Chapters coulometry

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

Two other techniques that depend only on base SI units are coulometry and isotope-dilution mass spectrometry. Coulometry is discussed in Chapter 11. Isotope-dilution mass spectroscopy is beyond the scope of an introductory text, however, the list of suggested readings includes a useful reference. 

Electrochemical methods covered in this chapter include poten-tiometry, coulometry, and voltammetry. Potentiometric methods are based on the measurement of an electrochemical cell s potential when only a negligible current is allowed to flow, fn principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell by measuring its potential and solving the Nernst equation the presence of liquid junction potentials, however, necessitates the use of an external standardization or the use of standard additions. 

Curran, D. J. Constant-Current Coulometry. Chapter 20 in Kissinger, P. T. Heineman, W. R., eds. Eaboratory Techniques in Electroanalytical Chemistry. Marcel Dekker, Inc. New York, 1984, pp.539—568. 

An electrode of surface area 100 pm or less is called a microelectrode and provides a means of decreasing the double-layer capacitance which can affect our coulometry experiments so badly. Microelectrodes are also useful when the cell considered is also tiny, as, for example, is the case when performing in vivo voltammetry (see next chapter) with biological samples. For example, a nerve ending is typically 10-100 pm in diameter, so electroanalytical experiments using a conventional electrode would be impossible. 

In the previous chapter, we encountered a form of coulometry known as stripping . We can combine both stripping and voltammetry in the powerful technique of stripping voltammetry. As we have seen, the potential of the working electrode is ramped during a voltammetric or polarographic experiment. The resultant current represents the rate at which electroactive analyte reaches the surface of the electrode, that is, current / a flux j. 

Recently flow coulometry, which uses a column electrode for rapid electrolysis, has become popular [21]. In this method, as shown in Fig. 5.34, the cell has a columnar working electrode that is filled with a carbon fiber or carbon powder and the solution of the supporting electrolyte flows through it. If an analyte is injected from the sample inlet, it enters the column and is quantitatively electrolyzed during its stay in the column. From the peak that appears in the current-time curve, the quantity of electricity is measured to determine the analyte. Because the electrolysis in the column electrode is complete in less than 1 s, this method is convenient for repeated measurements and is often used in coulometric detection in liquid chromatography and flow injection analyses. Besides its use in flow coulometry, the column electrode is very versatile. This versatility can be expanded even more by connecting two (or more) of the column electrodes in series or in parallel. The column electrodes are used in a variety of ways in non-aqueous solutions, as described in Chapter 9. 

This chapter is an overview of voltammetric methods used in analytical laboratories and as an example of coulometry the Karl Fischer method is presented. 

The excess, unreacted Cu+ is then titrated by coulometry, which is described in Chapter 17. In Experiment B, the superconductor is dissolved in l M HC1 containing excess 1 mM FeCl2 4H20. Bi5+ reacts with the Fe2+ but not with Cu3+.33... 

Thin-layer CV is frequently used in conjunction with an optically transparent thin-layer electrode to obtain spectra, E°, and n for redox couples by the spectropotentiostatic technique and thin-layer coulometry, as described earlier in this chapter. CV is used initially to locate the redox couple and give an estimate of E°. Once the CV is obtained, appropriate potentials can be selected for the spectropotentiostatic experiment and potential-step coulometry. A typical thin-layer cyclic voltammogram for the Schiff base complex Co(sal2en) in nonaqueous solvent is shown in Figure 3.34. 

Controlled-current electrolysis in flowing solution has been extremely useful for analytical purposes. The prevalent techniques are constant-current coulometry and coulometric titrations, which are discussed in Chapter 25. 

Controlled-potential coulometry (CPC) provides a direct measure of the number of electrons transferred in an electrode reaction. It is widely employed in molten salts for this purpose. Background information about CPC is given in Chapter 3 of this... 

Often the first step in the electrochemical characterization of a compound is to ascertain its oxidation-reduction reversibility. In our opinion, cyclic voltammetry is the most convenient and reliable technique for this and related qualitative characterizations of a new system, although newer forms of pulse polarography may prove more suitable for quantitative determination of the electrochemical parameters. The discussion in Chapter 3 outlines the specific procedures and relationships. The next step in the characterization usually is the determination of the electron stoichiometry of the oxidation-reduction steps of the compound. Controlled-potential coulometry (discussed in Chapter 3) provides a rigorously quantitative means for such evaluations. 

With respect to chemical steps prior to the electron-transfer step, chrono-potentiometiy offers a convenient technique. The methods of measurement and the quantitative relationships are outlined in Chapter 4. Post-electron-transfer reactions to the electron-transfer step are most conveniently characterized by cyclic voltammetry (see Chapter 3). Although the techniques of cyclic voltammetry and chronopotentiometiy both provide a means for the qualitative detection of adsorption processes at an electrode, the coulostatic method and chrono-coulometry are the methods of choice for quantitative measurements of adsorption. 

In this chapter, the fundamental electrochemical principles of potentiometry, voltammetry and/or amperometry, conductance, and coulometry will be summarized and clinical apphcations presented. Next, optodes and biosensors will be discussed. The chapter concludes with a discussion of in vivo and minimally invasive sensors. 

W e now turn our attention to several analytical methods that are based on oxidation/reduction reactions. These methods, which are described in Chapters 18 through 23, include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry, and voltammetry. Fundamentals of electrochemistry that are necessary for understanding the principles of these procedures are presented in this chapter. 

Electrogravimetry and coulometry are moderately. sensitive and among the most accurate and precise techniques available to the chemist. Like the gravimetric techniques discussed in Chapter 12, electmgravimetiy requires no preliminary calibration against chemical standards becau.se the functional relationship between... 

Chapter 22 Bulk Electrolysis Electrogravimetry and Coulometry 633 Chapter 23 Voltammetry 665... 

Part IV is devoted to electrochemical methods. After an introduction to electrochemistry in Chapter 18, Chapter 19 describes the many uses of electrode potentials. Oxidation/reduction titrations are the subject of Chapter 20, while Chapter 21 presents the use of potentiometric methods to obtain concentrations of molecular and ionic species. Chapter 22 considers the bulk electrolytic methods of electrogravimetry and coulometry, while Chapter 23 discusses voltammetric methods including linear sweep and cyclic voltammetry, anodic stripping voltammetry, and polarography. 

The first group consists of conventional electroanalytical techniques such as cyclic voltammetry (CV), chronoamperometry, chronopotentiometry, coulometry, and electrochemical impedance spectroscopy (EIS), all of which provide general information about the doping process (see also Chapters 4 and 5). Below are listed some typical questions that can be answered using the above group of techniques ... 

Let us begin by pointing out the basic differences between voltammetry and the two types of electrochemical methods that we discussed in earlier chapters. V oltammetry is based on the measurement of tlie current that develops in anelecirochomical cell under conditions where concentration polarization exists. Recall from Section 22H-2 that a polarized electrode is one to which we have applied a voltage In excess of that predicted by the Nernst equation to cause oxidation or reduction to occur. In contrast, poientiomeirlc measurements are made at currents that approach zero and where polarization is absent. Voltammetry differs from coulometry in that, with coulometry, measures are lakcn to minimize or compensate for the effects of concentration polarization. Furthermore, in voltammetry there is minimal consumption of analyte, whereas in coulometry esse ntially all of the analyte is converted to another Slate. 

F.lectrochemical detectors of several types arc currently available from instrument manufacturers, These devices are based on ampcromelry, voltammetry, coulometry, and conductometry. The first three of these methods are discussed in Chapters 24 and 25,... 

All instrumental analytical methods except coulometry (Chapter 15) require calibration standards, which have known concentrations of the analyte present in them. These calibration standards are used to establish the relationship between the analytical signal being measured by the instrument and the concentration of the analyte. Once this relationship is established, unknown samples can be measured and the analyte concentrations determined. Analytical methods should require some sort of reference standard or check standard. This is also a standard of known composition with a known concentration of the analyte. This check standard is not one of the calibration standards and should be from a different lot of material than the calibration standards. It is run as a sample to confirm that the calibration is correct and to assess the accuracy and precision of the analysis. Reference standard materials are available from government and private sources in many countries. Government sources include the National Institute of Standards and Technology (NIST) in the US, the National Research Council of Canada (NRCC), and the Laboratory of the Government Chemist in the UK. 

Other analytical techniques. Electroanalytical methods can also be used to differentiate between ionic species (based on valence state) of the same element by selective reduction or oxidization. In brief, the electroanalytical methods measure the effect of the presence of analyte ions on the potential or current in a cell containing electrodes. The three main types are potentiometry, where the voltage difference between two electrodes is determined, coulometry, which measures the current in the cell over time, and voltammetry, which shows the changes in the cell current when the electric potential is varied (current-voltage diagrams). In a recent review article, 43 different EA methods for measuring uranium were mentioned and that literature survey found 28 voltammetric, 25 potentiometric, 5 capillary electrophoresis, and 3 polarographic methods (Shrivastava et al. 2013). Some specific methods will be discussed in detail in the relevant chapters of this tome. 

The aim of this chapter is to describe both the high precision methods for the determination of total alkalinity At (potentiometric titration) and total dissolved inorganic carbon Ct (coulometry) that are the methods of choice today, and also a simpler method for total alkalinity (back titration) which does not require sophisticated computerised equipment. Thermodynamic calculations of carbonate speciation are also covered. 

In this section the theory and methodology of electro-analytical chemistry are explored. Chapter 22 provides a (general foundation for the study of subsequent chapters in this section. Terminology- and conventions of electrochemistry as well as theoretical and practical aspects of the measurement of electrochemical potentials and current s are. presented. Chapter 23 comprises the many methods and applications of potentiometry. and constant-potential coulometry and constant-current coulornetrv are discussed in Chapter 2 4. The many facets of the important and widely used technique of voltammetry are presented in ( hapter 2.5. which concludes the section. 

In some cases, multienzyme cascades are used where an artificial metabolic pathway has been created on an electrode to take a specific substrate and electrocataly tic ally convert it to produce energy [5,12-14]. With these systems, coulometry is useful for determining coulombic efficiency, as well as combining coulometry with nuclear magnetic resonance (NMR) analysis to determine reaction intermediates and products, and elucidate bottlenecks in the artificial metabolic pathway, or to determine the energy density of a fuel cell. Please refer to Chapter 5 for further information regarding multienzyme cascades. 

In this monograph, an effort has been made to provide sufScient information about the theoretical background, instrumental considerations, and experimental techniques of controlled-potential coulometry so as to enable the practicing analytical chemist to make use of this method for his own particular requirements. Detailed procedures for specific analyses will not be given here, but a considerable number of typical applications to inorganic analysis have been critically treated in the final chapter. While this modest monograph is not intended in any way as a textbook it is hoped that it may serve to stimulate some interest in this interesting and useful technique. 

---