Cyclic voltammograms transfer

For multielectron-transfer (reversible) processes, the cyclic voltammogram consists of several distinct peaks if the E° values for the individual steps are successively higher and are well separated. An example of such a mechanism is the six-step reduction of the fullerenes C60 and C70 to yield the hexaanion products and C7q. Such six successive reduction peaks are observed in Figure 2-4. 

For quasi-reversible systems (with 10 1 > k" > 10 5 cm s1) the current is controlled by both the charge transfer and mass transport. The shape of the cyclic voltammogram is a function of k°/ JnaD (where a = nFv/RT). As k"/s/naD increases, the process approaches the reversible case. For small values of k°/+JnaD (i.e., at very fast i>) the system exhibits an irreversible behavior. Overall, the voltaimnograms of a quasi-reversible system are more drawn-out and exhibit a larger separation in peak potentials compared to those of a reversible system (Figure 2-5, curve B). 

FIGURE 2-6 Cyclic voltammograms for a reversible electron transfer followed by an irreversible step for various ratios of chemical rate constant to scan rate, k/a, where a = nFv/RT. (Reproduced with permission from reference 1.)... 

Example 2-2 The following cyclic voltammogram was recorded for a reversible couple Calculate the number of electrons transferred and the formal potential for the couple. 

F. la-c. Cyclic voltammograms of dissolved and stance confined ferrcx ne in a< tonitrile/0.1 M TBAP. a. 4 X 10 M dissolved ferrocene at Pt. b. 4-ferrocenyl-phenylacetamid monolayer bound to Pt (ref. ). c. Poly-vinylferrocene dip coated on Pt,r = 1 x lO raolcm. Straight arrows indicate diffusional events. Curved arrows electron transfer events (from ref. ). 

Figure 17.12 Direct electrocatal3ftic oxidation of D-fnictose at a glassy carbon electrode painted with a paste of Ketjen black particles modified with D-fructose dehydrogenase from a Gluconobacter species. The enzyme incorporates an additional heme center allowing direct electron transfer from the electrode to the flavin active site. Cyclic voltammograms were recorded at a scan rate of 20 mV s and at 25 + 2 °C and pH 5.0. Reproduced by permission of the PCCP Owner Societies, from Kamitaka et al., 2007.

FIG. 2 Cyclic voltammogram of the ferricenium transfer across the water-DCE interface at lOmVs. The electrochemical cell featured a similar arrangement to Fig. 1(b), but the organic phase contained 2mM of ferrocene. Heterogeneous oxidation of Fc occurred in the presence of 0.2mM CUSO4 in the aqueous phase. Supporting electrolytes were lOmM 02804 and lOmM BTPPATPBCl. The transfer of the standard tetramethylammonium (TMA+) under the same condition is also superimposed. 

FIG. 1 Cyclic voltammogram of acetylcholine cation transfer from water to DCE. The liquid-liquid interface was supported at a 20/xm-diameter hole formed in the 12/xm-thick polyester film. The sweep rate was lOmV/s. (Reprinted with permission from Ref. 3a. Copyright 1989 Elsevier Science S.A.)... 

More recently, Manzanares et al. [17] presented additional experimental evidence of the enhanced cation transfer across a hemispherical water-1,2-DCE interface covered with DSPC by using a syringe experimental set up described elsewhere [9,28]. Figure 7 shows the cyclic voltammograms measured in the cell [17],... 

Dinitrophenol (DNP) gives a single wave in NPV and a pair of anodic and cathodic peak currents in CV at the NB/W interface in pH range studied. Figure 7 shows a cyclic voltammogram of 0.6 mM DNP (NB) at O.IM TPenATPB (NB)-O.IM LiCl, 50 mM phosphate buffer, pH 6.3 (W), that can be assigned to one-proton transfer assisted by A present in NB. The E]j2 vs. pH plot is shown in Fig. 8, in which the results obtained with... 

As mentioned above, the distribution of the various species in the two adjacent phases changes during a potential sweep which induces the transfer of an ion I across the interface when the potential approaches its standard transfer potential. This flux of charges across the interface leads to a measurable current which is recorded as a function of the applied potential. Such curves are called voltammograms and a typical example for the transfer of pilocarpine [229] is shown in Fig. 6, illustrating that cyclic voltammograms produced by reversible ion transfer reactions are similar to those obtained for electron transfer reactions at a metal-electrolyte solution interface. 

FIG. 6 Typical cyclic voltammogram obtained for the transfer of pilocarpine hydrochloride at the water-DCE interface. The organic phase contains 0.01 M tetrabutylammonium tetrakis(4-chloro-phenyl)borate, the aqueous solution is 0.01 M HCl + 0.2 mM pilocarpine hydrochloride, and the sweep rate is fixed at 10, 25, 75, 100, and 150mV/s. (Reprinted from Ref. 229.)... 

The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. 

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

Cyclic voltammograms of DTT-TTF, 86a and 86b, exhibited two reversible one-electron transfer processes corresponding to the successive formation for the stable cation radical and dication <2003JMC1324>. 

The Prussian blue/Prussian white redox activity with potassium as the countercation is observed in cyclic voltammograms as a set of sharp peaks with a separation of 15-30 mV. These peaks, in particular the cathodic one, are similar to the peaks of the anodic demetallization. Such a set of sharp peaks in cyclic voltammograms correspond to the regular structure of Prussian blue with homogeneous distribution of charge and ion transfer rates throughout the film. This obvious conclusion from electrochemical investigations was confirmed by means of spectroelectrochemistry [10]. 

In this context it is noteworthy to refer to the unsaturated analogue l,2-di(9-anthryl)ethene [32] (Weitzel and Mullen, 1990 Weitzel et al., 1990). Like [6] (Becker et al., 1991), compound [32] forms a stable dianion and tetra-anion upon reduction. In the cyclic voltammogram of [32], the first two electrons are transferred at nearly the same potential, pointing to an effective minimization of the Coulombic repulsion between the charged anthryl units (Bohnen et al, 1992). This situation, which again corresponds to that in [6], could imply a torsion about the central olefinic bond (Bock et al., 1989). 

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