Voltage sweep rate

In Fig. 9.5, curve (a) is the cyclic voltammogram (CV) for the oxidation of tri-p-anisylamine (TAA) in 0.1 M Et4NCl04-AN, and curves (b) and (c) are the absorbance-potential curve and its derivative curve, respectively. They were obtained using an OTE with a vapor-deposited platinum film [8]. The excellent agreement of curves (a) and (c) shows that the reaction in the CV is purely the oxidation and re-reduction of the TAA. However, the two curves are different in that the peak current for curve (a) is proportional to the square root of the voltage sweep rate, while the peak height of curve (c) is inversely proportional to the square root of the voltage sweep rate. 

Fig. 9.9 CV and frequency-potential curves for the oxidation and re-reduction processes of the electropolymerized polyaniline film [26] (a) in 0.5 M LiCICVAN, and (b) in aqueous 0.5 M NaCl04+HCIO4 (pH = 1). Voltage sweep rate 5 mVs-1, quantity of film deposition 0.4Ccm-2, and SSCE = saturated NaCl calomel electrode.

The electrohydrodimerisation of acrylonitrile to give adiponitrile (a one-electron process at high substrate concentrations, Scheme 1.8A and Chapter 6) is an example of how an industrially important electrosynthetic process has been investigated following recent instrumental developments, viz. the application of ultramicroelectrodes at low-voltage sweep rates. Use of conventional electrodes would have required substrate concentrations in the mM range but, under these conditions, acrylonitrile undergoes a different reaction - a two-electron electrochemical reduction of the alkene residue (Scheme 1.8B). The switchover between the two reactions occurs at about 1 mol dm-3 substrate concentration. 

Fig. 6.8 Typical cyclic voltammetry waveform corresponding to a voltage sweep rate (v) of 0.1 V s 1 initial — final = 0.5 V- nd Eswitch — —0.5 V.

Quasi-reversible electron reactions are observed for many aromatic compounds at high voltage sweep rates and, at low voltage sweep rates, for compounds that have only small electronic n -systems composed of one or two double bonds. 

For preliminary work, a circular disc electrode with a diameter in the 1-5 mm range is usually used. The substrate concentrations are typically between 1 and 10 mM and the voltage sweep rates are usually between 50 mV s 1 and 500 V s 1. Sometimes the voltage sweep is continued to include several E — t half-cycles. 

In addition to investigating the effect of changing the voltage sweep rate and of addition of nucleophiles/bases (oxidation) or electrophiles/acids (reductions), the preliminary work often also includes investigation of how the voltammograms are affected by changes in the substrate concentration. 

Fig. 11.4. Typical linear sweep voltammogram (LSV) data for the ICA film in background electrolyte LSV data is recorded (a) initially after film formation (b) after cycling and repeated holding first at a potential corresponding to complete oxidation (1 hour) and then at a potential for complete reduction of the film (1 hour) for an overall period of 2 days (c) same as (b) but for 5 days (d) in 0.1 M NaCI04 rather than 0.1 M LiCI04. The voltage sweep rate was 2.5 mV/s"1 in all cases.

Fig. 14 Cyclic voltammograms for the reduction of lucigenin (B +) at room temperature in (a) dichloromethane, and (b) DMF containing BU4NBF4 (0.1 M) voltage sweep rate 86 mV (Ahlberg et al., 1981)...

The LSV observables, the voltage sweep rate dependence (df /dlogv), the substrate concentration dependence (df /dlogCA), and the dependence of the peak potential upon the concentration of an additional reactant dlogCx) are given by eqns (60), (61) and (62), respectively. The symbols a, b,... 

Parker, 1981c. Measured by derivative cyclic voltammetry in solvent containing Me4NBFi (0.05 M) at 18.5°C. vj refers to the voltage sweep rate at which the derivative peak ratio is equal to 0.500. 

Fig. 28 Derivative cyclic voltammograms for the oxidation of 4,4 -dimethoxy-stilbene in acetonitrile at 25°C (a) and — 30°C (b). Voltage sweep rate = 100 V s". (BU4NBF4) = 0.1 M. (Aalstad et al., 1981a)...

Without any further refinement, the cell shown in Fig. 2 is suitable for measurements at voltage sweep rates from about 0.1 to 500 V s using working electrodes with surface diameters ranging from about 2 to 0.1 mm (see also Sec. II.D). 

The perspective in using very high voltage sweep rates during CV has been demonstrated by several authors [125,143,183-187]. Historically, it is of interest to notice that Perone in 1966 already used sweep rates up to 50,000 Vs [183] in studies of the electron transfer... 

It follows from the discussion of electron transfer reactions in Sec. II.B that a reversible process when studied by CV inevitably passes into the quasi-reversible regime at some value of V when v is allowed to increase. For k° = 3cms a value typical for many aromatic hydrocarbons, for example [71], it is seen from Eq. (27) that this happens at approximately v = 170 Vs Thus, studies of electron transfer rates in this region require voltage sweep rates in the range of 200-1000 Vs Ultramicroelectrodes are superb for this purpose, as demonstrated, for example, in a study of the oxidation of ferrocene [190]. Peak potential separations recorded at sweep rates between 500 and 3000 Vs at an electrode with a surface diameter of 10 pm over a range of substrate concentrations resulted in k° equal to 1.10, 1.13, and 1.13 cm s A more recent example is the oxidation of selenanthrene and related compounds [191]. 

The application of phase-selective SHAC voltammetry to organic systems has proved to be fruitful [147,148]. This method is capable of measuring even when the radical cation undergoes a moderately rapid reaction. Another approach for obtaining E° for the formation of reactive radical cations is the application of CV with voltage sweep rates in the kV s range [149-155]. 

Anthracenes without substituents in the 9,10-positions as well as most benzenes and naphthalenes give rise to irreversible oxidation in, for example, MeCN and CH2CI0 during CV at moderate voltage sweep rates. 

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