cu clusters nucleate at the steps between different Au terraces. Thus, the pair of shoulders in the cyclic voltammogram is correlated with this surface process. 

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

Both the TPD spectra (Fig. 5.2b) and the cyclic voltammograms (Fig. 5.2c) show clearly the creation of two distrinct oxygen adsorption states on the Pt surface (vs. only one state formed upon gas phase 02 adsorption, Fig. 5.2b, t=0). 

This can be carried out in vitro (in brain slices, cultured cell preparations) or in vivo and involves penetrating the experimental tissue with a carbon-fibre electrode of 5-30 pm in diameter (Fig. 4.9). This serves as an oxidising electrode and the Faradaic current generated by the oxidation of solutes on the surface of the electrode is proportional to their concentration. Obviously, only neurotransmitters which can be oxidised can be measured in this way so the technique is mainly limited to the study of monoamines and their metabolites. The amplitude of each peak on the ensuing voltammogram is a measure of solute concentration and individual peaks can be identified because different... 

Figure 3. Cyclic voltammograms of ascorbic acid at a freshly polished, active (a) and a deactivated (b) glassy carbon electrode surface. See text for details.

Figure 6. Simulated cyclic voltammogram for the oxidation of ascorbic acid without Inclusion of ec catalysis by the surface qulnone functionalities. Filled circles represent the simulated data and an experimental curve Is shown with a line for comparison. A scan rate of 100 mV s was assumed for experimental and simulated data.

The additivity principle was well obeyed on adding the voltammograms of the two redox couples involved even though the initially reduced platinum surface had become covered by a small number of underpotential-deposited mercury monolayers. With an initially anodized platinum disk the catalytic rates were much smaller, although the decrease was less if the Hg(I) solution had been added to the reaction vessel before the Ce(lV) solution. The reason was partial reduction by Hg(l) of the ox-ide/hydroxide layer, so partly converting the surface to the reduced state on which catalysis was greater. 

Figure 3.2 Cyclic voltammograms for H adsorption on Pt(lll) and Pt(lOO). Two different methods have been applied. In (a) and (b), the H particles were assumed not to interact in the expression for the configurational entropy. In (c) and (d), the more elaborate model involving Metropolis Monte Carlo was applied. As can be seen, for these homogenous surfaces, the simple method suffices. The figure is adopted from [Karlberg et al., 2007a], where the full details of the calculations can also be found.

Interestingly, it has been shown that some adatoms can be selectively deposited on step sites, taking advantage of the enhanced reactivity of these sites. Figure 7.5 shows the voltammogram of a Pt(775) surface in 0.5 M H2SO4. The hard sphere model for... 

Figure 7.5 Cyclic voltammogram of a Pt(775) electrode in 0.5 M H2SO4 solution and a hard sphere model of this surface. Sweep rate 50 mV/s. In the hard sphere model, four atoms forming the (110) step site have been identified in black.

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