Electrochemical methods operation principle

Principles and Characteristics Contrary to poten-tiometric methods that operate under null conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise occur spontaneously. It is thus possible to analyse ions and organic compounds that can either be reduced or oxidised electrochemi-cally. Polarography, which is a division of voltammetry, involves partial electrolysis of the analyte at the working electrode. 

A variety of electrochemical methods have been incorporated into automated systems. The most widely used Mec-trochemical approach involves ion-selective electrodes. These electrodes have replaced flame photometry for the determination of sodium and potassium in many analyzers and have lately found direct application in the measurement of other electrolytes and indirect application in the analysis of several other serum constituents. The operating principle of ion-selective electrodes is given in some detail in Chapter 4. The relationship between ion activity and the concentration of ions in the specimens must be established with calibrating solutions, and frequent recalibration must be done to compensate for alterations of electrode response. 

Since the Co(II) to Co(III) complexes were redox active, an electrochemical method of analysis seemed viable for the quantification of the two species in the reaction. The specific electrochemical technique developed to monitor the activation reaction allowed the simultaneous quantitative measurement of (salen)Co(II) and (salen)Co(III) species in the medium. The principle of the method is based on the electro-oxidation of both species on a platinum-rotating electrode linearly polarized with respect to a standard electrode [7]. The electrochemical reactions operative with this cyclic voltammetry technique involve the single electron oxidation of each species and occur at the revolving surface of the electrode. With this salen ligand system, the Co(II) to Co(III) transformation was determined as being fully reversible, while the Co(III) to Co(IV) reaction was irreversible. 

All analysts are familiar with the principles of potentiometry and potarography and indeed, most analytical laboratories will contain a pH meter and a polarograph. However, electrochemical methods arc, in general, not very important in modern analysis. In contrast, there arc spccifiG applications such as trace metal ion analysis in water and effluents and also some other aspects of environmental analysis for which electrochemical methods are particularly attractive. This is because (1) some methods, especially anodic stripping voltammetry, have a very high sensitivity for heavy-metal ions and the lowest detection limit of from 10 to mol dm is well below that of other available methods (2) electrochemical methods are well suited for modification to on-line and/or portable devices for analysis in the held. Whether the analysis is based on current, conductivity or the response of an ion-selective electrode, both the cell and the control electronics are readily miniaturized and operate on low power Hence, this chapter considers the principles of the electroanalytical methods important in environmental and on-line analysis, together with biochemical applications of electrochemical sensors. 

The current interruption (or current interrupt) method is generally used for measuring ohmic losses caused mainly by proton or anion transport resistance in batteries, fuel cells, and other electrochemical cells. The principle of this technique is that the ohmic losses vanish much faster than do the electrochemical overpotentials on the electrodes when the current is interrupted [42]. As shown in Figs 5.5 and 5.6, the cell is operated at a constant current (i) it is... 

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

Flow injection analysis (FIA) is a robust method for automating complex chemical analyses (Ruzicka and Hansen, 1988). It is relatively simple and can be adapted for use with a variety of detectors, including spectrophotometers, fluorometers, mass spectrometers, and electrochemical analyzers. It has been used on board ships to determine dissolved nutrients (Johnson et al., 1985) and trace metals (Sakamoto-Arnold and Johnson, 1987 Elrod et al., 1991). Unsegmented continuous flow analysis (CFA) systems based on the principles of FIA can operate in situ over the entire range of depths found in the ocean (Johnson et al., 1986a, 1989). 

Reference methods for blood gas and electrolyte determinations have been described in detail by the IFCC. A schematic diagram characteristic of a typical instrument is shown in Figure 27-4. Electrochemical principles and structural features of electrodes are discussed in Chapter 4. Prominent manufacturers of blood gas equipment include Roche Scientific Corp., Bayer Diagnostics, Instrumentation Laboratory, Nova Biomedical, and Radiometer America, Inc. Readers are referred to these manufacturers for details and operational features of specific instruments. 

Voltammetric measurements are not simply restricted to analytical laboratories. The applications of these methods are more numerous than is at first obvious. A large number of analytical instruments, whether portable or not, intended to make precise measurements of substrates present in gas mixtures, vapours or solutions are equipped with electrochemical sensors. These devices operate on the principle of the 2- or 3- electrode cell enclosed in the sensor housing. 

It is also possible to measure oxygen and other gas fugacities with electrochemical sensors. These methods use the same theoretical principles that apply to measurement of Eh. The design and operation of such sensors is described in detail by Sato (1971) and Huebner (1987). We will illustrate the technique briefly, by describing an oxygen fugacity sensor. 

This point is very important in the case of computerized systems for both the performing of polarization curves and the processing of experimental data without an operator. The success of the application of some numerical analysis techniques [34, 37] depends on the absence of problems concerning the convergence of numerical sequences used by the method adopted. Such problems may arise when the interval width of the potential difference AE is so small that the available experimental data do not contain the information required for a correct use of the numerical technique. In this case, the evaluation of the electrochemical parameters /<, a and by other methods not subject to convergence criteria is, in principle, physically unacceptable because in the region examined the law (2) cannot be deemed valid. This particular problem has been dealt with by the... 

The method developed by M.M. Faktor et al. is widely used in semiconductor industry. Based on the same principle an equipment was constructed by POLARON for the British Post. The scheme of the system is shown in Fig. 13. The equipment is suitable for the continuous and automatic determination and is used to monitore the impurity profile in the surface layer of semiconductors with a resolution of about 10 nm in the range 1-50 /xm. Using this equipment operating on electrochemical principle the number of charge carriers can be determined in the surface layer. In addition to this n- and p-type semiconductors can be distinguished (Fig. 14). 

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