Cyclic voltammograms oxidation potentials

Electrochemistry of the protonated compounds supported the reactions given in Scheme 2. The series of monoprotonated complexes exhibits a reversible two-step one-electron reduction of the protonated anthraquinone moiety (AqH) in the cyclic voltammograms, whose potentials are largely shifted in the more positive direction than those of nonprotonated forms (Table 1). The reversible oxidation waves of non-and monoprotonated complexes are derived from the metal-centered oxidation of the ferrocenyl and ftilvene complex moieties. In [l,5-Fc FvAqH2 ], the redox reaction... 

The unusual cyclic voltammograms and responses to large-amplitude potential steps of a variety of conducting polymer films have prompted a number of groups to develop nucleation models for their oxidation. The key features that they have sought to explain are the peaks observed in anodic chronoamperometry (see Fig. 14), and the dependence of the anodic peak position on scan rate207 and the time spent in the undoped state.20 ... 

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. 

Figure 5.2. NEMCA and its origin on Pt/YSZ catalyst electrodes. Transient effect of the application of a constant current (a, b) or constant potential UWR (c) on (a) the rate, r, of C2H4 oxidation on Pt/YSZ (also showing the corresponding UWR transient)3 (b) the 02 TPD spectrum on Pt/YSZ4,7 after current (1=15 pA) application for various times t. (c) the cyclic voltammogram of Pt/YSZ4,7 after holding the potential at UWR = 0.8 V for various times t.

Fig. 17. Cyclic voltammogram of the water-soluble Rieske fragment from the bci complex of Paracoccus denitrificans (ISFpd) at the nitric acid modified glassy carbon electrode. Protein concentration, 1 mg/ml in 50 mM NaCl, 10 mM MOPS, 5 mM EPPS, pH 7.3 T, 25°C scan rate, 10 mV/s. The cathodic (reducing branch, 7 < 0) and anodic (oxidizing branch, 7 > 0) peak potentisds Emd the resulting midpoint potential are indicated. SHE, standEU d hydrogen electrode.

Figure 5.9 Schematic cyclic voltammogram showing the electro-oxidation of the electrode (dashed box). The curve was generated from measurements by Jerkiewicz et al. [2004] of Pt in 0.5 M H2SO4 with a reversible hydrogen reference electrode (RHE). For each separable potential range, an atomistic model of the electrode structure is shown above.

Electrochemically active compounds can be evaluated using a potentiometer to generate a cyclic voltammogram for the analyte. Cyclic voltammetry will allow the analyst to determine whether the compound can be oxidized or reduced, to choose the appropriate potential to use in the electrochemical detector, and to establish whether oxidation or reduction is irreversible. Irreversible oxidation or reduction of the analyte could be predictive of problems with electrode poisoning and reduced sensitivity of the electrochemical detector over time. Turberg et al. used EC detection at an applied potential of -1-600 mV to analyze for ractopamine. 

The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. 

While thick oxide formation on Ru occurs together with Oz evolution at a potential of 1.15 Vsce, the initial steps of oxide formation are expected to occur at more cathodic potentials of roughly 0.3 0.8 V [72]. Structures in the cyclic voltammogram in this potential region were attributed by Vucovik et al. [73] to hydroxide or oxide adsorption. These oxides are reversibly reduced at a potential of 0.1 V. The presence of a thin oxide layer on Ru at potentials cathodic of 1.15 V was demonstrated by... 

Thick anodic iridium oxide films are formed by repetitive potential cycling between properly chosen anodic and cathodic limits [89]. The coloration (bleaching) transition is reflected in the cyclic voltammogram by a significant increase (decrease) of the electrode pseudo-capacity at a potential around 0.7 Vsce in acid electrolytes. At potentials above 0.7 V the thick film appears dark blue, while below 0.7 V the film is almost clear. 

On the basis of obtained data of cyclic voltammograms for 3d metals oxides electrodeposition the optimal conditions (current density, potential, process time, electrolyte composition, temperature) for dense oxide films (Ni, Cr and Co) deposition on steel foil have been elaborated. Data relating to several best films are summarized in Table 1. 

Cyclic voltammetric methods In these, the potential is swept linearly with time and the oxidation or reduction of the surface species can be followed by measuring the resultant current. Great care is needed in the interpretation of cyclic voltammograms and examples are given in chapter 2. 

Melendres et ai (1991) reported the in-situ study of the electrode/oxide and oxide/electrolyte interfaces for a copper electrode in pH 8.4 borate buffer under potential control. The grazing-angle incidence arrangement employed by the authors is shown in Figure 2.81(a) and a cyclic voltammogram of the Cu electrode in the buffer is shown in Figure 2.81(b). 

Figure 2.82 (a) Reflectivity of Cu-on-Si electrode at various potentials in borate buffer solution (pH 8.4). A.B.C and D correspond to potentials indicated in the cyclic voltammogram of Figure 2.81(b). Solid lines represent calculated curves while symbols correspond to experimental data, Open circles, A, -0,12 V open squares, B, -0.80V both y-axes are reflectivity x 10". Filled circles, C, 0.40 V, reflectivity x 10" 3 open diamonds, D, —0.80 V, reflectivity x 10 s. (b) Schematic of multi-layer mode) for Cu-on-Si electrode (not to scale). The oxide film is represented as Cu20. From Melendres et ai (1991). 

Figure 3.14 The integrated oxide formation charge, relative to the charge under the hydride formation region, ( o/Gh as a function of anodic potential, for the cyclic voltammogram in Figure 3.1. From Angerslein-Kozlnwska et ai (1973)...

Table 3.1 Peak potential, charge passed and corresponding oxide coverage for the various oxide regions in the anodic sweep of the Pt cyclic voltammogram...

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