Electrochemical quasi-reversible processes

Square planar Ni11 complexes (50a) and (50b) of the quinoxaline-2,3-dithiolate ligand are oxidizable in chemically reversible, electrochemically quasi-reversible processes to yield Ni111 species, also featuring the (dxy)1 configuration.198 Interestingly, the difference in protonation state makes for a 0.20V difference in oxidation potential ((50a) +0.12V (50b) +0.32V vs. SCE), consistent with the less basic S-donors in the thione form. 

The effects of mercury film electrode morphology in the anodic stripping SWV of electrochemically reversible and quasi-reversible processes were investigated experimentally [47-51], Mercury electroplated onto solid electrodes can take the form of either a uniform thin film or an assembly of microdroplets, which depends on the substrate [51 ]. At low sqtrare-wave frequencies the relationship between the net peak crrrrent and the frequency can be described by the theory developed for the thin-film electrode because the diffusion layers at the snrface of microdroplets are overlapped and the mass transfer can be approximated by the planar diffusion model [47,48],... 

Cyclic Square Wave Voltammetry (CSWV) is very useful in determining the reversibility degree and the charge transfer coefficient of a non-Nemstian electrochemical reaction. In order to prove this, the CSWV curves of a quasi-reversible process with Kplane = 0.03 and different values of a have been plotted in Fig. 7.17. In this figure, we have included the net current for the first and second scans (Fig. 7.17b, d, and f) and also the forward, reverse, and net current of a single scan (first or second, Fig. 7.17a, c, e) to help understand the observed response. 

So, a totally irreversible process could be mistaken for a quasi-reversible one with a 0.5 (Fig. 7.17f). In order to discriminate the reversibility degree of the electrochemical reaction, it is necessary to take into account that for a quasi-reversible process the peak corresponding to more cathodic potentials in the second scan (denoted as RC by [29]) is higher than that located at more anodic ones (denoted as RA by [29]) when a 3> 0.5, whereas the opposite is observed for a fully irreversible electron transfer for any value of a (see also Table insert, Fig. 7.20). 

Blaedel and Engstrom [48] noted that for a quasi-reversible process the current could be simply expressed in terms of the rate constant and mass-transport coefficient. Application of a square wave step in the rotation rate of a RDE (i.e., PRV, see Section 10.4.1.3) resulted in modulation of the diffusion-limited current and hence modulation of the mass-transfer coefficient. By solving the appropriate quadratic equation it was possible to derive a value for the heterogeneous rate constant for the electrochemical cathodic, kf, or anodic, kb, process of interest. Values for the standard heterogeneous rate constant and transfer coefficient were subsequently... 

The auraferraborane clusters of System 7 show a marked dependence on the identity of the MPhj ligand. The As complex is easier to reduce than the corresponding P complex and the reduction product is more stable (tj/j 0.1 s for the As complex, and tj/2 0.01 s for the P complex ). The major difference, however, is found for the oxidations while the P complex shows two well-defined electrochemically and chemically quasi-reversible processes in CV, the As complex only gives one well-defined oxidation peak at a potential ca 0.3 V higher than F ,i for the P complex (i.e. close to F ,2 for the P complex). A difference of ca 0.3 V in the redox potential upon a change from PPhj to AsPhj seems very unlikely based on the other data in Table 18, and the explanation of the observation is probably that a small pre-peak in the CV of the As complex (at a potential close to F ,i for the P complex) is in fact the first oxidation of the As complex. The unexpected small size may be due to a reorganization process prior to electron transfer, which is much slower for the As than for the P complex. 

The one-electron electroreduction of hexacyanoferrate(III) to hexacyanoferrate(Il) (or ferricyanide to ferrocyanide) is a classic example of a quasi-reversible process. Despite being initially regarded as an outer-sphere adiabatic charge-transfer process [44], the electrode kinetics of the [Fe(CN)6] transformation exhibits a pronounced dependence on the properties of the electrode, and reproducible results can be achieved only when the working electrode surface is reproducibly pretreated and cleaned, either by polishing or electrochemically [47,48]. Metal electrodes provide notably faster heterogeneous electron transfer kinetics of [Fe(CN)6] as compared to carbon surfaces, possibly due to differences in the density of electronic states (DOS) for these materials, which can impose a notable influence on the rate of adiabatic charge transfer [49]. 

In case of quasi-reversible processes when the rate of the reaction is controlled by both diffusion and the charge transfer steps, it is necessary to assess the influence of such kinetic parameters as the exchange current density (/q) and the charge transfer coefficient (a). Furthermore, additional variants to be considered appear related to different mechanisms of the electrochemical process. Therefore, in the simplest case, it is most convenient to analyze reversible processes when -> oo and only the diffusion overvoltage ri is observed in the system. Despite the fact that characteristics of the latter processes differ quantitatively from the first one, general peculiarities remain the same. 

By introducing redox-active N-methyl-4,4/-bipyridinium ion (mbpy+) to the oxo-centered triruthenium cores, a series of triruthenium derivatives bearing two or three axially coordinated mbpy+ were prepared by Abe et al. [12, 13]. Electrochemical studies indicated that these mbpy+-containing triruthenium complexes afforded a total of seven to nine reversible or quasi-reversible redox waves in acetonitrile solutions at ambient temperature. Of these redox waves, four or five one-electron redox processes arise from RU3 -based oxidations or reductions involving five or six formal oxidation states, including... 

For Ag(lOO) crystals, a similar electrochemical behavior was observed with quasi-reversible adsorption/desorption of Cd and surface alloying, faster than for Ag(lll) [286]. Electrochemical and AFM experiments have shown that the alloying process consisted of two steps a very fast reaction occurring within a few atomic layers, and a much slower one, represented by a solid-state diffusion process [244]. 

A preliminary electrochemical investigation of l,2-dimethyl-3-thio-formylindolizine in acetonitrile using mercury electrodes indicated that an initial quasi-reversible electrochemical reduction was followed by an irreversible process with possible loss of hydrosulfide ion.170... 

A reversible criterion will be presented in order to clearly establish the experimental conditions for which a charge transfer process can be considered as reversible, quasi-reversible, or fully irreversible. Note that this criterion can be easily extended to any electrochemical technique. This section also analyzes the response of non-reversible electrode processes at microelectrodes, which does not depend on the electrochemical technique employed, as stated in Chap. 2. 

The effect of the reversibility of electrochemical reaction on the theoretical Qp —t curves calculated from Eq. (6.131) is shown in Fig. 6.22. For reversible processes (k°t > 10), the charge-time curves present a stepped sigmoid feature and are located around the formal potential of the electro-active couple. Under these conditions, the charge becomes time independent (see Eq. (6.132)). As the process becomes less reversible, both the shape and location of the Qp — t curves change in such a way that the successive plateaus tend to disappear and a practically continuous quasi-sigmoid, located at more negative potentials as k°r decreases, is obtained. For k°r < 0.1, general Eq. (6.131) simplifies to Eq. (6.134), valid for irreversible processes and leads to a practically continuous Qp — t curve. 

The influence of the reversibility of the electrochemical reaction on the SW net charge-potential curves ( (Gsw/Gf) - (Eindex is plotted in Fig. 7.48 for different values of the square wave amplitude ( sw = 25,50,100, and 150mV) and three values of the dimensionless surface rate constant (1° ( k°t) = 10,0.25, and 0.01), which correspond to reversible, quasi-reversible, and fully irreversible behaviors. Thus, it can be seen that for a reversible process (Fig. 7.48a), the (Gsw/Gf) — (Eindex EL°) curves present a well-defined peak centered at the formal potential (dotted line), whose height and half-peak width increase with Esw (in line with Eqs. (7.118) and (7.119)), until, for sw > lOOmV, the peak becomes a broad plateau whose height coincides with Q s. This behavior can also be observed for the quasi-reversible case shown in Fig. 7.48b, although in this case, there is a smaller increase of the net charge curves with sw, and the plateau is not obtained for the values of sw used, with a higher square wave amplitude needed to obtain it. Nevertheless, even for this low value of the dimensionless rate constant, the peak potential of the SWVC curves coincides with the formal potential. This coincidence can be observed for values of sw > 10 mV. 

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