Electrolysis cells, coulometry

The instrumentation for potentiostatic coulometry consists of an electrolysis cell, a potentiostat, and a device for determining the charge consumed by the analyte. 

Figure 22-11 Electrolysis cells for potentiostatic coulometry. Working electrode (a) platinum gauze, (b) mercury pool. (Reprinted with permission from J. E. Harrar and C. L. Pomernacki, Anal. Chein., 1973, 45, 57. Copyright 1973 American Chemical Society.)...

The methods of coulometry are based on the measurement of the quantity of electricity involved in an electrochemical electrolysis reaction. This quantity is expressed in coulombs and it represents the product of the current in amperes by the duration of the current flow in seconds. The quantity of electricity thus determined represents, through the laws of Faraday, the equivalents of reactant associated with the electrochemical reaction taking place at the electrode of significance. In the analytical chemistry sense, the process of coulometry, carried out to the quantitative reaction of the analyte in question, either directly or indirectly, will yield the number of analyte equivalents involved in the sample under test. This will lead to a quantitative determination of the analyte in the sample. Analytical coulometry can be carried out either directly or indirectly. In the former the analyte usually reacts directly at the surface of either the anode or cathode of the electrolysis cell. In the latter, the analyte reacts indirectly with a reactant produced by electrolytic action at one of the electrodes in the electrolysis cell. In either case, the determination will hinge on the number of coulombs consumed in the analytical process. 

Practically every serious worker in the field of controlled-potential coulometry has, at one time or another, designed an electrolysis cell to fit a particular set of circumstances. Many of these designs have found their way into the literature but, nonetheless, have not stimulated any noticeable commercial production of such apparatus. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

From this equation we see that increasing k leads to a shorter analysis time. For this reason controlled-potential coulometry is carried out in small-volume electrochemical cells, using electrodes with large surface areas and with high stirring rates. A quantitative electrolysis typically requires approximately 30-60 min, although shorter or longer times are possible. 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

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. 

Controlled-potential coulometry has been applied extensively to precision determinations [69] and to the establishment of n values for electrode reactions. The technique has also been used for the elucidation of electrode reactions [70,71]. It is a very useful technique when used in conjunction with a thin-layer cell where complete electrolysis is rapid (see Sec. II.E). 

Equation 11.7.11 is the same as the coulometry equation (11.3.11), and it indicates that determinations of Nq or n are possible without the necessity of knowing Dq. The electrolysis rate constant in a thin-layer cell, p, can be quite large. For example, for D = 5 X 10 cm /s and / = 10 cm, p = 49 s and the electrolysis will be 99% complete in less than 0.1 s. In actual experiments the measured charge will be larger than that given by (11.7.9) to (11.7.11), because contributions from double-layer charging, electrolysis of adsorbed species, and background reactions will be included (see Section 14.3.7)... 

Another popular mode for transmission experiments involves a thin-layer system (9, 10, 13, 18) like that shown in Figure 17.1.2. The working electrode is sealed into a chamber (e.g., between two microscope slides spaced perhaps 0.05-0.5 mm apart) containing the electroactive species in solution. The chamber is filled by capillarity, and the solution within it contacts additional solution in a larger container, which also holds the reference and counter electrodes. The electrolytic characteristics of the cell are naturally similar to those of the conventional thin-layer systems discussed in Section 11.7. One can do cyclic voltammetry, bulk electrolysis, and coulometry in the ordinary way, but there is also a facility for obtaining absorption spectra of species in the cell. 

Bulk electrolysis for the purpose of electrosynthesis or for a coulometric determination of the number of electrons (w) associated with a half-cell reaction of the kind A B + neT (A is the compound being oxidized and B is product, with charges being omitted for simplicity, and n is the overall number of electrons transferred per molecule of A oxidized as determined by coulometry and application of Faraday s Law of electrolysis). 

Counting Electrons Coulometry and Faraday s Law of Electrolysis 21-7 Commercial Applications of Electrolytic Cells Voltaic or Galvanic Cells 21-8 The Construction of Simple Voltaic Cells... 

Table 21-1 shows the amounts of several elements produced during electrolysis by the passage of 1 faraday of electricity. The use of electrochemical cells to relate the amount of reactant or product to the amount of current passed is called coulometry. 

In cases where electrolysis of the solution bulk is not desired (as in voltammetry or chronoamperometry), the working electrode area is kept reasonably small (nominal dimensions of square millimeters). Such an electrode is termed a microelectrode. On the other hand, in electrolysis procedures (as in thin-layer cells or coulometry), the ratio of electrode area to solution volume (A/V) must be maximized. Large-area working electrodes [grids or porous electrodes such as reticulated vitreous carbon (RVC)j are used in such cases. 

In the preceding section, we mostly considered cases wherein only a thin segment of the electroactive region (whether the solution or the film phase) was electro-chemically altered. This situation must be contrasted with those in which exhaustive electrolysis is involved. An example is constant-potential coulometry (Fig. 20.4) wherein the entire solution contained within the cell is electrolyzed. As mentioned earlier, this is ensured by the use of a large A/V ratio and efficient solution agitation. The underlying coulometric equation derives from Faraday s law of electrolysis and can be expressed as... 

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