Chemical polarography

Analysis of Trace or Minor Components. Minor or trace components may have a significant impact on quaHty of fats and oils (94). Metals, for example, can cataly2e the oxidative degradation of unsaturated oils which results in off-flavors, odors, and polymeri2ation. A large number of techniques such as wet chemical analysis, atomic absorption, atomic emission, and polarography are available for analysis of metals. Heavy metals, iron, copper, nickel, and chromium are elements that have received the most attention. Phosphoms may also be detectable and is a measure of phosphoHpids and phosphoms-containing acids or salts. 

Standard addition. A known amount of the constituent being determined is added to the sample, which is then analysed for the total amount of constituent present. The difference between the analytical results for samples with and without the added constituent gives the recovery of the amount of added constituent. If the recovery is satisfactory our confidence in the accuracy of the procedure is enhanced. The method is usually applied to physico-chemical procedures such as polarography and spectrophotometry. 

Because of these advantages, polarography became very popular immediately after its inception in 1922. For the development of this method, Heyrovsky was awarded a Nobel prize in 1959. Over the period from 1922 to 1960, several tens of thousands of papers concerned with the use and improvement of polarography were published. However, interest in this method declined markedly in the 1960s, due primarily to a drastic increase in the requirements to be met by methods of chemical analysis. With the production of new superpure materials and increasing awareness for ecological problems, it became necessary to develop much more sensitive methods of analysis able to detect the different impurities down to a level of 10 M. 

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. 

Polarography with superimposed AC signal. As examples of chemical analytical importance we shall consider (a) sinusoidal AC, (b) AC bridge, (c) square-wave and (d) Kalousek polarography. 

Kounaves and Zirino [ 145] studied cadmium-EDTA complex formation in seawater using computer-assisted stripping polarography. They showed that the method is capable of determining the chemical speciation of cadmium in seawater at concentrations down to 10 8 M. 

Polarography of Organic Cations, M.I. James and P.H. Plesch, Chemical Communications, 1967, 508-510. 

Polarography of Carbonium Ions in Acid Solution, Part I, The Triphenylmethyl Ion in Sulphuric Acid, P.H. Plesch and I. Sestakova, Journal of the Chemical Society, (B), 1970, 87-92. 

Polarography of Organic Cations in Acid Solutions. Part 3. Three 1,1 -Diarylethyl Ions in Sulphuric and Methanesulphonic Acids, Kabir-ud-Din and P.H. Plesch, Journal of the Chemical Society, Perkin II, 1978, 892-895. 

The Polarography of Oxonium Ions, Part II. The Half-wave Potentials of Five Tertiary Ions. G.E. Holdcroft, Kabir-ud-Din, and P.H. Plesch, Journal of Chemical Research, 1980, 390-391. 

The dominant tendency of my studies has been not so much to obtain and describe organic compounds but... to penetrate their mechanisms.. . . For undertaking this kind of problem, the classic methods of organic chemistry are far from sufficient. Physicochemical procedures become more and more necessary. I have been led to use especially optical methods (the Raman effect and ultraviolet spectra) and electrochemical techniques (conductibility, electrode potentials, and especially polarography).. . . The notion of reaction mechanism led almost automatically to envisioning the electronic aspect of chemical phenomena. From 1927, and working in common with Charles Prevost, I have directed my attention on the electronic theory of reactions." 56... 

Voltammetry is the term given to electrochemical techniques which monitor the relationship between the voltage applied to an electrode system and the current that flows as a result of the reaction. It covers a wide range of different electrode techniques, many of which are specifically designed to monitor a particular chemical reaction. Voltammetry is generally divided into two main subdivisions of polarography and amperometry. 

Current maximum suppresser A chemical, usually a surfactant (detergent), added to polarography solutions in order to decrease the incidence of polaro-graphic peaks (see Section 6.8.1). 

In order to overcome the drawbacks of DC polarography, various new types of polarography and voltammetry were developed. Some new polarographic methods are dealt with in this section. They are useful in chemical analyses as well as in studying electrode reactions. 

Since the invention of d.c. polarography [10, 11], numerous inorganic and organic compounds have been studied by means of this method in Heyrovsky s school and extensive knowledge gathered about the electrochemical properties of these compounds. Among them, many cases were discovered where the polarographic wave appeared to be influenced by the existence of chemical equilibria between the electroactive substance and other, in most cases electroinactive, species in the electrolyte solution, more particularly by the finite rate at which these equilibria relax after the electrochemical perturbation. In fact, the chemical reaction serves as either a source or a sink to deliver or to consume the electroactive reactants and products, in addition to diffusion. 

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