Potential half-peak

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 ... 

Generally the peak height/ ip, is linearly dependent on the analyte concentration however, the impression given by the idealized Fig. 3.64 that Ep agrees with E) is less correct, but the mid-value between Ep and the half-peak potential at the high side EiP appears to be a better, although not strict, approximation of E (see later). 

Table 5. Oxidation potentials (half peak potentials, Epl/2) of fluoro-ethylamines and related amines...

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,... 

First the reversibility criteria [332] (1) The anodic and cathodic peak potentials must be independent of the scan rate v (2) the difference between the anodic and cathodic peak potentials must be such that EP a - E c = 59 mV at 298.15 K and for n = 1 (3) the difference between the anodic or cathodic peak potential and the corresponding half-peak potential must be 57 mV at 298.15 K and for n = 1, for example, Ep.a - Ep/2,a = 57 mV (4) the anodic and cathodic peak currents must be equal, that is, /p,a//P)C = 1 and (5) the peak currents must be proportional to the square root of the scan rate, zp a v1/2. 

Half-peak potentials (Ep/o) from cyclic voltam-metiy of alkanes and alkenes in acetonitrile... 

Table 6.5. Half-peak potentials from cyclic voltammetry of phenols and...

As noted for heteroatom attachment in 8-oxo-dG and C8-arylamine adducts, attachment of the Ph moiety to the C-8 site of dG enhances the one-electron donor characteristics of the purine nucleoside. The redox properties of 8-/>-X-Ph-dG (X = OH, OCH3, CH3, H, CN, CHO) adducts have been studied by cyclic voltammetry in anhydrous DMF. The C8-aryl adducts exhibited irreversible one-electron oxidation peaks with half-peak potentials ( p/2) ranging from 0.85 V versus saturated calomel electrode (SCE) for 8-/>-PhOH-dG (X = OH) up to 1.11 V/SCE for 8- -CHO-Ph-dG. All adducts were oxidized more readily than dG, which gave p/2= 1.14 V/SCE in DMF (Table 2). 

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. 

The LSV current voltage curve, often called an LSV wave, for a reversible charge transfer reaction is shown in Fig. 2. The features of interest which are labelled are the peak current (/p), the peak potential (Fp), the reversible potential (Exev), and the half-peak potential (iJp/2). 

These three relationships provide useful experimental criteria for reversible LSV waves. A plot of Ipjvl/2C0 vs. vx/1 is expected to be linear with zero slope. The peak and half-peak potentials for Nemstian charge transfer are independent of v and Ep — Epa is expected to be equal to 56.5/n mV at 298.1 K. 

A comparison of LSV slopes obtained from peak and half-peak potential measurements6... 

This term denotes a potential whose nature depends on the technique used. Typical characteristic potentials are the half-wave potential in polarography, the quarter-transition-time potential in chronopotentiometry, and the peak or half-peak potential in stationary-electrode voltammetry. Regardless of its nature, the characteristic potential always depends on the identity of the electroactive substance, on the kinetics or thermodynamics of the electron-transfer process, and of course on the experimental conditions for any particular technique and under any completely defined set of experimental conditions the value of any characteristic potential is a reproducible property of the electroactive substance. 

Eq. (2.32) (= Ef1 + lny/DR/Do ) Another interesting point of the voltammogram is the half peak potential, which is the potential at which the current is the half of the peak current. This value is of importance when the peak of the /- curve is very broad and/or badly defined, and it is given by... 

Note that both the peak and half peak potentials are independent of the scan rate in agreement with Eqs. (5.57)—(5.58). This behavior can be seen in Fig. 5.4, where the voltagrams corresponding to a Nemstian process for different scan rates in the range 50-500 mV s-1 have been plotted. 

Another useful parameter of the voltammetric curves is the half-peak potential p/2> which is the potential at which the registered current reaches half its maximum value. For a reversible process Em is located half-way between Ep and p/2. 

Fig. 9. Correlation of half peak potentials with energies of two visible charge transfer bands. In order of increasing Ep/2 the compounds are Fephen2(CN)2, Fephen2 (CN)2 BF3, Fephen2(CNBF3)2, Fephen2(CNBCl3)2 and Fephen2(CNBBr3)2.

For aromatic hydrocarbons which form carbenium ions upon protonation, the order of oxidation potentials conforms to the rule. The peak potential for the proton adduct in methylene chloride/7% CF3SO3H was found (Hammerich and Parker, 1974) to be displaced about 1 5 V towards more positive potentials than that of the parent hydrocarbon for a series of 9,10-disubstituted anthracenes. We can use this potential difference to estimate p/2 for protonated hydrogen at about 1 5 V versus the Pt/H2 electrode and, accordingly, a methyl cation coordinated to a hydrogen molecule (= CHj ) would be expected to have its half peak potential somewhere around this value. For protonated hydrocarbons, half peak potentials fall in the region of 1 9- 2 2 V versus the Pd/H2 electrode. 

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,... 

Theoretical linear sweep voltammogram for a reversible charge transfer and a planar electrode, using the dimensionless auxiliary function x at) of Table 6.3. The half-wave potential Ey2 and the half-peak potential p/2 (scan-dependent) are shown. 

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). 

Comparison of Eq. (225) and Eq. (193) or (211) shows that the problem is identical to that presented for steady-state methods. Thus, the same mathematical derivations show that any characteristic potential figure (Ep or half-peak potential Ep/2, and so on) varies linearly with the logarithm of A = k0. Indeed, one obtains by simple transposition in Eq. (219),... 

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