Polarography, principles

Principles and Characteristics A substantial percentage of chemical analyses are based on electrochemistry, although this is less evident for polymer/additive analysis. In its application to analytical chemistry, electrochemistry involves the measurement of some electrical property in relation to the concentration of a particular chemical species. The electrical properties that are most commonly measured are potential or voltage, current, resistance or conductance charge or capacity, or combinations of these. Often, a material conversion is involved and therefore so are separation processes, which take place when electrons participate on the surface of electrodes, such as in polarography. Electrochemical analysis also comprises currentless methods, such as potentiometry, including the use of ion-selective electrodes. 

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. 

Applications As the basic process of electron transfer at an electrode is a fundamental electrochemical principle, polarography can widely be applied. Polarography can be used to determine electroreductible substances such as monomers, organic peroxides, accelerators and antioxidants in solvent extracts of polymers. Residual amounts of monomers remain in manufactured batches of (co)polymers. For food-packaging applications, it is necessary to ensure that the content of such monomers is below regulated level. Polarography has been used for a variety of monomers (styrene, a-methylstyrene, acrylic acid, acrylamide, acrylonitrile, methylmethacrylate) in... 

The difficulties in conventional polarography as mentioned in Section 3.3.1.1, especially the interference due to the charging current, have led to a series of most interesting developments by means of which these problems can be solved in various ways and to different extents. The newer methods concerned can be divided into controlled-potential techniques and controlled-current techniques. A more striking and practical division is the distinction between advanced DC polarography and AC polarography. These divisions and their further classification are illustrated in Table 3.1. In treating the different classes we have not applied a net separation between their principles, theory and practice, because these aspects are far too interrelated within each class. 

Rapid DC polarography These principles can be 5. Linear-sweep 1. Polarography with 2. Oscillographic ... 

Fig. 3.41. Sinusoidal AC polarography. (a) measuring principle, (b) fundamental harmonic ac polarogram (i included).

Heyrovsky, J., and J. Kuta, Principles of Polarography, Academic Press, New York, 1966. 

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... 

In analytical practice, some methods using definitive measurements, in principle, are also caUbrated in an experimental way (e.g. spectrophotometry, polarography) to provide reliable estimates of S. 

In principle, E° can be determined by the widely used electroanalytical techniques (e.g. polarography, cyclic voltammetry [25]). The combination of the techniques is also useful. It has been demonstrated recently where potentiom-etry, coulometry, and spectrophotometry have been applied [26]. The case of the cyclic voltammetry is examined below. 

For example, (a) Kolthoff, I.M., Lin-gane, J. J. Polarography, 2nd edn., Interscience Publishers, New York, 1952 (b) Meites, L. Polarographic Techniques, 2nd ed., Interscience Publishers, New York, 1965 (c) Heyrovsky, J., Kuta, J. Principles of Polarography, Academic Press, New York, 1966 (d) Bond, A. M. Modem Polarographic Methods in Analytical Chemistry, Marcel Dekker, New York, 1980 ... 

Polarography. In principle, it should be possible to relate the halfwave potentials Exjl for oxidation of organometallic molecules, or of the critical oxidation potential Ec, to the energy of the highest occupied orbital, since oxidation (or reduction) in many of these systems involves simple removal or addition of an electron to this orbital. 

Polarography has been largely replaced by voltammetry with electrode materials that do not present the toxicity hazard of mercury. Principles described for the mercury electrode apply to other electrodes. Mercury is still the electrode of choice for stripping analysis, which is the most sensitive voltammetric technique. For cleaning up mercury spills, see note 18. 

Thus, the contributions of several orders can be accessed experimentally by means of a detection device tuned to the proper frequency. Applications of this principle have thus far been limited to the so-called second-order techniques second harmonic a.c. polarography [25], faradaic rectification [26], and the recently developed demodulation technique [27],... 

Another technique related to the principle of multi-potential step perturbation is square-wave polarography [52], However, because its theory is not essentially different from that of the much more popular a.c. method, it will not be treated here. 

We therefore advise that the reader should consult a recent series of papers published by Galvez et al. [171, 172] encompassing all the mechanisms mentioned in Sect. 7.1, elaborated for both d.c. and pulse polarography. The principles of the Galvez method are clearly outlined in the first part of the series [171]. It is similar to the dimensionless parameter method of Koutecky [161], which enables the series solutions for the auxiliary concentration functions cP and cQ exp (kt) and

combined directly with the partial differential equations of the type of eqn. (203). In some of the treatments, the sphericity of the DME is also accounted for. The results are usually visualized by means of predicted polarograms, some examples of which are reproduced in Fig. 38. Naturally, the numerical description of the surface concentrations at fixed potential are also immediately available, in terms of the postulated power series, and the recurrent relationships obtained for the coefficients of these series. 

Electrochemistry finds wide application. In addition to industrial electrolytic processes, electroplating, and the manufacture and use of batteries already mentioned, the principles of electrochemistry are used in chemical analysis, e.g.. polarography, and electrometric or conductometric titrations in chemical synthesis, e.g., dyestuffs, fertilizers, plastics, insecticides in biolugy and medicine, e g., electrophoretic separation of proteins, membrane potentials in metallurgy, e.g.. corrosion prevention, eleclrorefining and in electricity, e.g., electrolytic rectifiers, electrolytic capacitors. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

Heyrovsky, Jaroslav, and Kuta, Jaroslav (1965). Principles of Polarography. Prague Publishing House of the Czechoslovak Academy of Sciences. 

Refs. [i] Kolthoff IM, Lingane JJ (1952) 2nd edn. Polarography. Polarographic analysis and voltammetry. Amperometic titrations. Interscience, New York, vol. 2, pp 887 [ii] Heyrovsky J, Kuta J (1966) Principles of polarography. Academic Press, New York, pp 267 [Hi] Classification and nomenclature of electroanalytical techniques (1976) Pure Appl Chem 45 81 [iv] Bard AJ, Faulkner LR (2001) Electrochemical methods, 2nd edn. Wiley, New York, pp 437... 

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