Cyclic voltammetry peak potentials

The experimental kinetic data obtained with the butyl halides in DMF are shown in Fig. 13 in the form of a plot of the activation free energy, AG, against the standard potential of the aromatic anion radicals, Ep/Q. The electrochemical data are displayed in the same diagrams in the form of values of the free energies of activation at the cyclic voltammetry peak potential, E, for a 0.1 V s scan rate. Additional data have been recently obtained by pulse radiolysis for n-butyl iodide in the same solvent (Grim-shaw et al., 1988) that complete nicely the data obtained by indirect electrochemistry. In the latter case, indeed, the upper limit of obtainable rate constants was 10 m s", beyond which the overlap between the mediator wave and the direct reduction wave of n-BuI is too strong for a meaningful measurement to be carried out. This is about the lower limit of measurable... 

Reduction of sulphonium salts polarographic half-wave potentials, Ey. ref. [54], in water cyclic voltammetry peak potentials, Ep ref. [55], in acetonitrile at glassy carbon, scan rate 50 mV s. ... 

Aliphatic ketones are oxidised in both acetonitrile [1,2] and trifluoracetic acid [3] at potentials less positive than required for the analogous hydrocarbons. The oxidation process is irreversible in both solvents and cyclic voltammetry peak potentials are around 2.7 V V5. see. Loss of an electron from the carbonyl oxygen lone pair is considered to be the first stage in the reaction. In acetonitrile, two competing processes then ensue. Short chain, a-branched ketones cleave the carbon-carbonyl bond to give the more stable carbocation, which is then quenched by reaction with... 

Variation of cyclic voltammetry peak potential separation with the heterogeneous kinetic parameter i//... 

Indeed, in cyclic voltammetry, peak potentials Ep play a role identical to that of halfwave potentials E1/2 in steady-state methods. As for the later methods, peak potentials vary linearly with the logarithm of dimensionless kinetic parameters A. or A in Table 5, provided these latter have values sufficiently large when compared to unity [94]. These linear variations, which may be used for determination of reaction orders, stem from the same mathematical reasons as explained in the case of E1/2. Yet the physical reason is quite different as evidenced by the case of the simple EC sequence in Eqs. (222) and (223) ... 

Binding constants in solution are usually determined from potentiometric titrations or plots of cyclic voltammetry peak potentials vs. CyD concentration after assessing the guest/host ratio in the complex [3, 4]. Potentiometric measurements are more frequently used, and ion-selective electrodes are employed, for the direct measurement of the guest activity in the solution. Measurements of pH allow the evaluation of concentrations of several reaction components. 

Figure 1 displays derivative cyclic voltammograms for the oxidation of Cp Mn(CO)2(NCMe) (1 left) and Cp Mn(CO)2(Pl%3) (2 right) at a voltage sweep rate v - 0.2 V/s. Cyclic voltammetry peak potentials correspond to the intersections between the DCV curves and the base line. The reversible potentials for the oxidation of 1 and 2, taken as the midpoints between the anodic and cathodic CV peaks, are located at -0.12 and +0.22 V vs the ferrocene/ferricinium couple (Fc), respectively. The unity ratio of the cathodic to anodic derivative peak currents demonstrates the chemical reversibility of the 1/1 and 2/2- couples. 

The cyclic voltammetry procedure reported by Kohen and others (2000) evaluates the overall reducing power of low-molecular-weight antioxidants in a biological fluid or tissue homogenate. The electrochemical oxidation of a certain compound on an inert carbon glassy electrode is accompanied by the appearance of the current at a certain potential. While the potential at which a cyclic voltammetry peak appears is determined... 

If k[ = n-F-v/R-T (i.e. if the chemical complication is neither too slow nor too fast and, consequently, the kinetics of the chemical complication are of the same order as the time scale of cyclic voltammetry) the potential of the forward peak, which has been localized at more anodic potentials than E0 by the chemical complication, shifts towards less anodic values with the scan rate according to the relationship ... 

In cyclic voltammetry, the potential applied to the working electrode is varied linearly (Fig. 2.1) between potentials Ex and E2, E2 being a potential more positive (for oxidation) or negative (for reduction) than the peak maximum observed for the oxidation/reduction reaction concerned. At E2, the voltage scan is reversed back to E3 or to another end potential value, E3. The application of this type of potential ramp can be done in a number of ways, varying the starting potential Eu the reverse potential E2, the end potential E3 and the scan rate. The latter is the rate that is applied to vary the potential as a function of time, commonly represented in Vs 1 or mVs"1. 

Comparative anodic polarization data (Fig. 27) obtained by Conway and Liu (285-287) for chemically and anodically formed nickel oxide show a Tafel slope of 33 mV on anodically formed nickel oxide, lower than that for the chemically formed film (60 mV), with better activity than the former. Pseudocapacitance profiles obtained from an analysis of the potential relaxation data are shown in Fig. 28. The initial descending part of the C o versus rj profiles of Fig. 28a appears to be connected with the positive end of the well-known cyclic-voltammetry peak for Ni(II) - Ni(III) oxidation in nickel oxide. This peak goes into an ascending current versus potential line for O2... 

Staircase voltammetry is in fact a modified, discrete linear scan (or cyclic) voltammetry. The potential scan can be reversed in SV, similarly as it is done in cyclic voltammetry, and then a cyclic staircase voltammogram can be obtained. Staircase voltammograms are peaked-shaped the same as linear scan voltammo-grams. There are some differences between these voltammetries anyway. A linear scan (or cyclic) voltammogram forms a continuous current vs. potential curve, while each staircase voltammogram consists of a number oii-E points. Also, the peak heights obtained under conditions of identical scan rates in linear scan and staircase voltammetries (v = AE/At) may differ considerably. 

Cyclic voltammetric behaviour of redox polymers including PVF has been studied previously in acetonitrile and in water solutions [18]. In acetonitrile, PVF exhibits stable, symmetrically shaped cyclic voltammetry peaks at potentials characteristic of oxidation and re-reduction of ferrocene sites in the polymer film. In aqueous electrolyte solutions, non-symmetrical peaks are evident in both anodic and cathodic branches. Differences in PVF behaviour in the two solvents have been attributed to solvent uptake in the polymer film (lower for aqueous solutions), changes in site-site interaction parameters for the polymer film (attractive for aqueous electrolytes and repulsive for acetonitrile electrolytes), and differences in deswelling processes in aqueous solution in the reduction half of the cycle as compared with the oxidation half (Figure 2.4). Acetonitrile is a better swelling solvent for PVF than water [18] and break in of the spin-coated films usually requires more cycling in water than in acetonitrile. 

Electrode potential The potential difference across the interface of an electronic conductor and an electrolytic solution. The absolute value is not measurable but a relative value can be assigned with reference to an SHE. Peak potential ( p) In cyclic voltammetry the potential at which the current exhibits a maximum (-f-ve and -ve). 

Fig. 2. Cyclic Voltammetry. (A) Potential variations vs time ( see text for the definitions ). (B) Cyclic Voltammogram. The concentration profiles are represented for different potential/time locations, for A (solid line) and A (dashed line). and cathodic and anodic peak potentials. (C) Effect of the scan rate on the overall shape ( see text ).

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... 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

Fig. 8 Reactions of various carbocations with Kuhn s anion [2 ] as compared with their reduction potentials (peak potentials measured vs. Ag/Ag in acetonitrile by cyclic voltammetry cf. Tables 1 and 8 and Okamoto et al., 1983). SALT, salt formation COV, covalent bond formation ET, single-electron transfer. 

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