Three-electrode voltammetric analytical

Fig. 13.2 Typical three-electrode voltammetric analytical system.

Voltammetric methods in these methods, a potential is applied to the working electrode using a three-electrode setup (see section 1.6). The electrical current, resulting from charge transfer over the electrode-electrolyte interface, is measured and reveals information about the analyte that takes part in the charge transfer reaction. The potential applied can be constant (chronoamperometry, section2.5), varied linearly (cyclic voltammetry, section 2.3) or varied in other ways (Chapter 2). 

Figure 23-2 shows the components of a simple apparatus for carrying out linear-sweep voltammetric measurements. The cell is made up of three electrodes immersed in a solution containing the analyte and also an excess of a nonreactive electrolyte called a supporting electrolyte. (Note the similarity of this cell to the one for controlled-potential electrolysis shown in Figure 22-7.) One of the three electrodes is the working electrode, whose potential versus a reference electrode is varied linearly with time. The dimensions of the working electrode are kept small to enhance its tendency to become polarized. The reference electrode has a potential that remains constant throughout the experiment. The third electrode is a...

As it can provide some of the most basic electrochemical information related to the reactivity of the selected analyte (peak potential and peak current) most instruments that perform amperometry can also perform some of the most basic voltammetric techniques. These techniques determine the current as a function of the potential applied to the WE (in a conventional three-electrode cell) and can be performed with relatively simple instrumentation [105,106]. As different signals can be combined in the input ports of the instrument, multiple variations of the technique have been developed including cyclic voltammetry, linear sweep voltammetry, linear sweep stripping voltammetry, stripping voltammetry [107, 108], fast-scan cyclic voltammetry [109], square-wave voltammetry [110],and sinusoidal voltammetry [111]. 

Again for the titration of Ce(IV) with Fe(II) we shall now consider constant-potential amperometry at one Pt indicator electrode and do so on the basis of the voltammetric curves in Fig. 3.71. One can make a choice from three potentials eu e2 and e3, where the curves are virtually horizontal. Fig. 3.74 shows the current changes concerned during titration at e1 there is no deflection at all as it concerns Fe(III) and Fe(II) only at e2 and e3 there is a deflection at A = 1 but only to an extent determined by the ratio of the it values of the Ce and Fe redox couples. The establishment of the deflection point is easiest at e2 as it simply agrees with the intersection with the zero-current abscissa as being the equivalence point in fact, no deflection is needed in order to determine this intersection point, but if there is a deflection, the amperometric method is not useful compared with the non-faradaic potentiometric titration unless the concentration of analyte is too low. 

Other analytical techniques. Electroanalytical methods can also be used to differentiate between ionic species (based on valence state) of the same element by selective reduction or oxidization. In brief, the electroanalytical methods measure the effect of the presence of analyte ions on the potential or current in a cell containing electrodes. The three main types are potentiometry, where the voltage difference between two electrodes is determined, coulometry, which measures the current in the cell over time, and voltammetry, which shows the changes in the cell current when the electric potential is varied (current-voltage diagrams). In a recent review article, 43 different EA methods for measuring uranium were mentioned and that literature survey found 28 voltammetric, 25 potentiometric, 5 capillary electrophoresis, and 3 polarographic methods (Shrivastava et al. 2013). Some specific methods will be discussed in detail in the relevant chapters of this tome. 

In this section, the effect of chemical reactions coupled with electron transfer processes studied by three pulse methods is discussed, namely in normal pulse (NP), differential pulse (DP), and square-wave polaro-graphic/voltammetric techniques. These methods, especially DPV, belong to the most frequently employed voltammetric methods in contemporary analytical practice. In recent years, criteria for elucidation of electrode mechanisms have been also developed for these techniques. Under favorable conditions (in pure kinetic zone), the electrode mechanisms for simple reaction systems can be established without difficulties. 

The use of CEP-coated microelectrodes offers several advantages over the use of conventionally sized electrodes. The use of microelectrodes means that volt-ammetric measurements can be carried out using much lower concentrations of supporting electrolytes. With the FIA system for detection of ions this enables much lower (sometimes three orders of magnitude lower) detection limits to be obtained [31]. Also, with microelectrodes, faster switching routines can be used without distortion of the voltammetric response [223]. The use of microelectrodes also enables microarrays to be employed [77,224]. This enables multicomponent analyses and pattern recognition to be used for analytical purposes. 

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