Analytical techniques coulometry

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

Precision of analytical techniques should only be described verbally or comparatively (e.g., the precision of coulometry is high or the precision of spectrophotometry is better then that of OES ). 

Three-dimensional electrode — This term is used for electrodes in which the electrode-solution interface is expanded in a three-dimensional way, i.e., the - electrode possesses a significantly increased surface area due to nonplanarity, so that it can be housed in a smaller volume. This can be achieved by constructing corrugated electrodes, reticulated electrodes, -> packed bed electrodes (see also - column electrodes), -> carbon felt electrodes, or fluidized bed electrodes. Three-dimensional electrodes are important for achieving high conversion rates in electrochemical reactions. Therefore they are especially important in technical electrochemistry, wastewater cleaning, and flow-through analytical techniques, e.g., - coulometry in flow systems. However, the - IR-drop within three-dimensional electrodes is an inherent problem. 

The detection and determination of water (or moisture) in a wide variety of industrial and chemical products is of great interest. Analytical techniques for water determination include chemical, spectroscopic, thermal, electrical, gravimetric, and physical techniques. One of the most versatile techniques is based on coulometry (130). This technique generally involves absorption of the water onto a hygroscopic material, from which it is subsequently electrolyzed. The current required for the electrolysis is directly proportional to the water absorbed. One such commercially available instrument based on this principle is the Du Pont 902 moisture evolution analyzer (MEA), illustrated in Figure 8.36. 

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. 

Controlled potential bulk electrolysis or coulometry, as it is often called, is widely used to determine the overall number of electrons involved in an electrode process. It is also used to prepare a sufficient quantity of the reaction products to enable them to be identified by the application of conventional analytical techniques. 

Electrolysis is the basis of the analytical techniques of polarography, voltammetry, amperometry and coulometry (see Topic C9). Electrolysis may also be used for the deposition, production and purification of materials. For example. 

Starting with voltammetry, we get a well defined current-potential curve for the reaction to be studied. The first step is then to find the identity of the product(s). Large scale electrolysis, followed by conventional analytical techniques, often gives the answer. Quantitative coulometry, where we react a known amount of material and measure the electricity consumed. 

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. 

J. E. Harrar, "Techniques, Apparatus, and Analytical AppHcations of ControUed-Potential Coulometry," in A. J. Bard, ed., Electroanalytical Chemisty, Vol 8, Marcel Dekker, New York, 1975. 

The techniques of voltammetry/polarography, atomic absorption, ICP, etc., have in most cases supplanted the coulometric approach for the determination of inorganic analytes. Coulometry and the use of coulometry in food analysis have recently been reviewed [473,476]. 

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

In the laboratory, electroanalysis is used for two main purposes, either for direct measurement of a physico-chemical property that is informative with respect to the identity and/or amount of the analyte, or for detecting the course of conversion of the analyte or indicating the separate appearance of analyte components, which is informative with respect to their identity and amount. In the former instance we are dealing with conductometry, voltammetry and coulometry and in the latter with various titrations and mostly separational flow techniques such as chromatography and flow injection analysis. 

In the previous section, we introduced the way that coulometry can be employed as an analytical tool, looking speciflcally at some simple forms of the technique. We saw that the charge passed was a simple function of the amount of material that had been electromodified, and then looked at ways in which the coulometric experiment was prone to errors, such as non-faradaic currents borne of electrolytic side reactions or from charging of the double-layer. 

In this present section, we will look at more specific forms of coulometry. Other than electrolysis of a bulk solution, the most common technique is stripping, which is the method of choice when only a tiny amount of analyte is in solution. 

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. 

The Karl Fischer titration is a specialised type of coulometric titration. Coloumetry itself is a useful technique, but is not used as a mainstream technique for pharmaceutical analysis. Essentially coulometry is based on the electrolytic reduction of the analyte, i.e. the analyte is reduced by electrons supplied by a source of electrical power and the amount of charge passed in order to convert the analyte to its reduced form is equivalent to the amount of analyte present in solution. 

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. 

Lingane was a leader in the field of - electro analytical chemistry and wrote, with Kolthoff, the definitive, two volume monograph, Polarography [i] that remains a useful reference work. He also helped develop other electroanalytical techniques, like controlled potential electrolysis, -> coulometry, -> coulometric titrations, and developed an early electromechanical (Lingane-Jones) potentiostat, He wrote the widely-used monograph in this field, Electroanalytical Chemistry (1st edn., 1953 2nd edn., 1958). Lingane received a number of awards, including the Analytical Chemistry (Fisher) Award of the American Chemical Society in 1958. Many of his Ph.D. students, e.g., -> Meites, Fred Anson, Allen Bard, Dennis Peters, and Dennis Evans, went on to academic careers in electrochemistry. 

Virtually any electrochemical technique may be used for either analytical or mechanistic (our focus) studies. The merits and limitations of each technique and the information that can be gleaned are discussed for direct-current (d.c.) polarography, pulse polarography, alternating-current (a.c.) polarography and cyclic voltammetry. Con-trolled-potential coulometry is technically not a voltammetric technique (there is no variation of potential), and this technique is considered in 12.3.5. 

Coulometry is the name given to a group of other techniques that determine an analyte by measuring the amount of electricity consumed in a redox reaction. There are two categories referred as potentiostatic coulometry and amperostatic coulometry. The development of amperometric sensors, of which some are specific for chromatographic detection, open new areas of application for this battery of techniques. Combining coulometry with the well known Karl Fischer titration provides a reliable technique for the determination of low concentrations of water. 

Analytical Applications of Coulometry. The major advantage of coulometry is its high accuracy, because the reagent is electrical current, which can be well controlled and accurately measured. Coulometry is used for analysis, for generation of both unstable and stable titrants on demand , and for studies of redox reactions and evaluation of fundamental constants. With careful experimental technique, it is possible to evaluate the Faraday constant to seven significant figures, for example. 

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