Voltammograms, electron-transfer reactions

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. 

This is another indication of the large potential sensitivity of the CV and LSV methods. Equation (2.46) shows that, in order to increase the measurable area under the voltammogram, the scan rate need only be increased. This will be applicable so long as the kinetics of the surface electron transfer reaction are sufficiently fast. 

Figure 13.5 Cyclic voltammogram of a simple electron transfer reaction.

Another advantage of SWV over CV can be seen when dealing with a separate multi-electron transfer reaction. The CV current wave of each or each group of electrons always contains the contribution from the previous electron transfer, particularly the diffusion-controlled current. Separating currents from different electron transfers can be tedious, if not impossible. It can be even worse when we have to take into account the capacitive charging current. Since both capacitive and diffusion-controlled currents are absent or at least minimized on the 7net vs E curve of an SW voltammogram, current waves from each electron transfer are much better resolved and more accurate information can be obtained. 

Fig. 8 Typical cyclic voltammograms of pure electron transfer reactions (a) effect of quasi-reversibility ks decreases from solid to dashed line) (b) effect of relative values of...

Evans and. Gilicinski [61b] used cyclic voltammetry assisted by a simulation method to determine the rate constants for a homogeneous electron-transfer reaction Oxy + Red2 Red + Ox2. They measured cyclic voltammograms for the mixtures of Ox and Ox2. If the above reaction does not occur, the reduction-reoxidation peaks... 

The diffusion of the electroactive ions is both physical and due to electron transfer reactions.45 The occurrence of either or both mechanisms is a function of the electroactive species present. It has been observed that the detailed electrochemical behaviour of the electroactive species often deviates from the ideal thin film behaviour. For example, for an ideal nemstian reaction under Langmuir isotherm conditions there should be no splitting between the anodic and cathodic peaks in the cyclic voltammogram further, for a one-electron charge at 25 °C the width at half peak height should be 90.6 mV.4 In practice a difference between anodic and cathodic potentials may be finite even at slow scan rates. This arises from kinetic effects of phase formation and of interconversion between different forms of the polymer-confined electroactive molecules with different standard potentials.46... 

In the general case, referred to as quasi-reversible, the electron transfer reaction in Equation 6.6 does not respond instantaneously to changes in . In other words, [R]x=o and [Ojx o are determined not only by the value of — °, but also by the magnitudes of k° and a through Equations 6.10 and 6.11. Typical voltammograms for quasi-reversible electron transfers are shown in Fig. 6.11. There are no simple analytical expressions for ped — °, z ped, ped — p/d, A p and — z°x/z ped for quasi-reversible electron transfers. Values for a given set of v, k° and a are, when needed, most conveniently obtained by digital simulation. 

It follows from the above discussion that an electron transfer reaction that appears reversible at one (low) sweep rate may change to a quasi-reversible or even an irreversible process at higher sweep rates. This should be kept in mind since the application of LSV and CV in kinetics studies usually includes the recording of voltammograms at a number of different sweep rates (see below), and the analysis of the data is usually based on the assumption that the electron transfer is reversible. 

There are a number of molecules for which two- or more electron transfers can be detected (i.e., stepwise processes). For molecules capable of giving two-electron transfer reactions (EE mechanism see reaction scheme (3.II)), the addition of the second electron occurs, in the more typical case, with greater difficulty than the first, so the single pulse voltammogram presents two well-separated waves because of the difference between the two formal potentials defined as... 

Fig. 14.32. Cyclic voltammogram of coenzyme Q within the bilayer electrode. Phosphate buffer (pH 7.4, ionic strength 0.15), scan rate =100 mV/s. (Reprinted from Y. Xiaoling, J. Cullson, S. Sun and F. M. Hawkridge, Interfacial Electron Transfer Reactions of heme Proteins, Charge and Field Effects in Biosystems, M. J. Allen, S. F. Cleary, and F. M. Hawkridge, eds., vol. 2, p. 87, Plenum, 1989.)...

An example of a voltammogram for two consecutive one-electron reductions is shown in Fig. 4. When the two electron transfer reactions are as well separated as those shown in Fig. 4, the difference between the peak potentials for the first and the second electron transfer is to a good approximation equal to the difference in standard potentials E — E . When the two electron transfer reactions are closely spaced, the difference in standard potentials may be determined from the half-peak width of the overlapping waves [56]. 

Figure 7. Simulated (DigiSim) cyclic voltammograms at v = 1 V s for an eC mechanism with E° = -1.5V and k = 2s (dash), 10s (dot), 10 s (dash-dot), 10" s (dash-dot-dot), 10 s (short dash), and 10 s (short dot), corresponding to A. = 0.0513, 0.257, 2.57, 2.57 x 10 , 2.57 X 10, and 2.57 x 10, respectively. The solid line corresponds to the simple electron transfer reaction shown in Fig. 3, where also other simulation parameters are given.

Given the value of v, it is seen that both the shape and the position of the voltam-mogram depend on the magnitude of k. On the other hand, given the value of k, it is intuitively understood that the effect of the chemical reaction will gradually diminish if the sweep rate is allowed to increase. In limit, the experiment time is so short that the chemical reaction does not have the time to manifest itself and, consequently, the voltammogram observed is just that for the electron transfer reaction, Eq. (1). 

As mentioned earlier, the characteristic features of processes in this category are that Ep is close to that for the no-reaction case, Eq.(l), and that the peak current ratio —ip /ip varies from approximately unity to zero. The observation of oxidation current for B during the backward sweep shows that the material conversion is low. By comparison of the voltammograms for the eC and the eCen mechanisms in Fig. 8, it is seen that the second electron transfer reaction in the eCeh mechanism gives rise to only little additional current, illustrating that only a small fraction of B has been converted to C. 

The other noteworthy oxidoreductase system is the redox activity of polyethylene oxide modified cyt c in [EMIM][CF3MeS02l by optical waveguide spectroelectro-chemical analysis [109]. Cyt c was effectively dissolved in IL due to the polyethylene oxide side chains and found to be stable over a period of a month [109]. But the report also confirms that organic bulky ions in ILs do not offer any advantage rather are disadvantageous for electron transfer reactions of proteins having shielded cofactor inside the folded polypeptide domain since no cyclic voltammogram of polyethylene oxide coated cyt c was detected in IL. 

Diffusion coefficient of the substrate (Dg) and diffusion coefficient of the electron-exchange (D ) were calculated from cyclic and disk current voltammograms by using the Koutecky-Levich equation and Fick s first law (14, 15) (Table II). Dg in the polymer domains was estimated as 10 - 10 cm /sec, much smaller than in solution (10 cm /sec). Dg is affected by charge density of the polymer domain, e.g., the diffusion of cations is suppressed in the positively charged domain composed of cationic polyelectrolyte, while anions moves faster. A larger Dg value was observed, of course, for the porous film and not for the film with high density. On the other hand, Dg in the polymer domain was also very small, i.e. 10" - 10" cm /sec. This may be explained as follows. An electron-transfer reaction always alters the... 

As suggested above, by recording an approach curve or voltammogram with the tip close to a substrate, one can study the rates of electron transfer reactions at electrode surfaces (Chapter 6). Because mass transfer rates at the small tip electrodes are high, measurements of fast reactions without interference of mass transfer are possible. As a rule of thumb, one can measure k° values (cm/s) that are of the order of Did, where D is the diffusion coefficient (cm2/s). For example, k° for ferrocene oxidation at a Pt electrode in acetonitrile solution was measured at a 1 /xm radius tip at a d of about 0.1 /xm yielded a value of 3.7 cm/s (24). The use of small tips and small currents decreases any interference from uncompensated resistance effects. 

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