Quasireversible processes voltammetry

Since in cyclic voltammetry the potential scan rate commonly ranges from 0.02 V s-1 to 50 V s-1, it follows that for a quasireversible process ... 

Due to the similarity with surface electrode processes, a quasireversible reaction in thin-film voltammetry exhibits a quasireversible maximum and splitting of the net peak. The reasons causing these voltammetric features are the same as for surface... 

The Pu +/Pu + couple for a series of Pu(IV)/(EDTA ) based complexes, where EDTA = ethylenediaminetetraacetate, has been studied as a function of pH and EDTA concentration [118]. The voltammetry was also studied with citrate and carbonate ions present in solution. At a relatively low pH of 2.3 and equimolar Pu +/EDTA concentrations a quasi-reversible one-electron reduction is observed for Pu(EDTA) at E /2 = 0.342 V versus SHE. The quasireversibility of this process remains as the pH is raised to 4.6. Additional voltammetry studies are discussed in the paper for the higher coordinate Pu + species, Pu(EDTA)-L (where L = EDTA, carbonate, citrato), all of which show irreversible electron-transfer behavior. 

One of the first reports on the quasireversible electrochemistry of redox proteins appeared in 1977 when Eddowes and Hill demonstrated (10) cyclic voltammetry of horse heart cytochrome c at a gold electrode in the presence of 4,4 -bipyridyl (Bipy) in solution. In the voltammo-grams (Fig. 1), the peak-to-peak separations were close to 60 mV and the faradaic currents varied linearly with (scan rate), indicating a quasireversible one-electron transfer process controlled by linear diffusion of redox species to the electrode surface. The midpoint potential... 

Method ofMirkin and Bard (33). If the voltammetry is based on steady-state currents, one can analyze a quasireversible wave very conveniently in terms of two differences, Eu — and IE3/4 - Ey. Mirkin and Bard have published tables correlating these differences with corresponding sets of and a hence one can evaluate the kinetic parameters by a look-up process. Reference 33 contains a table for uniformly accessible electrodes, which applies to a spherical or hemispherical UME. A second table is given for voltammetry at a disk UME, which is not a uniformly accessible electrode. 

By using the potential program given by (6.5.1) and (6.5.2) in the equations for linear scan voltammetry in Section 6.4, the i-E curves for the quasireversible one-step, one-electron process can be derived. In this case the wave shape and are functions of v, a, and Ex- As before, however, if Ex is at least 90/n mV beyond the cathodic peak, the effect of Ex is small. In this case the curves are functions of the dimensionless parameters a and either A (see equation 6.4.3) or an equivalent parameter, if/ defined by ... 

More recently, Wudl and coworkers have reported a terthienyl utilizing a central Ceo-functionalized thieno[3,4-fo]pyrazine (Scheme 12.13) [87]. Terthienyl 59 was electropolymerized to produce polymer 60 as greenish-brown thin films on either platinum or ITO. The CV of 60 was reported to exhibit three reversible and one quasireversible redox waves at —0.43, —0.93, —1.39, and —1.94 V (vs. Ag/Ag" "). An g of 0.215 eV was determined from the onsets of the p- and n-doping redox processes (—0.050 and —0.265 V, respectively). The p- and n-doping onset potentials were also determined by differential pulse voltammetry (DPV) to give an even lower Eg of 0.176 eV. 

In contrast to classical cyclic voltammograms, AC-cyclic voltammograms have a clear baseline, which is advantageous for quantitative measurements. By extending AC linear sweep voltaimnetry by a reverse scan, AC cyclic voltammetry is obtained. If the surface concentrations of the electroactive species are the same at the same potential for forward and reverse scans, the peaks for forward and reverse scans are expected to be identical. If the DC process is not fully reversible, the surface concentrations of the electroactive species are different at a given DC potential for forward and reverse scans— that is, for quasi-reversible systems a displacement of the jjeaks for forward and reverse scan can be observed. This displacement can be used to derive kinetic parameters of the electrode reaction. For instance, the derivation (1, p. 393] (or Eq. 5-33) for sluggish one-step heterogeneous quasireversible and/or... 

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