Coulometry flow cell method

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. 

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. 

As a new class of materials, ionic liquids require special analytical methods. In the case of imidazolium halides and similar compounds the most common impurities are amines, alkyl halides and of course water. Seddon et al. described a method for the detection of residual amines using the strong UV absorbance of copper tetramine complexes. These complexes are readily formed by the addition of Cu2+ ions [24]. The detection of both amines and alkyl halides is possible by NMR spectroscopy but with limited resolution [25]. By far the most powerful analytical method is liquid chromatography combined with UV detection. This sensitive method allows the detection of traces of amines and halides [26]. Unreacted amines can be also detected by ion chromatography combined with a suppressor module. In this case detection is achieved using a continuous flow conductivity cell since amines are protonated and thus detectable. For traces of other ionic impurities ion chromatography is also the most powerful analytical tool [27]. Finally, residual water can be quantified using Karl Fischer titration or coulometry [28]. 

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. 

The basic electric circuit in flow-through voltammetry and coulometry is the same in principle. The indicating electrode is polarized against a reference electrode to a constant potential. In the close vicinity of the indicating electrode a constant convection is maintained e.g. by a stirrer. The sample flows through the cell and the electric current proportional to the concentration of the species to be measured is recorded. The basic difference between the two methods is in the physical meaning of the current. 

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