Voltammetry basics

Mechanistic analysis, perspectives in modem voltammetry basic concepts and, 32, 1... 

Perspectives in Modern Voltammetry Basic Concepts and Mechanistic Analysis... 

The term polarography basically refers to a method, where the current flowing across the electrochemical interface is recorded as a function of the applied electrode potential, historically in most cases a mercury electrode is involved. Thus polarography might be called also voltammetry. This sometimes results in confusing terms like e.g. AC voltammetry, which is obviously equivalent to AC polarography (see following entry). (Data obtained with this method are labelled DCP.)... 

PARC Anal. Instruments Division, Basics of Voltammetry and Polarography, Application Note-P2, Princeton Applied Research, Princeton, NJ, 1980, p. 7, Fig. 10. 

As has already been stated, voltammetry is usually the predecessor to other techniques capable of furnishing more detailed information. However, the importance of the technique should not be underrated it is generally the very first experiment performed by an clectrochemist on a new system. Without the valuable information furnished by voltammetry, an electro-chemist would waste a large amount of time having to use other, less straightforward, means of determining the basic parameters of the system. 

The anodic oxidation of furans has been studied from both synthetic and theoretical points.285 Because of the many possibilities for delocalization, tetraphenylfuran is a special case but it does allow certain basic processes to be monitored by cyclic voltammetry.286 In nitrobenzene two steps are seen ... 

Voltammetry, perspectives in modern basic concepts and mechanistic analysis, 32, 1 Volumes of activation, use of, for determining reaction mechanisms, 2, 93... 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Chloroform extraction of uranium quinoline complex Uranium adsorbed as azide on basic ion exchange column, uranium desorbed with 1 M hydrochloric acid Uranium adsorbed on bismuthol(II) modified anion exchange resin, desorbed with 0.1 M cysteine Uranium, by square wave adsorptive stripping voltammetry... 

Topics discussed above are some basic principles and techniques in voltammetry. Voltammetry in the frequency domain where i-E response is obtained at different frequencies from a single experiment known as AC voltammetry or impedance spectroscopy is well established. The use of ultramicroelectrodes in scanning electrochemical microscopy to scan surface redox sites is becoming useful in nanoresearch. There have been extensive efforts made to modify electrodes with enzymes for biosensor development. Wherever an analyte undergoes a redox reaction, voltammetry can be used as the primary sensing technique. Microsensor design and development has recently received... 

As noted in Section 2.2.5, the effect of dimerization may also be seen on the second wave, the wave that corresponds to the reduction of the radicals formed at the first wave. The example presented in Figure 2.35 shows the cyclic voltammetry of benzaldehyde in basic ethanol.26 The second wave represents the reduction of the benzaldehyde anion radicals formed at the first wave that have escaped dimerization. In other words, Scheme 2.29 should be completed by Scheme 2.30. 

FIGURE 2.35. Cyclic voltammetry of benzaldehyde (2mA/) in basic ethanol (pH 12). Adapted from Figure 1 of reference 26, with permission from Elsevier. 

Several electrochemical techniques may yield the reduction or oxidation potentials displayed in figure 16.1 [332-334], In this chapter, we examine and illustrate the application of two of those techniques cyclic voltammetry and photomodulation voltammetry. Both (particularly the former) have provided significant contributions to the thermochemical database. But before we do that, let us recall some basic ideas that link electrochemistry with thermodynamics. More in-depth views of this relationship are presented in some general physical-chemistry and thermodynamics textbooks [180,316]. A detailed discussion of theory and applications of electrochemistry may be found in more specialized works [332-334],... 

Before discussing the voltammogram obtained with the triangular waveform of figure 16.3, which is simply a plot of the observed current intensity versus the applied potential, it is useful to describe some experimental details of a cyclic voltammetry experiment [335-337] and to recall some basic theory of dynamic electrochemistry [180,332], A typical cell (figure 16.4) consists of... 

CH3CN, dimethylsulfoxide, dimethylfor-mamide (DMF) and pyridine, of course, is reversible at the timescale of cyclic voltammetry the first unambiguous studies appeared in 1965, the radical being identified by electron spin resonance (ES R) [34, 35]. The reversibility has been demonstrated by cyclic voltammetry in pyridine even in a basic medium, the second reduction step occurring at a much more negative potential is irreversible [36]. In the presence of proton donors, and, of course, in protic solvents, it is known that O is unstable and that the reduction of O2 proceeds via a two-electron step [10, 27, 37]. The superoxide ion is moderately basic... 

Voltammetry at a glassy carbon (GC) electrode was used to study of the electrochemical deposition of CdTe from the Lewis basic l-ethyl-3-methylimidazolium chloride/tetrafluoroborate room temperature ionic liquid [208]. 

Compared to studies in acidic media, studies on the electrochemical behavior of 1102 " in basic media are more limited. The report from Morris [52] describing voltammetry results for hydroxo and carbonato uranyl complexes is a recent example. Previous studies have been performed mostly in carbonate and bicarbonate solutions. Wester and Sullivan have studied the reduction of 1102 in these solutions to find an electrochemically irreversible process but disproportionation of U(V)02" " was evidenced only in the bicarbonate solutions [67]. 

The electrochemical behavior of Np ions in basic aqueous solutions has been studied by several different groups. In a recent study, cyclic voltammetry experiments were performed in alkali ([OH ] = 0.9 — 6.5 M) and mixed hydroxo-carbonate solutions to determine the redox potentials of Np(V, VI, VII) complexes [97]. As shown in Fig. 2, in 3.1 M LiOH at a Pt electrode Np(VI) displays electrode processes associated with the Np(VI)/Np(V) and Np(VII)/Np(VI) couples, in addition to a single cathodic peak corresponding to the reduction of Np(V) to Np(IV). This latter process at Ep —400 mV (versus Hg/HgO/1 M NaOH) is chemically irreversible in this medium. Analysis of the voltammetric data revealed an electrochemically reversibleNp(VI)/Np(V)... 

The relatively low value is attributed to the absorption of the ferrocene fragment at 280 nm. The isoelectrofocusing of Fc-HPR confirmed its homogeneity with the expectedly shifted isoelectric point to a more basic pH (cf. 8.1 and 8.5 for HRP and Fc-HRP, respectively). Cyclic voltammetry experiments indicated the ferrocene moiety in Fc-HRP (Fig. 12). New anodic and cathodic peaks are around 360 and 300 mV, respectively (Fig. 12b). The redox potential of 327 mV for Fc-HRP is shifted anodically compared with that for Fc-Hemin in water (264 mV). This may indicate that the ferrocene unit is shielded by amino acid residues located in the vicinity of the active site. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

As will be seen, the rate at which the potential is changed (i.e., the sweep rate) becomes veiy important. For complex reactions, it may have to be so slow (0.01 mV s 1) that cyclic voltammetry approaches a potentiostatic (rather than a potentiody-namic) technique. On the other hand, too large a sweep rate may yield parameters that are not those of the steady state and hence are difficult to fit into a mechanism of consecutive reactions in which the attainment of a steady state (d6/dt = 0) at each potential is a basic assumption. Thus, determining the mechanisms of reactions that are to function in steady-state devices such as fuel cells or reactors is more likely to... 

The summary given by this author rests on the work of several theorists who followed the work of Sevcik. Among the most outstanding of these is Paul Delahay who. with Strassner and others in 1951 1953 contributed much to the basic theory of linear sweep voltammetry with partial interfacial control. Students interested in programs for such simulations should contact Prof. David K. Gosser, Chemistry Department, City College of New York, NY, 10031. 

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