Polarography, Potentiometry

Other methods of instmmental analysis include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

Numerous methods have been pubUshed for the determination of trace amounts of tellurium (33—42). Instmmental analytical methods (qv) used to determine trace amounts of tellurium include atomic absorption spectrometry, flame, graphite furnace, and hydride generation inductively coupled argon plasma optical emission spectrometry inductively coupled plasma mass spectrometry neutron activation analysis and spectrophotometry (see Mass spectrometry Spectroscopy, optical). Other instmmental methods include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

Practically all commonly employed electroanalytical methods can be used in environmental analysis the choice of method depends on the character of the compound to be determined and of the matrix in which it occurs, as well as on sensitivity and selectivity requirements. The principal methods are voltammetry and polarography, potentiometry, coulo-metry and conductometry. 

Analytical chemistry is that branch of science that deals with the determination of the composition of matter, its elements, ions, radicals and compounds, by chemical or physical methods. It is, therefore, one of the bases on which the whole structure of chemistry is erected. The methods employed are very numerous and include the following chromatography, electro-analysis, elementary analysis, gas analysis, gas chromatography, gravimetric analysis, colorimetry, mass analysis, micro-analysis, polarography, potentiometry, qualitative and quantitative analysis, spectral analysis, thermal analysis, spot analysis and many others. 

The method of vibrational spectroscopy has found useful application to groups such as the NOg" ion< but studies of such entities as NiClf" or PtCll" by the laser Raman technique are still awaited. While NMR, ESR, and magnetic susceptibility studies of molten salt systems have been made, only the last have been of much use in coordination assignment. The techniques used to measure stability constants of complexes, such as polarography, potentiometry, and chromatography, are beginning to be applied to molten salt systems but the results to date require caution in interpretation. 

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. 

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. 

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 calibrated by indirect reference measurements using least squares estimating to provide reliable estimates of b (spectrophotometry, potentiometry, ISE, polarography). 

In order to determine the stability constants for a series of complexes in solution, we must determine the concentrations of several species. Moreover, we must then solve a rather complex set of equations to evaluate the stability constants. There are several experimental techniques that are frequently employed for determining the concentrations of the complexes. For example, spectrophotometry, polarography, solubility measurements, or potentiometry may be used, but the choice of experimental method is based on the nature of the complexes being studied. Basically, however, we proceed as follows. A parameter is defined as the average number of bound ligands per metal ion, N, which is expressed as... 

Brugmann [784] discussed different approaches to trace metal speciation (bioassays, computer modelling, analytical methods). The electrochemical techniques include conventional polarography, ASV, and potentiometry. ASV diagnosis of seawater was useful for investigating the properties of metal complexes in seawater. Differences in the lead and copper values yielded for Baltic seawater by methods based on differential pulse ASV or AAS are discussed with respect to speciation. 

Riley, T. Watson, A., Polarography and Other Voltammetric Methods, Wiley, Chichester, 1987. Svehla, G., Automatic Potentiometrie Titrations, Pergamon, Oxford, 1977. 

An amperometric technique relies on the current passing through a polarizable electrode. The magnitude of the current is in direct proportion to the concentration of the electroanalyte, with the most common amperometric techniques being polarography and voltammetry. The apparatus needed for amperometric measurement tends to be more expensive than those used for potentiometric measurements alone. It should also be noted that amperometric measurements can be overly sensitive to impurities such as gaseous oxygen dissolved in the solution, and to capacitance effects at the electrode. Nevertheless, amperometry is a much more versatile tool than potentiometry. 

A variety of spectroscopic methods that produce different signals from bound and free substrate, such as UV-vis spectroscopy, IR spectroscopy and NMR spectroscopy, have been established for this purpose. Electrochemical methods, such as potentiometry and polarography, have been applied as well (1). 

Other electroanalytical methods The use of h.v.t. in conjunction with electroanalytical techniques of the potentiometry-polarography type has been described in detail (Kesztelyi, 1984), so that it need not be discussed here. That author, however, ignores a very useful cell for electrosynthesis under vacuum (Schmulbach and Oommen, 1973) and the electrochemical techniques developed by Szwarc and his co-workers and others in the context of anionic polymerisation, which we have mentioned above. 

The most common analytical methods used were gas chromatography, HPLC, AA spectrophotometry, polarography, colorimetry, and potentiometry with ion-selective electrodes. In this study GC/MS and other more expensive instrumentation were avoided. If sorbent tubes could not be used for gaseous substances, then the less desirable miniature bubblers or impingers were considered. Although these devices are inconvenient they were often used because no better alternatives were available. Bags were used in a few cases where the analyte could not be retained on a sorbent because of volatility and a small tendency to sorb. Filters were used for particulates. Combinations of collection devices were used if we felt that both particulates and vapor might be present in the analyte. 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

J. Thompson, 1893) and the development of chemical thermodynamics (G. N. Lewis, 1923). Building on this foundation, the utilization of electrochemical phenomena for thermodynamic characterization and analysis of molecules and ions (electroanalytical chemistry) began at the beginning of this century [po-tentiometry (1920) and polarography (1930)]. Relationships that describe the techniques of potentiometry and polarography derive directly from solution thermodynamics. In the case of polarography, there is a further dependence on the diffusion of ionic species in solution. The latter is the basis of conductivity measurements, another area that traces its origin to the nineteenth century. These quantitative relationships make it possible to apply electrochemistry to... 

For chemists, the second important application of electrochemistry (beyond potentiometry) is the measurement of species-specific [e.g., iron(III) and iron(II)] concentrations. This is accomplished by an experiment in which the electrolysis current for a specific species is independent of applied potential (within narrow limits) and controlled by mass transfer across a concentration gradient, such that it is directly proportional to concentration (/ = kC). Although the contemporary methodology of choice is cyclic voltammetry, the foundation for all voltammetric techniques is polarography (discovered in 1922 by Professor Jaroslov Heyrovsky awarded the Nobel Prize for Chemistry in 1959). Hence, we have adopted a historical approach with a recognition that cyclic voltammetry will be the primary methodology for most chemists. 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

Verfahren I Mikrocoulometrie. — II , Linear-sweep -Voltammetrie. — III Potentiostatische Verfahren. — IV Chrono-potentiometrie. — V Oscillopolarographie ((dEfdt — /( )). — VI Square-wave-Polarographie. 

In this section it will be useful to discuss some common experimental methods used in the determination of stability constants of rare earth complexes. Ethylenediamine tetraacetate anion (EDTA) is a hexadentate and forms complexes with trivalent rare earth ion readily. The pioneering studies of Schwarzenbach [4] on the determination of stability constants of rare earth EDTA complexes by potentiometry and polarography can be considered to illustrate the principles involved in the determination of stability constants. 

Details of the application of polarography and various elaborations of this technique to the determination of stabihty constants are described by Heyrovsky and Kuta, Nancollas, Hartley etal. Beck and Nagypal, and Cukrowski. A major advantage of polarography is its usefiilness as a complement to potentiometry for determining stabihty constants. ... 

Techniques such as potentiometry, polarography, and microcalorimetry have been chosen in exploiting the benefits of immobilized enzymes (see Chapter 4). Enzymes incorporated into membranes form part of enzyme electrodes. The surface of an ion-sensitive electrode is coated with a layer of porous gel in which an enzyme has been polymerized. When the electrode is immersed in a solution of the appropriate substrate, the action of the enzyme produces ions to which the electrode is sensitive. For example, an oxygen electrode coated with a layer containing glucose oxidase can be used to determine glucose by the amount of oxygen consumed m the reaction, and urea can be estimated by the combination of a selective ammonium ion-sensitive electrode and a urease membrane. 

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