Voltage-Sweep

Now commence the voltage sweep using a scan rate of 5 mV per second, or with a manual polarograph, increase the voltage in steps of 0.05 V. The recorder plot will take the form shown in Fig. 16.4 if a manual instrument is used, then since the current oscillates as mercury drops grow and then fall away, the plot will have a saw-tooth appearance, and for measurement purposes a smooth curve must be drawn through the midpoint of the peaks of the plot. 

Linear sweep voltammetry at the dme. In linear sweep voltammetry (LSV) at the dme a continuously changing rapid voltage sweep (single or multiple) of the entire potential range to be covered is applied in one Hg drop. Originally the rapidity of the sweep (about 100 mV s 1) required the use of an oscilloscope,... 

One specific variant of the technique is known as direct current cyclic voltammetry (DCCV), in which the voltage sweep is over a limited range and a short time and is immediately reversed. The cycle is repeated many times and the pattern of current change is monitored. The technique uses relatively simple electrodes and is used to study redox reactions and there are a range of sophisticated variants of the technique. 

Fig. 7.79. Current-voltage sweep curves on platinum single-crystal faces (100), (111), and (110) at 25 °C. Sweep range 50-1550 mV, sweep rate 0.1 V/s. (Reprinted from F. G. Will, J. Electrochem. Soc. 112(4) 451, 1965. Reproduced by permission of The Electrochemical Society, Inc.)...

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 reduction peaks in curve 3 of Figure 7 are presumed to be due to the transfer of an electron from the conduction band to the oxidized compound. After the voltage sweep of curve 3 the current reverts to curve 1 and the 470 nm photocurrent reverts to its original value. The process is repeatable and delays of several minutes between photoexcitation and voltage sweep show little change in curve 3. 

Fig. 5.6. Principal block diagram of the electronic unit. (1) Signal generation, (2) amplification of /photo, (3) controller for CC mode, (4) dc voltage sweep for I/V mode and (5) communication interface.

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. 

The term voltammetry refers to measurements of the current as a function of the potential. In linear sweep and cyclic voltammetry, the potential steps used in CA and DPSCA are replaced by linear potential sweeps between the potential values. A triangular potentialtime waveform with equal positive and negative slopes is most often used (Fig. 6.8). If only the first half-cycle of the potential-time program is used, the method is referred to as linear sweep voltammetry (LSV) when both half-cycles are used, it is cyclic voltammetry (CV). The rate by which the potential varies with time is called the voltage sweep (or scan) rate, v, and the potential at which the direction of the voltage sweep is reversed is usually referred to... 

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.

The peak separation, A p = °x — E 1, is approximately 57 mV, the exact value depending on the potential Eswitch he- when the voltage sweep is reversed. Equation 6.41 relates to the limiting case when E° - ESwitch = oo. 

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. 

Another limitation of solid electrodes has been their complex diffusion-current response relative to time with slow-sweep voltammetry. The development of a capillary hanging-mercury-drop electrode (HMDE) by Kemula and Kublik,4,5 together with modem electronic instrumentation, allowed the principles of voltage-sweep voltammetry and cyclic voltammetry to be established. The success has been such that this has become one of the most important research tools for electrochemists concerned with the kinetics and mechanisms of electrochemical processes. These important contributions by Nicholson and Shain6 7 rely, as have all electrochemical kinetic developments, on the pioneering work by Eyring et al.8... 

Figure 3.9 Linear voltage-sweep voltammogram with reversal of sweep direction to give a cyclic voltammogram. Initial sweep direction to more negative potential.

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