Half-peak potential measurements

Ep/2 is the so-called half peak potential, measured on peak voltammograms as the potential at half the peak current. 

As concerns the mechanism of anodic oxidation of 5 at sacrificial anodes, it can be noted that the process occurs at potentials close to those of the oxidation of the corresponding diorganomagnesium compounds (1). For example, half-peak potentials for the oxidation of 5b and lb in THF containing 0.25 M TBAP at a lead electrode measured at a scan rate of 0.3 V s are equal to Jip/2 = —1.73 and —1.72 V vs. 0.01 M Ag /Ag, respectively. However, the oxidation mechanism for both compounds is different, as shown... 

The curve is characterized by the peak height h, the peak potentials Fpc(red) and pa(ox) (or the half-peak potentials, which may be easier to measure exactly than the peak potential because of the somewhat flattened shape of the peak), and the peak height of the anodic peak. Further information may be obtained by semiintegration whereby a curve is obtained that resembles a polarographic curve and that contains all the information of the original data, not only peak heights and peak potentials. 

It is often convenient to report an empirical and conveniently measured half-peak potential Ep/2, at which the current has reached ip/2j one-half of its maximum value. This Ep/2 depends on experimental conditions (the potential scan speed v, and whether the electrode reaction is reversible, irreversible,... 

Both derivative CV and SHAC voltammetry require specialized instrumentation. A much more simple experimental procedure has been described for electrode potential measurements which can be done with respectable precision using rudimentary instrumentation. The measurement of peak potentials during LSV is normally carried out to a precision of the order of 5 mV. This is because the peak resembled a parabola with a rather flat maximum. On the other hand, the half-peak potential where the current is half the peak value, has just as much thermodynamic significance and can be measured to about 1 mV using x-y recording with a suitable expansion on the potential axis. When used in conjunction with a digital data retrieval system the method is as precise as derivative cyclic voltammetry (Aalstad and Parker, 1980). 

In LSV or CV the experimental data are usually presented as anodic peak potentials Ep) or half-peak potentials (Ep/o). The latter quantity, which for a one-electron process at 298 K is related to Ep through Eq. (54), may more easily be measured to a high degree of precision owing to the steepness of the voltammetric wave in the potential region corresponding to Ep/2-... 

The current is measured throughout the experiment and the resulting current-potential curve (voltammogram) displays the typical shape shown in the lower plot in Figure lA, also reporting main parameters. They are ip = peak current, i.e., the maximum current value Ep = peak potential, i.e., the potential corresponding to ip Ep/i = half-peak potential, i.e., the potential at which i = /p/2. 

The half-peak potential of the prepeak shifted to a negative value along with an increase in pK of the added acid. In the presence of mixed acids in the solution, stepwise prepeaks may appear when their pK values are different enough from each other. On the other hand, when the acid strengths and diffusion coefficients were nearly the same for all the acids, only one prepeak could be observed at an identical potential with a height proportional to the sum of the total acid concentration for all the acids. In such cases, the total acid determination can be made by measuring the prepeak height. 

Controlled potential electrooxidations at potentials corresponding to the half-peak potential (Ep/2) for oxidation peak la caused the characteristic UV spectrum of 5-HT (X max in 0.01 M HCl = 285 (sh), 272, 215 nm) to disappear and the solution to become purple. The spectrum of this solution is quite complex (X max 530, 350, 300, 272, 220 nm). The measured coulo-metric n-values at Ep/2 were 3.0 0.3. Cyclic voltammograms recorded after such an electrolysis showed that the reversible peaks IIIr/Ila are present along with a small peak Ila and a new oxidation peak Ilia (Figure 3). 

The peak-shaped response of differential-pulse measurements results in unproved resolution between two species with similar redox potentials, hi various situations, peaks separated by 50 mV may be measured. Such quantitation depends not only upon the corresponding peak potentials but also on the widths of the peak. The width of the peak (at half-height) is related to the electron stoichiometry ... 

Plotting the peak potentials as a function of the logarithm of the scan rate allows one to determine the transfer coefficient from the slope of the resulting straight fine in the total irreversibility region. Another possibility for determining a is to measure the half-height peak widths ... 

E = Faraday constant). The equilibrium potential E is dependent on the temperature and on the concentrations (activities) of the oxidized and reduced species of the reactants according to the Nemst equation (see Chapter 1). In practice, electroorganic conversions mostly are not simple reversible reactions. Often, they will include, for example, energy-rich intermediates, complicated reaction mechanisms, and irreversible steps. In this case, it is difficult to define E and it has only poor practical relevance. Then, a suitable value of the redox potential is used as a base for the design of an electroorganic synthesis. It can be estimated from measurements of the peak potential in cyclovoltammetry or of the half-wave potential in polarography (see Chapter 1). Usually, a common RE such as the calomel electrode is applied (see Sect. 2.5.1.6.1). Numerous literature data are available, for example, in [5b, 8, 9]. 

Taking account of the charge associated with peak IIIa, this implies that about 1 pC of the 5 pC of peak Ic measured in scan 20 is still due to gold oxide reduction, corresponding to 95% CoTSPc coverage. Therefore, it can be concluded that peaks Ic and IIIa in scan 20 are related to the same reversibly behaved redox system. Reversibility can be proven by the fact that the half-wave potentials for both waves are the same, and that the peak potential for the anodic wave (IIIa) does not shift with scan number. 

A polarogram of the type shown in Fig. 2 immediately tells us that the substrate is the electroactive species in a certain potential region below that of the anodic limit of the SSE. The potential at half the plateau value of the current is denoted the half-wave potential (E,, 2) of the substrate and is a measure of how easily the compound is oxidized. With a knowledge of the half-wave potential of the substrate it is now easy to link products isolated from macroelectrolyses with the electrode processes possible in the system. This is done by the technique of controlled potential electrolysis (Sect. 4.4). From a peak polarogram, the peak potential (IT ) may be used in the same way asEx, 2. 

As a final point we call attention to the fact that a plot of Y vs. E has the same form as an a.c. voltammogram, i.e. it comprises a peak centred on the half-wave potential—this because the measured current is inversely proportional to Rct. 

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