Reverse scan cyclic voltammetry

In the case of dissociative electron transfer to aromatic compounds, electron transfer is not necessarily concerted with bond dissociation. The substrate 7t-radical-anion may be an intermediate whose existence can be demonstrated by fast scan cyclic voltammetry in aptotic solvents. At fast scan rates, reversible electron transfer occurs. At slower scan rates, die anodic peak height falls and a second reversible electron transfer step appears due to formation of the radical-anion of the compound formed by replacement of the substituent by hydrogen. Cleavage of the... 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

With faster scan cyclic voltammetry, a new two-electron anodic peak was detected, at more negative potentials, for the first stage of the oxidation process, with an accompanying cathodic peak on the reverse scan (11). The ratio of the forward to the reverse peak currents increased towards unity as the scan rate was raised to —200 V s 1 (Fig. 15). This behavior was attributed to the initial two-electron process being accompanied by a fairly rapid follow-up chemical reaction and was successfully analyzed in terms of an EqCi process (quasi-reversible electron transfer followed by a first-order irreversible chemical process), with a rate constant for the chemical step, k, = 250 s 1. 

The voltammogram for the reduction of AN in a DMF/0.1 M TBAPF solution at a tip (radius, 2.5 fim) shows a reduction wave at —2.0 V vs. a QRE (Figure 16.4.5). The tipgenerated species, AN, is so unstable that in most experiments, such as fast scan cyclic voltammetry, one does not observe its oxidation in a reversal experiment. However... 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Tire deprotonation of thiazolium salts (see Section II) under argon at room temperature allowed the characterization of nonfused DTDAF of types 52 and 53 by cyclic voltammetry. Their very good donor properties were confirmed by two quasi-reversible peaks of equal intensity (93CC601). It is noteworthy that upon a second scan the first oxidation peak was shifted from -0.03 to -0.04 V. Upon further scans the voltam-mogram remains unchanged. Tliis interesting feature has been observed previously with TTF analogs. It was demonstrated that the neutral form... 

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

The reversibility of the carrier was tested by cyclic voltammetry. The scan of the solvent and supporting electrolyte is shown in Fig. 13, with and without dissolved oxygen. The oxygen reduction occurs at about — 0.43 V. (vs. SCE). The scan with the complex added, but the solution free of dissolved oxygen is shown as Fig. 14. The carrier is seen to be reduced at about 0.04 V, well within the window of the solvent and electrolyte, and well before reduction of molecular oxygen. 

A comparison of the products of AP hydrolysis of HQDP (HQ), PP, and 1-NP using cyclic voltammetry revealed that HQ produced well-defined peaks, and that the oxidation of HQ is reversible. More importantly, no apparent passivation of the electrode surface was observed even at high millimolar concentrations after 50 scans. Following a series of investigations, this non-fouling nature of HQ was attributed to the non-accumulation of its oxidation products on the electrode surface and the good diffusional properties of HQ at the electrode-solution interface. Another positive feature of HQDP as a substrate for AP is a tenfold greater oxidation current response of HQ compared to those obtained in the presence of PP or 1-NP. Overall, HQDP provides a suitable and attractive alternative substrate system for AP in the development of amperometric immunosensors. 

Cyclic voltammetry shows the Cu(i)/Cu(0) couple to be highly reversible in dimethylformamide (DMF) even at very slow scan rates. Thus, dark blue solutions of the formally Cu(0) complex can be electrochemically... 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

Then, ionic diffusion towards self-affine fractal electrode was experimentally investigated by using cyclic voltammetry in a mixture of 30 wt % glycerol and 70 wt % (0.01 M K4[Fe(CN)6] + 0.5 M Na2S04) solution. The cyclic voltammograms obtained from each electrode at various scan rates clearly showed one set of well-defined current peaks assigned to be a reversible redox couple as follows ... 

FIGURE 2.4. EC reaction scheme in cyclic voltammetry. Derivation of the rate constant from the anodic-to-cathodic peak current ratio in zone KO. In this example the scan is reversed 200 mV (at 25° C) after the peak. 

As already stated, other electrochemical techniques have been used to derive thermodynamic data, some of them considered to yield more reliable (reversible) redox potentials than cyclic voltammetry. This is the case, for instance, of second harmonic alternating current voltammetry (SHACV) [219,333], Saveant and co-workers [339], however, concluded that systems that appear irreversible in slow-scan CV are also irreversible in SHACV experiments. We do not dwell on these matters, important as they are. Instead, we concentrate on a different methodology to obtain redox potentials, which was developed by Wayner and colleagues [350-352]. 

The most popular voltammetric technique is probably cyclic voltammetry (CV), partly because of its early development in theory and the availability of the corresponding commercial equipment. In this technique, the electrode potential is first scanned linearly with time from a starting potential, where no reaction occurs, passing E°, towards another potential, and then reversed back to the starting potential. In this case, the time variable can be conveniently represented by the scan rate, v. 

In linear sweep voltammetric techniques the applied electrode potential is varied from an initial value E to a final value f at a constant scan rate v (single sweep voltammetry). Once the value is reached the direction of the scan can be reversed, maintaining the same scan rate v, and the potential brought back to the initial value (cyclic voltammetry). In the two cases the form of the potential-time impulse can be represented as shown in Figure 1. 

In the case when the preceding chemical reaction occurs at a rate of the same order as the intervention time scale of cyclic voltammetry, the repercussions of the chemical complication on the potential of the electrode process are virtually negligible, whereas there is a significant effect on the current. In particular, it is characteristic of this mechanism that the forward current decreases with the scan rate much more than the reverse current. This implies that the current ratio ipr/ipf is always greater than 1, increasing as scan rates are increased. 

The simplest and most useful case that one can study by single potential step chronoamperometry is that in which " /2> 0/i (z.e. AEor = E2 - E 5 180 mV). This means that the primarily electrogenerated species Red converts to a new species Ox, which is more easily reducible than the initial species Ox. As seen in Section 1.4.3, in cyclic voltammetry such a system exhibits a single reversible process in the forward scan. 

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

Many other substrate-nucleophile couples have been similarly investigated in liquid ammonia and found to conform to the Sr I mechanism quantitatively. Among them, several systems give rise to unusual cyclic voltammetry patterns, exhibiting dips between the two waves of the starting halide (Pinson and Saveant, 1978) or trace crossing upon scan reversal (Amatore et al., 1980a). Examples of such behaviour are shown in Fig. 16. 

During cyclic voltammetry, the potential is similarly ramped from an initial potential E but, at the end of its linear sweep, the direction of the potential scan is reversed, usually stopping at the initial potential E (or it may commence an additional cycle). The potential at which the reverse occurs is known as the switch potential ( >.) Almost universally, the scan rate between E and Ex is the same as that between Ex and E. Values of the scan rates Vforwani and Ubackward are always written as positive numbers. 

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

---