Controlled-potential electrolysis equipment

It is evident that controlled potential electrolysis is, in principle, the preferred technique. In large-scale preparations, potentiostatic electrolysis with wholly electronic equipment may be economically unfeasible, but electromechanical systems may be constructed more easily. 

In some cases, the cathodic reduction of dichlorosilane gives the corresponding disilene. For example, the electrolysis of dimesityldichlorosilane in a divided cell equipped with a mercury pool cathode and silver anode under controlled potential conditions (-3.2V vs Ag/Ag+) affords tetramesityldisilene in 20% yield (Scheme 42) [90]. 

The method of complete electrolysis is also important in elucidating the mechanism of an electrode reaction. Usually, the substance under study is completely electrolyzed at a controlled potential and the products are identified and determined by appropriate methods, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis. In the GC method, the products are often identified and determined by the standard addition method. If the standard addition method is not applicable, however, other identification/determination techniques such as GC-MS should be used. The HPLC method is convenient when the product is thermally unstable or difficult to vaporize. HPLC instruments equipped with a high-sensitivity UV detector are the most popular, but a more sophisticated system like LC-MS may also be employed. In some cases, the products are separated from the solvent-supporting electrolyte system by such processes as vaporization, extraction and precipitation. If the products need to be collected separately, a preparative chromatographic method is use-... 

In general, controlled-current electrolyses need less expensive equipment. Only a controlled-current source in combination with a coulomb integrator is necessary. Therefore, in industry, electroorganic reactions are always performed at a fixed current density. In the laboratory, it is advisable to start with controlled-potential electrolyses using a potentiostat and a three-electrode electrolysis cell (Fig. 22.8). In this way, the reaction can be controlled at the redox potential of the substrate determined analytically, and the selectivity of the process can be studied at different potentials. After determination of the selectivity controlling factors, it is usually possible to change over to current control by proper selection of the current density and the concentration of the substrate. Using a continuous process, the concentration can be fixed at the desired value. Thus, selectivity can also be obtained under these conditions. 

Several finishing modes are applicable. The carbon dioxide can be retained in an absorption tube packed with Ascarite (sodium hydroxide mixed with asbestos) and weighed, or absorbed in a solution and titrated. The iodine produced in Eq. (14) can also be determined by titration. Alternatively, the iodine vapor can be led by a stream of nitrogen into an electrolysis cell, where iodine is reduced at controlled potential and the amount of electricity is recorded. Figure 6 shows, from right to left, the complete equipment for oxygen determination, which comprises the combustion train, the furnace for the oxidation of carbon monoxide, and the assembly for electrometric finish. 

Electrolytic procedures in which no effort is made to control the potential of the working electrode use simple and inexpensive equipment and require little operator attention. In these procedures, the potential applied across the cell is maintained at a more or less constant level throughout the electrolysis. 

The reactions were carried out in a divided cell of conventional design equipped with a platinum gauze anode and magnetically stirred Hg-pool cathode and immersed in an ice bath. The reference electrode was a silver wire immersed in sat. aq KCl in a Fischer Remote Reference Junction and positioned 1 mm from the cathode surface. Cathode potentials were controlled by a solid-state potentiostat. AIM LiCl solution (50 mL) in the appropriate solvent was placed in the cathode compartment the anolyte was identical, but contained also 95% hydrazine (5 mL). The solvent was purified by pre-electrolysis in an Nj stream for 30 min at a cathode potential of — 1.1 V versus Ag/AgCl for the 1,1-dibromocyclopropanes and —2.0V for 7,7-dichlorobicyclo[4.1.0]heptane. The dihalide (5-7 mmol) was added and electrolysis was allowed to proceed until the current had decayed to background. The catholyte was poured into HjO, extracted with pentane, and the combined extracts were washed with H O and dried (MgSOJ. The pentane was carefully distilled and gave monohalocyclopropane in 80-90% yield. 

Fig. 3 Scheme of potentiostatic operation for a preparative electrolysis, using in principle a simplified cyclovoltammetry equipment. The potential of the working electrode is measured by a Luggin capillary, coupled with a reference electrode (RE, see Sect. 2.5.1.6). The control circuit in the potentiostat adjusts the cell current until the potential of the working electrode is equal to the voltage at the control input. 

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