Anodic peak potential

Thus, the peak separation can be used to determine the number of electrons transferred, and as a criterion for a Nemstian behavior. Accordingly, a fast one-electron process exhibits a AEp of about 59 mV Both the cathodic and anodic peak potentials are independent of die scan rate. It is possible to relate the half-peak potential (Ep/2. where the current is half of the peak current) to the polarographic half-wave potential, El/2 ... 

The conspicuous separation between the cathodic and anodic peak potentials was initially interpreted in terms of the simple theory for redox polymers as a kinetic effect of slow heterogeneous charge transfer the thermodynamic redox potential of the whole systems was calculated from the mean value between Ep and Ep ... 

Typical results of these authors are shown in Figure 3.46. A small faradaic re-oxidation wave was observed at very high sweep rates, i.e. up to 4400 Vs"1. The standard potential of the C02/C0 2 couple was evaluated as the middle of the interval between the cathodic and anodic peak potentials at 4400 Vs giving —2.21 V vs. SCE. 

The cyclic voltammograms in Figure 3.73 show an interesting characteristic in that, at potentials greater than the anodic peak potential, the current tends towards a constant value. The charge passed, Q, in charging a capacitor C to a potential V is (see section 2.1.1.) ... 

Except for very electron-rich donors that yield stable, persistent radical cations, the ox values are not generally available.64 Thus the cation radicals for most organic donors are too reactive to allow the measurement of their reversible oxidation potentials in either aqueous (or most organic) solvents by the standard techniques.65 This problem is partially alleviated by the measurement of the irreversible anodic peak potentials E that are readily obtained from the linear sweep or cyclic voltammograms (CV). Since the values of E contain contributions from kinetic terms, comparison with the values of the thermodynamic E is necessarily restricted to a series of structurally related donors,66 i.e.,... 

The current responses may be displayed as a function of time, as in Figure 1.1c, or as a function of potential, as in Figure 1.1c. The latter presentation is generally preferred and is what is meant in short by the phrase cyclic voltammetry. The fact that the response is symmetrical about the potential axis provides a clear indication of the reversibility of the system, in both the chemical sense (the electron transfer product is chemically stable) and the electrochemical sense (the electron transfer is fast). If the electron transfer product were unstable, the anodic current would be less than the cathodic current, eventually disappearing for high instabilities. For a slow electron transfer and a chemically stable product, the current-potential pattern is no longer symmetrical about the vertical axis, the anodic peak potential being more positive than the cathodic peak potential. 

Table 1 Anodic peak potentials of common spin traps, determined by cyclic voltammetry in acetonitrile, unless otherwise stated.

Anodic peak potentials for cyclic voltammetry of amines in acetonitrile, NaC104, scan rate 6 V s . Ref. [74],... 

Owing to the presence of surface waves upon reoxidation, anodic peak potentials are reported. Controlled potential electrolysis used to generate Ceo due to insolubility of Ceo in MeCN. 

TABLE 7. Cyclic voltammetric anodic peak potentials for methyltriarylphosphonium tosylates. Reproduced by permission of VCH from Reference 40... 

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.

Fig. 3. Peak potentials V versus SCE of S-chloro-8-methoxyquinoline in dependence of the sweep rate v without and in the presence of C02. Curve c, cathodic peak potential without C02 a, anodic peak potential without C02 b, cathodic peak potential in the presence of C02. (Reprinted from Ref. 16 with permission from Acta Chemica Scandinavica.)...

Cyclic voltammetric methods, or other related techniques such as differential pulse polarography and AC voltammetry,3 provided a convenient method for the estimation of equilibrium constants for disproportionation or its converse, comproportionation. In this respect, the experimentally measured quantity of interest in a cyclic voltammetric experiment is E>A, the potential mid-way between the cathodic and anodic peak potentials. For a one-electron process, E,A is related to the thermodynamic standard potential Ea by equation (4).13 In practice, ,/2 = E° is usually a good approximation. 

In cyclic voltammetry, for example, E1/2 is, to a good approximation, equal to the potential midway between the cathodic and anodic peak potentials of a reversible voltammogram. Normally, D0 DR so the term involving the diffusion coefficient is small (a few millivolts) and E1/2 is an accurate measure of E0, but in some instances the difference is significant the diffusion coefficients should be measured and a correction applied. 

Data were obtained in acetonitrile solution containing 0.1 mol dm-3 Bu"NBF4 as supporting electrolyte. Solutions were 3 x 10"3 mol dm-3 in compound and potentials were determined with reference to SCE at 21 1°C at 50 mV s"1 scan rate. The CVs of [28], [29] and [31] consisted of a main current wave (reversible for [30] and [32] and EC mechanism for [28], [29] and [31]) corresponding to the Fc+/Fc couple and minor current waves (irreversible or quasi-reversible) from the oxidation of the amino groups. p, represents the anodic current peak potential of the Fc+/Fc couple. "Anodic shifts of the anodic peak potential of the Fc+/Fc couple produced by the presence of metal cations (1 or 2 equiv added as their perchlorate salts). For [28], [29] and [31], after addition of cations, the current waves from the respective amino groups disappeared and that of the Fc+/Fc couple became reversible. Obtained in methanol, instant oxidation by silver cations. 

From Eqs. (5.57) and (5.59), it follows immediately that the midpoint between cathodic and anodic peak potentials coincides with the half-wave potential (see also Fig. 5.4) ... 

Unfortunately, this method is not generally applicable. In cases where the product of one-electron transfer (S+1) is unstable or has a shorter lifetime than required for a reversible electrode reaction, the anodic peak potential is shifted toward less positive values, depending on the scan rate, the rate constant of the decay reaction, and the reaction order. The oxidation potentials derived under these conditions are not reliable. On occasion, the peak potentials for nonreversible reactions are quoted in the literature, sometimes to as many as three significant figures. The critical reader will realize that nonreversible peak potentials provide a basis only for qualitative comparisons. Since few, if any, strained ring compounds have stable one-electron oxidation products, their oxidation is not reversible, and reliable oxidation potentials cannot be determined by the CV method. 

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