Half-wave, anodic, potentials

Table 13. Polarographic half wave anodic potentials by a two step oxidation of arylated phenols in H0Ac/H20 (9 1) 5% NaOAc against an aqueous Ag/AgCl electrode...

Solvent CH2CI2 scan rate 50 mV s. Anodic peak (ap) potential. Half-wave reduction potential. log A" = AEIQ.Q59 AE = —Two oxidation peaks are present. Reversible wave. Measured under the same experimental conditions. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

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 ... 

Figure 6.7 shows a typical special feature of the polarization curves. In the case of reversible reactions (curve 1), the anodic and cathodic branches of the curve form a single step or wave. In the case of irreversible reactions, independent, anodic and cathodic, waves develop, each having its own inflection or half-wave point. The differences between the half-wave potentials of the anodic and cathodic waves will be larger the lower the ratio fH. ... 

The diffusion problem can be solved to predict that one wave in NPV or one pair of anodic and cathodic peak currents in CV are to be observed the half-wave potential is expressed by... 

For a solution with only ox or only red, then il a or il c, respectively, disappears from this equation and for the cathodic or anodic half-wave potentials we obtain the same relationship (cf., eqns. 3.15 and 3.16) ... 

In the controlled (constant) potential method the procedure starts and continues to work with the limiting current iu but as the ion concentration and hence its i, decreases exponentially with time, the course of the electrolysis slows down quickly and its completion lags behind therefore, one often prefers the application of a constant current. Suppose that we want to oxidize Fe(II) we consider Fig. 3.78 and apply across a Pt electrode (WE) and an auxiliary electrode (AE) an anodic current, -1, of nearly the half-wave current this means that the anodic potential (vs. an RE) starts at nearly the half-wave potential, Ei, of Fe(II) - Fe(III) (= 0.770 V), but increases with time, while the anodic wave height diminishes linearly and halfway to completion the electrolysis falls below - / after that moment the potential will suddenly increase until it attains the decomposition potential (nearly 2.4 V) of H20 -> 02. The way to prevent this from happening is to add previously a small amount of a so-called redox buffer, i.e., a reversible oxidant such as Ce(IV) with a standard... 

Useful experimental parameters in cyclic voltammetry are (i) the value of the separation of the potentials at which the anodic and cathodic peak currents occur, A = Pia — PiC, and (ii) the half wave potential, 1/2, the potential mid-way between the peak potentials. A value of AE of c. 0.057 V at 25°C is diagnostic of a Nernstian response, such as that shown in Figure 2.87. More generally, if n electrons are transferred from R, then the separation will be 0.057/n V. It should be noted that the expected value for AE of 0.57/nV has no relationship to the usual Nernstian slope of RT/nF = 0.059/n V at 25UC. 

Some of the parameters that can be extracted from the cyclic voltammogram, all shown in figure 16.7, are the anodic and cathodic peak currents (z p,a and zP C), the anodic and cathodic peak potentials (EP a andEP C), the anodic and cathodic half-peak potentials (.Ep/2,aand Ep/2,c), and the half-wave potential (.E /2). For our purposes, E /2 is the most important. It is defined as the average of EP a and p C,... 

Table 13 Anodic half-wave or peak potentials (vs SCE) of hydroxylamines RNHOH or the corresponding anions RNHO-.

In polarography, we obtained the half-wave potential E// by analysing the shapes of the polarographic wave. E1/2 is a useful characteristic of the analyte in the same way as E . In cyclic voltammetry, the position o/both peaks (both forward and back in Figure 6.13 cathodic and anodic, respectively, in this example) gives us thermodynamic information. Provided that the couple is fully reversible, in the thermodynamic sense defined in Table 6.3, the two peaks are positioned on either side of the formal electrode potential E of the analyte redox couple, as follows ... 

Benzoquinones are conveniently prepared in solution by the anodic oxidation of catechols. 1,2-Quinones are unstable in solution but they have a sufficient lifetime for the redox process to be reversible at a rotating disc electrode. Reaction involves two electrons and two protons and the half-wave potential varies with pH at 25 °C according to Equation 6.1. Some redox potentials for catechols and hy-droquinones are given m Table 6.6. 

Table 6.8. Half-wave potentials for the oxidation of 5-membered heterocyclic rings at a rotating platinum anode in acetic acid, sodium acetate...

Anodic stripping voltammetry (ASV). This is an electrochemical technique in which the element to be analyzed is first deposited on an electrode and then redissolved, that is, stripped, from the electrode to form a more concentrated solution. For example, a drop of mercury hanging from a platinum electrode in a solution containing the species to be measured has been used as the deposition electrode. A potential slightly more negative than the half-wave potential for the ion of interest is applied to deposit the element on the electrode. After deposition of the metal for a given... 

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. 

TABLE 8.8 Cathodic Peak Potentials and Anodic Half-Wave Potentials (in V Versus Fc+/Fc)ofSc3N C68)Sc3N C7S, and Dy3N C7S Obtained by Cyclic V oltammetry in o-DCB + 0.05 M (w-Bu)4NPF6 from the Cyclic Voltammograms Comparison with... 

Polarographic anodic half-wave potential HOMO... 

Like benzenoid hydrocarbons, pyridine-like heterocycles give well-developed two-electron waves on reduction at the dropping mercury electrode. The latter are polarographically much more reducible than the former. This can be explained easily in terms of the HMO theory It is assumed (cf. ref. 3) that the value of the half-wave potential is determined essentially by the energy of the lowest free 7r-molecular orbital (LFMO) of the compound to be reduced, and for models of hetero analogues this quantity is always lower than that for the parent hydrocarbons. Introduction of an additional heteroatom into the molecule leads to a further enhancement of the ease of polarographic reducibility.95 On the other hand, anodic oxidation of the heterocyclic compounds is so much more difficult in comparison with benzenoid hydrocarbons that they are not oxidizable under the usual polarographic conditions. An explanation in terms of the HMO theory is obvious. 

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