Rapid electrolysis

In electrogravimetry, also called electrodeposition, an element, e.g., a metal such as copper, is completely precipitated from its ionic solution on an inert cathode, e.g., platinum gauze, via electrolysis and the amount of precipitate is established gravimetrically in the newer and more selective methods one applies slow electrolysis (without stirring) or rapid electrolysis (with stirring), both procedures either with a controlled potential or with a constant current. Often such a method is preceded by an electrolytic separation using a stirred cathodic mercury pool, by means of which elements such as Fe, Ni, Co, Cu, Zn and Cd are quantitatively taken up from an acidic solution whilst other elements remain in solution. 

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. 

Fig. 5.34 A column electrode cell for rapid electrolysis. WE working electrode of carbon fiber or carbon powder RE reference electrode CE Pt counter electrode.

In order to determine the number of electrons, the flow-coulometric method described in Section 5.6.3 is also useful. The solution of the supporting electrolyte is flowing through the column-type cell for rapid electrolysis (Fig. 5.34) and the potential of the carbon fiber working electrode is kept at a value at which the de-... 

Fig. 9.4 A stopped-flow optical absorption cell equipped with two column-type cells for rapid electrolysis [7].

However, the cell in Fig. 9.2(b) has a disadvantage in that the concentration of the electrogenerated substance decreases with increasing distance from the OTE surface. Because of this, simulation of the reaction is very difficult, except for the first order (or pseudo-first-order) reactions. For more complicated reactions, it is desirable that the concentration of the electrogenerated species is kept uniform in the solution. With a thin-layer cell, a solution of uniform concentration can be obtained by complete electrolysis, but it takes 30 s. Thus, the thin-layer cell is applicable only for slow reactions. For faster reactions, a column-type cell for rapid electrolysis is convenient. Okazaki et al. [7] constructed a stopped-flow optical absorption cell using one or two column-type cells (Fig. 9.4) and used it to study the dimerization of the radical cations (TPA +) of triphenylamine and the reactions of the radical cation (DPA +) of 9,10-diphenylanthracene with water and alcohols. Using the stopped-flow cell, reactions of substances with a half-life of 1 s can be studied in solutions of uniform concentrations. 

Mercury cathode cell for rapid electrolysis) 14) A-J- Lindsey,... 

The rate of electrodeposition is dependent on a number of factors, and these are predictable to only a limited degree. However, the thickness of the diffusion layer must be minimized to obtain a rapid electrolysis. This is accomplished by vigorously stirring and by the use of electrodes with large surface areas. An increase in temperature enhances the rate of electrolysis because it increases the mobility of the electroactive species. The use of high ionic concentrations minimizes the iR drop between electrodes and also improves the electrolysis rate. The orientation and geometry of the electrodes is important to insure a uniform and adherent plate. Depolarizers frequently are introduced to prevent formation of interfering products from the counter electrode (see Table 3.5). 

For flow analysis incorporating electrolytic dissolution, very small characteristic masses, often below the ng level, are reported for metal determinations. This is a consequence of the analytical sensitivity and the small sample volume required, and is an attractive feature of in-line electrolytic dissolution. As a very small dissolved mass is required, rapid electrolysis (a few seconds) under a moderate current (mA) is sufficient. This was demonstrated in the flow-based spectrophotometric determination of aluminium in steels [29]. The analyte was oxidised and dissolved in a flowing acidic electrolytic solution that also acted as the sample carrier stream of the flow analyser. This innovation was further applied to the spectrophotometric determination of molybdenum in alloys [30]. In both applications, the anode was the polished metallic sample, and the cathode was a gold or silver coated electrode placed at the bottom of the electrolytic chamber (Fig. 8.4). A silicone rubber sheet (adapter) was placed between the solid sample and the chamber walls in order to avoid leakage and to define the sample surface area to be dissolved. This classical geometry is the most commonly used. 

OTTLE cells separate working and counter electrode compartments automatically, provided that leakage ( edge effects ) is minimized OTTLE cells useful for rapid electrolysis and investigation of slow follow-up reactions Open-circuit transients useful for kinetics of follow-up reactions... 

Figure 2.14 A stopped-flow optical absorption cell equipped with two column-type cells for rapid electrolysis. R, solution reservoir DP, Nj gas bubbling EC, electrochemical cell for flow electrolysis M, mixer FC, flow-type optical absorption cell L, light beam D, photodetector, CV, control valve (23, 24).

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