Electroanalytical method

In order to analyse, by polarography compounds which have been purified in a h.v.s., it is not necessary to have a cell which operates under vacuum. Methods have been described whereby a solution made up under vacuum can be admitted via a burette fitted with a pressure equaliser to a polarography cell operating at atmospheric pressure under nitrogen or, preferably, under argon (Kabir-ud-Din and Plesch, 1978). See also Chapter 3. 

The detection, identification and estimation of impurities by various chromatographic techniques is so well documented that few comments are required here. It must be remembered, however, that since we are concerned with extremely low concentrations, one cannot be sure of finding an impurity whose retention time on a column is close to that of the main compound, and also that a very small fraction of the main compound may undergo transformations on the column, especially if it is at an elevated temperature, so that spurious impurities may be produced in this way. If the main compound is sensitive to any of the components of air, especial precautions must be taken in transferring the sample from its evacuated container to the inlet of the chromatograph. 

One advantage of gas chromatography is the availability of detectors which respond specifically to certain types of compound. The best known are the electron capture detector for chlorine compounds and the flame photometric detector for nitrogen and phosphorus compounds. If one wants to detect very small molecules such as water or CSj, the standard flame ionisation detector must be replaced by a thermal conductivity detector. 

The history of electrochemistry received a welcome boost in 1988 when a symposium entitled Electrochemistry Past and Present was held at the third Chemical Congress of North America at Toronto. The published proceedings contain papers describing the historical development of many electroanalytical techniques.74 Kolthoff has given an account of the state of electroanalytical chemistry prior to World War II based on personal experience.75 

Electrolytic deposition was used as a qualitative analytical technique in the early years of current electricity, but it was not until 1864 that quantitative electrochemical analysis commenced with the development of electrogravimetry by Wolcott Gibbs.76,77 Electrolytic techniques of analysis were greatly refined by Edgar Fahs Smith at the University of Pennsylvania, who introduced the rotating anode and double-cup mercury cathode. Smith s book on electrochemical analysis ran to six editions.78 

One of the exciting developments associated with ion-selective electrodes has been the fabrication of microelectrodes capable of monitoring an intracellular ion concentration. The history of these developments from the mid-1950s has been reviewed.88 a symposium held in 1996 was devoted to the history of ion-selective electrodes. One paper discussed their development and commercialization,89 another described how the 1970s was the decade in which they really became established,90 a third outlined their industrial applications,91 and a fourth traced the evolution of blood chemistry analyses using them.92 The first attempts to construct biochemical sensors by immobilizing enzymes on electrodes date from the 1960s.93 

Polarography has been an extremely valuable electroanalytical method, although its importance has declined in recent years. The life and work of Jaroslav Heyrovsky, the inventor of the technique, was reviewed in the Toronto symposium,94 and also on the occasion of the centenary of his birth.95,96 The 75th anniversary of the discovery of polarography occurred in 1997. On that occasion the son of the inventor gave an account of the studies on the dropping mercury electrode and on the polarization 

Not all techniques that at one time showed promise eventually achieved widespread application. An example is coulometric titrimetry, whose history has been discussed.113 Reviews have also appeared of electroanalytical chemistry in molten salts114 and of the early years of the relatively new technique of spectroelectrochem-istry.115 The development of electrochemical instrumentation from its origins, through the electronic age to the computer age has been discussed.116 

The subject of electrochemistry deals with the study of the chemical interaction of electricity and matter generally, but it is the interaction with solutions that is of particular value in analytical biochemistry. The electrical properties of a solution depend upon both the nature of the components and their concentration and permit qualitative and quantitative methods of analysis to be 

Diverse analytical techniques, some highly sensitive, have been developed based on measurements of current, voltage, charge, and resistance in electrochemical systems. One variable is measured while the others are controlled. Electroanalytical methods can be classified according to the variable being measured. Table 15.2 provides a summary of the more important methods. The methods are briefly defined below and then discussed at length in subsequent sections. 

Conductometric titration Voltage, V(AC) Titrant volume vs. conductance 

Amperometry is the measurement of current at a fixed potential. An analyte undergoes oxidation or reduction at an electrode with a known, applied potential. The amount of analyte is calculated from Faraday s law. Amperometry is used to detect titration endpoints, as a detector for liquid chromatography and forms the basis of many new sensors for biomonitoring and environmental monitoring applications. 

A variety of electroanalytical methods are used as detectors for liquid chromatography. Detectors based on conductometry, amperometry, coulometry, and polarography are commercially available. 

In conductometry, an alternating (AC) voltage is applied across two electrodes immersed in the same solution. The applied voltage causes a current to flow. The magnitude of the current 

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The separation of the element or of the compound containing it may be effected in a number of ways, the most important of which are (a) precipitation methods (b) volatilisation or evolution methods (c) electroanalytical methods and (d) extraction and chromatographic methods. Only (a) and (b) will be discussed in this chapter (c) is considered in Part E, and (d) in Part C. 

R Kalvoda, Electroanalytical Methods in Chemical and Environmental Analysis, Plenum, New York, 1987... 

Electroanalytical methods in the study of chelation reactions. M. Kopanica and J. Zyka, Chelates Anal. Chem., 1972, 3,151-209 (196). 

Conway, B. E. Electroanalytical Methods for Determination of AI2O3 In Molten Cryolite 26... 

The main electroanalytical techniques are electrogravimetry, potentiometry (including potentiometric titrations), conductometry, voltammetry/polarography, coulometry and electrochemical detection. Some electroanalytical techniques have become very widely accepted others, such as polarography/voltammetry, less so. Table 8.74 compares the main electroanalytical methods. 

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]. 

F. Scholz (ed.), Electroanalytical Methods, Guide to Experiments and Applications, Springer-Verlag, Berlin (2002). 

There may be circumstances in which an electroanalytical method, as a consequence of the additional chemicals required, has disadvantages in comparison with instrumental techniques of analysis however, the above-mentioned advantages often make electroanalysis the preferred approach for chemical control in industrial and environmental studies. Hence, in order to achieve a full understanding of what electroanalysis can do in these fields first, it will be treated more systematically in Part A second, some attention will be paid in Part B to electroanalysis in non-aqueous media in view of its growing importance and finally, the subject will be rounded off in Part C by some insight into and some examples of applications to automated chemical control. 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

The application of electroanalysis in non-aqueous media to a certain analytical problem requires a well considered selection of the solvent together with a suitable electroanalytical method, which can be carried out on the basis of the solvent classes mentioned in Table 4.3 and of the related theories. The steps to be taken include the preparation of the solvent and the apparatus for the electroanalytical method proper, together with other chemicals, especially when the method includes titration. Much detailed information on the purification of the solvents and on the preparation of titrants and primary standards can be found in the references cited in Section 4.1 and in various commercial brochures1,84,85 and books17,86-89 we shall therefore confine ourselves to some remarks on points of major importance. 

Scanning Electron Microscopy and X-Ray Microanalysis Principles of Electroanalytical Methods Potentiometry and Ion Selective Electrodes Polarography and Other Voltammetric Methods Radiochemical Methods Clinical Specimens Diagnostic Enzymology Quantitative Bioassay... 

C.A. Wijayawardhana, H.B. Halsall, and W.R. Heineman, Milestones of electrochemical immunoassay at Cincinnati. Electroanalytical Methods for Biological Materials 329-365 (2002). 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Electroanalytical methods involve the measurement of either the electrical current flowing between a pair of electrodes immersed in the solution tested (voltammetric and amperometric methods) or an electrical potential developed between a pair of electrodes immersed in the solution tested (potentio-metric methods). In either case, the measured parameter (current or potential) is proportional to the concentration of analyte. See Figure 14.1. 

FIGURE 14.1 A representation of the general basis of electroanalytical methods. 

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