Cyclic voltammogram reversible redox system

Fig. 2 Illustration of a cyclic voltammogram for a reversible redox system...

Figure 5.13 Cyclic voltammogram of a reversible redox system. Dependence of peak heights on...

After the build-up of the diffusion layer at short times, for longer times (t — < ) the first term in Eq. (5.28) vanishes and the current density i becomes proportionally inverse to the radius r of the microelectrode and is therefore potential and time independent. This behavior leads to different forms of cyclic voltammograms. Instead of the diffusion peaks shown in Figure 5.12 for a reversible redox system, constant currents are obtained on a microelectrode. 

The cyclic voltammetry of a film formed on a Pt electrode reveals a highly reversible redox system. The cyclic voltammogram is highly symetric (IpJIpc 1-18) and the separation between the cathodic and anodic peaks is low ( pa — pc 60 mV) (Figure 14.32). The doping level, y, deduced from coulometric measurements has a value very close to that obtained for poly(3-heptylthiophene), respectively y = 0.15 and y = 0.16. At low scans, the anodic wave shows two components at 0.62 V and 0.8 V The fact that these potentials are slightly less anodic than those of poly(3-heptylthio-... 

Figure 1.20 Cont d. (ii) Cyclic voltammetry (a) potential scanning starting from an anodic sense (b) cyclic voltammogram of a reversible redox system (P, start potential EP, end potential to and tf, starting and final times of the scanning).

Cyclic voltammograms of the [Fe(CN)6] /Fe[(CN)g] redox couple with the bare and the DNA-modified electrodes are shown in Fig. 5 [14a]. The peak currents due to the reversible electrode reaction of the redox system on the bare Au electrode were significantly suppressed by the treatment with DNA. In contrast, the treatment with unmodified, native DNA made no suppression, and that with HEDS caused only a slight one, as seen in Fig. 

The cyclic voltammogram of a surface-bound species has already been considered in section 2.1 and we now turn to the situation where the redox species R/O is in solution. Consider the cyclic voltammogram in Figure 2.87. The figure is typical of a reversible electrochemical system, i.e. ... 

Figure 2.87 Schematic of the cyclic voltammogram expected from a reversible electrochemical redox system 0 + e + R having a standard reduction potential °. E is the potential of the working electrode, and I the current.

Coming to the comparison of the and 0-2 series, the possibilities of isomerization and/or decomposition of the precursor lacunary complexes and, presumably, of their metal ion-substituted derivatives, make it necessary to insure first that no fast conversion occurs, in particular from the a structure to the 2 one. Figure 13(a) compares the cyclic voltammograms of the two lacunary complexes in the pH 3 medium. The main difference appears on the third redox system a single two-electron, reversible wave is obtained for the complex in contrast, the corresponding system for the U2 isomer is clearly constituted by two. 

Fig. 1. Cyclic voltammogram of a reversible system, e.g., A + e a . Epc cathodic peak potential a anodic peak potential E reversible redox potential.

A redox couple in which both species rapidly exchange electrons with the working electrode is termed an electrochemically reversible couple. Such a couple can be identified from a cyclic voltammogram by measurement of the potential difference between the two peak potentials. Equation 3.25 applies to a system that is both electrochemically and chemically reversible ... 

Nearly all electrochemical studies of organic systems are initiated by a cyclic voltammetric examination of the redox behavior (Chap. 3, Sec. III.B). From several cyclic voltammograms obtained at widely separated scan rates, one not only can determine which redox processes are chemically reversible, but also can often identify intermediates and final products and ascertain the relationships among them. 

Now let us assume that the redox couple in one phase (e.g., 01/Rl in W) exists in excess. In this case, the interfacial concentrations of 01 and R1 are kept constant even if the ET reaction proceeds. Then, the Nemst equation (3), which is valid for reversible ET systems, is reduced to = const. + (RT/nF) ln([02]o/[R2]o). This expression is the same as that for a typical electrode reaction. Thus, the W phase containing 01 and R1 in excess may be regarded as a metal electrode, and the ET reaction is limited by the diffusion of 02 in O from the bulk to the interface. The reversible cyclic voltammogram obtained under these conditions has a peak separation of (59/ ) mV (at 25°C) in the same manner as that obtained for a typical electrode reaction. 

Polarograms and cyclic voltammograms for tris(bipy) complexes of Cr and Cr in DMF have been reported. On the basis of the half-wave potential shift caused by methyl substitution on the bipy ligands it was concluded that each excess electron of the reductant species of the redox systems Cr(bipy)3/Cr(bipy)3 Cr(bipy)3 /Cr(bipy)3 and Cr(bipy) /Cr(bipy)3 " occupies a ligand n orHtal. The electrochemical reduction of [CrPh2(bipy)2]I at Pt and Au electrodes in MeCN and Me2SO has been investigated. Two reversible one-electron reductions were observed, as in reaction... 

In order to improve the detection of short-lived intermediates, the potential step or chronoamperometric experiment can be replaced by a cyclic voltammet-ric experiment, which involves applying a triangular potential ramp. With a fast UVA is spectrometer, e.g. a diode array system, additional UVWis/NIR spectroscopic information as a function of the potential can be recorded simultaneously to the voltammetric data. However, recording cyclic voltammograms with the simple cell shown in Fig. II.6.4 is complicated by the presence of ohmic drop in the solution phase, which is amplified by poor cell design. In this kind of cell, the peak-to-peak separation in cyclic voltammograms of a reversible redox couple may increase by several hundreds of millivolts. Voltammetric data (and simultaneously recorded spectroscopic data) are therefore very difficult to interpret quantitatively. 

Fig. 10. Cyclic voltammogram of horse cytochrome c at pH 9.30. Solution contained 0.35 mM oxidized cytochrome c in a medium consisting of 0.10 M sodium perchlorate, 0.02 M sodium borate and 0.01 M 4,4 -bipyridyl as promoter. An Au electrode, area 0.0079 cm was used. Scan rate 5mVs , temperature 25 °C. Two chemically non-reversible redox processes are observed. Wave 2c is associated with reduction of the State IV conformer which prevails at this pH. Note the virtual absence of wave Ic, which would be observed for reduction of the State III conformer. The corresponding return wave 2a is not observed because the Fe(II) product reverts immediately to the State IIj, conformer resembling State HI of the Feflll) form. Instead, re-oxidation (wave la) is observed at a potential appropriate for the native State III system. From Ref. 62, redrawn with kind permission of the authors...

If a redox system remains in equilibrium throughout the potential scan, the EC reaction is said to be reversible. In other words, equilibrium requires that the surface concentrations of O and R are maintained at the values required by the Nemst equation. Under these conditions, the following parameters characterise the cyclic voltammogram of the redox process ... 

The oxidation of these systems is characterized by steep anodic waves, when charging begins, followed by a broad flat plateau as the potential increases. In the reverse scan, a potential-shifted cathodic wave typically appears at the negative end of the capacity-like plateau whose peak current is normally smaller than that of the anodic peak. This behavior is different from that of normal redox-active films. According to theory, the latter should show completely symmetrical and mirror-image anodic and cathodic waves with identical peak potentials = E = and currents [63-65]. However, cyclic voltammograms of films of conducting... 

Reversible half-wave potentials determined from cyclic voltammograms generally relate closely to thermodynamic values for the redox system under study. However, many types of interactions may exist between the surrounding medium and the neutral or electrogenerated species, and because of this, the determined values of E1/2 reflect thermodynamics only in the particular medium used for observation. Moreover, the half-wave potentials are meaningless in themselves and must be related (or compared) to some arbitrary standard (such as a reference electrode potential) in order to acquire significance. 

---