Redox reactions cyclic voltammograms

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. 

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

At high anodic potentials Prussian blue converts to its fully oxidized form as is clearly seen in cyclic voltammograms due to the presence of the corresponding set of peaks (Fig. 13.2). The fully oxidized redox state is denoted as Berlin green or in some cases as Prussian yellow . Since the presence of alkali metal ions is doubtful in the Prussian blue redox state, the most probable mechanism for charge compensation in Berlin green/Prussian blue redox activity is the entrapment of anions in the course of oxidative reaction. The complete equation is ... 

Although the initial steps of Schemes IA, IIA, and IIIA are strongly supported by the experimental data, the subsequent reactions and electron-transfer steps are based solely on the electrochemical measurements of Figures 1-3, 6 and 7. Intermediates have not been detected or isolated, but there is self consistency in the redox thermodynamics between the M/ OH systems and the M+/02 systems. The cyclic voltammograms also indicate the presence of common intermediates between the two systems. 

Diagnostic tests and quantitative criteria for cyclic voltammograms of reversible and irreversible redox reactions at 25°C... 

FIGURE 2.30. Redox catalysis induction of Srn1 reactions. Cyclic voltammetry in liquid ammonia + 0.1 M KC1 at —40°C of (a) redox catalyis of the reductive cleavage of 2-chlorobenzonitrile, RX, by 4-cyanopyridine, P. The dotted reversible cyclic voltammogram corresponds to P in the absence of RX. The solid line shows the catalytic increase of the current, (b) Transformation of the voltammogram upon addition of the nucleophile PhS. Adapted from Figure 1 in reference 23, with permisison from the American Chemical Society. 

Despite the problems that can afflict experimental cyclic voltammograms, when the method for deriving standard redox potentials is used with caution it affords data that may be accurate within a few tens of mV (10 mV corresponds to about 1 kJ mol-1), as remarked by Tilset [335]. Kinetic shifts are usually the most important error source The deviation (A If) of the experimental peak potential from the reversible value can be quite large. However, it is possible to estimate AEp if the rate constant of the chemical reaction is available. For instance, in the case of a second order reaction (e.g., a radical dimerization) with a rate constant k, the value of AEV at 298.15 K is given by equation 16.24 [328,339] ... 

Values of A , and k may be extracted from the polarographic data, although the treatment is complex. Examples of its use to measure the rate constants for certain redox reactions are given in Refs. 339 and 340 which should be consulted for full experimental details. The values obtained are in reasonable agreement with those from stopped-flow and other methods. The technique has still not been used much to collect rate constants for homogenous reactions. The availability of ultramicroelectrodes has enabled cyclic voltammograms to be recorded at speeds as high as 10 Vs". Transients with very short lifetimes (< ps) and their reaction rates may be characterised. ... 

The electrochemistry of dioxoosmium(VI) complexes has also been extensively studied. The tra 5-dioxoosmium(VI) complexes of polypyridyl and macrocyclic tertiary amine ligands display very similar proton-coupled electron transfer couples. In aqueous solutions at pH < 5-7 the cyclic voltammograms of n-a i-[0s (0)2(bpy)2] show a remarkable reversible three-electron couple and a one-electron Os coimle. In the Pourbaix diagram two break points are observed in the pH dependence of the Os couple, which correspond to the pAa values of Os —OH2 and Os —(OHXOH2) (Figure 10). The redox reactions are shown in Equations (41)-(43). At pH >8 the 3e Os wave splits into a pH-independent le Os wave and a 2e/2H" Os wave (Equations (44) and (45)). 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Figure 3.55 Cyclic voltammograms or Re(Bipy)(CO)3CI in CHjCN/O.l M tetrabutylammonium hexafluorophosphate as supporting electrolyte at a button Pt electrode, and with a sweep rate of 200mV s"(a) The switching potential characteristics of the coupled chemical reactions in the ahsence of C02. The lettered redox processes are discussed in the text, (b) The effect of saturating the solution with C02. From Sullivan et al (19155).

Cyclic voltammetry is a very valuable tool to use in following the course of redox reactions. An instructive demonstration of this is provided by studies on the oxidation of 9,10-diarylanthracenes in CH2C12—CF3C02H which can be considered to be nucleophile poor [81]. The use of CV in these studies is exemplified by Fig. 6. The first voltammogram [Fig. 16(a)] was recorded before bulk electrolysis was started. Peaks Ot and 02 correspond to the 1 e oxidations of 9,10-diphenyl-anthracene (DPA) and... 

A hypothetical cyclic voltammogram for the surface-confined redox reaction is shown in Figure 13.2. On the forward (positive-going) scan, an anodic wave is observed this wave is associated with oxidation of the -Fc groups (Fig. 13.1). This wave rises to a peak and then decays to zero current at potentials positive of the peak. This points out the first difference between a surface-confined redox reaction and a redox reaction in which the electroactive species is dissolved in solution. If the ferrocene were dissolved in the electrolyte solution... 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

The electrochemistry of both rans-dioxoosmium(VI) and rutheni-um(VI) macrocyclic tertiary amine complexes has been studied in great detail. Both trans-[RuVI(b)2(0)2]2+ and rans-[RuVI(14TMC)(0)2]2 + and its related complexes display similar cyclic voltammograms in aqueous solution (132, 212). At pH < 7, three reversible/quasi-reversible redox couples, corresponding to the redox reactions are observed ... 

The trans-dioxoosmium(VI) complexes have been found to show pH-dependent redox reactions 74, 75, 211). In acidic medium, the cyclic voltammogram of trans-[0sVI(L)(0)2]2+ showed a reversible three-electron couple and an irreversible one-electron couple. The Pourbaix diagram of fr[Pg.281]

Figure 5.8 Cyclic voltammograms for the Os2+/3+ redox reaction within spontaneously adsorbed [Os(OMebpy)2(p3p)Cl]+ monolayers. From right to left, the electrode materials are platinum, gold, carbon and mercury. The scan rate is 50 Vs-1, with a surface coverage of 1.0 0.1 x 10-1° mol cm-2 the supporting electrolyte is aqueous 1.0 M NaC104. Reprinted with permission from R. J. Forster, P. J. Loughman and T. E. Keyes,/. Am. Chem. Soc., 122,11948 (2000). Copyright (2000) American Chemical Society...

Figure 18 gives typical cyclic voltammograms taken in four redox systems at relatively heavily doped diamond electrodes [92]. Outer-sphere reactions proceed in... 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

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