Cyclic electrode reactions

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Figure Bl.28.4. Cyclic voltaimnogram for a simple reversible electrode reaction in a solution containing only oxidized species.

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

The cyclic voltammograms of ferrlcyanlde (1.0 mM In 1.0 M KCl) In Fig. 2 are Illustrative of the results obtained for scan rates below 100 mV/s. The peak separation is 60 mV and the peak potentials are Independent of scan rate. A plot of peak current versus the square-root of the scan rate yields a straight line with a slope consistent with a seml-lnflnlte linear diffusion controlled electrode reaction. The heterogeneous rate constant for the reduction of ferrlcyanlde was calculated from CV data (scan rate of 20 Vs using the method described by Nicholson (19) with the following parameter values D 7.63 X 10 cm s , D, = 6.32 X 10 cm s, a 0.5, and n =1. The rate constants were found to be... 

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 order to determine the electrochemical properties of the solvent, the electrode process in molten carbamide and in carbamide-MeCl (where Me - NH4, K) mixtures on inert electrodes (platinum, glassy carbon) were investigated using cyclic voltammetry. The electrode reaction products were analysed by spectroscopic methods. The adsorbtion of carbamide- NH4CI anodic product was investigated by differential capacity method. 

M. Tilset. Derivative Cyclic Voltammetry Applications in the Investigation of the Energetics of Organometallic Electrode Reactions. In Energetics of Organometallic Species, J. A. Martinho Simoes, Ed. NATO ASI Series C, Kluwer Dordrecht, 1991 chapter 8. 

For any process the ratio between the current of the reverse peak and that of the forward peak, /pr//pf, is particularly important. This current ratio is the parameter which allows one to judge the chemical reversibility of an electrode reaction. In fact, when such a ratio is equal to 1, the electrogenerated species Red is stable (at least on the cyclic voltammetric timescale). The /pr//pf ratio is easily calculated in computerized instrumentation or must be determined graphically in... 

Linear sweep and cyclic voltammetry (LSV and CV) are probably the most widely used techniques to investigate electrode reaction mechanisms. They are easy to apply experimentally, readily available in... 

To appreciate how the time-scale of the cyclic voltammogram, r, is a function of the scan rate v, variation of v therefore allows insights into the kinetics and mechanisms of electrode reactions. ... 

Figure 6.13 Schematic cyclic voltammogram for the reduction reaction at a solid electrode. As in Figure 6.12, the solution was under diffusion control, which was achieved by adding inert electrolyte and maintaining a still solution during potential ramping. The initial solution contained only the oxidized form of the analyte couple, so the upper (cathodic) peak represents the reaction, O + e - R, while the lower (anodic) peak represents the electrode reaction, RO + ne". Note also that the jc-axis represents overpotential, so the peaks are centred about .

Figure 6.18 Cyclic voltammograms as a function of scan rate to show the effects of a slow rate of electron transfer as caused by poor electronic conductivity through the working electrode reaction. This figure comprises traces simulated by the DigiSim program. The fastest scan rate v is shown outermost notice how the CV looks stretched as v increases. Figure courtesy of Dr Adrian Bott, Copyright Bioanalytical Systems, Inc., 2000.

As shown by the cyclic voltammetric response in Fig. 10, the peak potential separation of the initial Mn(II,II) — Mn(II,III) electrode reaction is much larger than that of the other steps. This suggests significant inner-shell reorganization and a small rate of heterogenous electron transfer for oxidation of the fully reduced Mn(II,II) state. Similar kinetic sluggishness is observed for Mn(III)/Mn(II) electron-transfer reactions of some mononuclear complexes (see Sects 16.1.2 and 16.1.3). 

The aging behavior of the porous zinc electrode in 6 M KOH in the presence of different electrolyte additives such as ZnO, LiOH, and KF has been studied using cyclic voltammetry by Shivkumar etal. [212]. The mechanism of the electrode reaction in all these electrolytes was investigated. 

The gas-liquid chromatography with mass spectrometric detection (GLC-MS) analysis of the electrolyzed solution has shown that thiophenol is the only reduction product and the S—S bond cleavage is quantitative. Such a mechanism of bond breaking was confirmed by electrochemical studies. In cyclic voltammograms, anodic and cathodic peak potentials were the same for thiophenol and diphenyl disulfides thus the same species were participating in these processes. Electrode reactions of diphenyl disulfide are given by the following equations [166] ... 

The electrode reaction of triamterene 15 was elucidated by means of DCP, Tast polarography, cyclic voltammetry, microcoulometry, controlled potential electrolysis, and spectroscopy (ultraviolet/visible (UVA is), NMR). Two steps of reduction independent of pH were observed two-electron reduction of 15 resulted in the formation of 17. The first reduction wave of 15 was assumed to be due to irreversible two-electron reduction forming unstable 16, which tautomerized to 17, and the second reduction wave was ascribed to two-electron reduction of 17 to the tetrahydro product, 18 (Scheme 2). 

As stated earlier, cyclic voltammetry is a good road map when one first comes to examine an electrode reaction. It gives the researcher some idea of the potential near which there is reactivity. Cyclic voltammetry should always be used to get some idea of things at the beginning of an electrode kinetic investigation.24... 

Fig. 5.21 Cyclic voltammograms for the electrode reaction Ox+ne <= Red, which is reversible (curve 1), irreversible but a=0.5 (curve 2), and reversible but accompanied by a conversion of Red to an electroinactive species (curve 3).

Cyclic voltammetry is one of the most useful techniques for studying chemistry in lion-aqueous solutions. It is especially useful in studying electrode reactions that involve an unstable intermediate or product. By analyzing cyclic voltammograms, we can elucidate the reaction mechanisms and can determine the thermodynamic and kinetic properties of the unstable species. Some applications were described in previous sections. Much literature is available concerning cyclic voltammetry dealing with the theories and practical methods of measurement and data analysis [66]. In this section, three useful cyclic voltammetry techniques are outlined. 

Moreover, each of the chemical and electrochemical reactions can have different reaction rates and reversibilities. All of them are reflected in cyclic voltammograms. If we measure cyclic voltammograms of an electrode reaction, changing parameters such as potential range, voltage scan rate, temperature, electrode material and solution composition, and analyze the voltammograms appropriately, we can obtain information about the electrode reaction. However, except for cases where the electrode process is very simple, it is not easy to analyze the cyclic voltammograms appropriately. 

Digital simulation software, which is now commercially available, is useful in analyzing cyclic voltammograms of complicated electrode reactions [67]. If we assume a possible reaction mechanism and can get simulated CV curves that fit the experimental CV curves, we can confirm the reaction mechanism and obtain thermodynamic and kinetic parameters concerning the electron transfer and chemical processes. By the development of simulation softwares, cyclic voltammetry has become a very powerful technique. On the contrary, without a simulation software, cyclic voltammetry is not as convenient.14)... 

---