Cells for Controlled Potential Electrolysis

Cells used in exhaustive electrolysis present more problems of electrodes symmetry than those for cyclic voltammetry, due to the long experimental times and high currents involved. 

The working electrode generally consists of a cylindrical platinum gauze or a mercury pool thereby offering the largest surface area possible to the redox process. 

The auxiliary electrode, which is normally a mercury pool, must be positioned in a compartment separate from the working electrode. Such a separation compromises the desired symmetric disposition of the electrodes. Normally, the compartments of a macroelectrolysis cell are separated by sintered glass frits, such that the catholyte and the anolyte are not mixed. In fact, if the working electrode is involved, for example, in a cathodic process, the auxiliary electrode will act as an anode. This implies that the auxiliary electrode will produce oxidized material (by anodic decomposition of the solvent itself, of the supporting electrolyte, of mercury-contaminated products or of electroactive residues diffused at the auxiliary electrode) that may subsequently be reduced at the working electrode, contaminating and falsifying the primary process. 

As for the case of cells for cyclic voltammetry, electrolysis cells are either of commercial type or in-house constructed according to the experience of each researcher. 

It is recommended to insert into the cell an electrode for cyclic voltammetry. This allows one to record a cyclic voltammogram of the species under study directly before and after exhaustive electrolysis, 

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Figure 6.14 Four-compartment cell for controlled-potential electrolysis and coulo-metric titrations. Two central bridge compartments can be emptied to sample compartment by application of nitrogen pressure and refilled by vacuum 1, polyethylene top 2, lock ring 3, combination glass-calomel electrode 4, N2 inlet tube 5, N2 outlet tube 6, Pt gauze electrode 7, cell rinse assembly 8, polyethylene spray shield 9, 0.1 M KC1 in 3% agar gel 10, Ag anode.

Figure 6.15 Mercuiy-pool cell for controlled-potential electrolysis with uniform potential and current distribution.

Fig. 11.2 - Some typical laboratory cells, (a) and (b) are cells for kinetic studies and (c) a cell for controlled potential electrolysis.

Figure 23-2 shows the components of a simple apparatus for carrying out linear-sweep voltammetric measurements. The cell is made up of three electrodes immersed in a solution containing the analyte and also an excess of a nonreactive electrolyte called a supporting electrolyte. (Note the similarity of this cell to the one for controlled-potential electrolysis shown in Figure 22-7.) One of the three electrodes is the working electrode, whose potential versus a reference electrode is varied linearly with time. The dimensions of the working electrode are kept small to enhance its tendency to become polarized. The reference electrode has a potential that remains constant throughout the experiment. The third electrode is a...

Figure 11.2.2 Typical cells for bulk electrolysis, (a) Undivided cell for controlled-potential separations and electrogravimetric analysis at a solid cathode. [From J. J. Lingane, Anal Chim. Acta, 2, 584 (1948), with permission.] ib) Undivided cell for coulometric analysis at mercury cathode with a silver anode. [Reprinted with permission from J. J. Lingane, J. Am. Chem. Soc.,...

Electrolysis cell for controlled potential separations with a mercury cathode. From J. J. Llngane, "Electroanalytlcal Chemistry", Interscience Publishers Inc., New York, 1953. 

Figure 3. Controlled-potential electrolysis cell for generation of radical ions in the cavity of esr spectrometer [from (16) by permission of the authors and the American Chemical Society]. 

For cases directly comparable to the cyclization originating from (27) above, the yields of the product were not as high. However, a related reaction used in the synthesis of an 11-substituted dibenzo[a,d]-cycloheptenimine derivative was very successful as shown in Scheme 11 (Eq. 2) [32]. In this reaction, a controlled potential electrolysis of (33) led to the formation of the tetracyclic (34) in an 85% isolated yield. The reaction was performed on a 1 g scale using an undivided cell, a graphite felt anode, a stainless steel cathode, a saturated calomel reference electrode, and a 1% NaBF4 in 70 30 THF/water electrolyte solution. The electrolysis was scaled up further with the use of a flow cell. In this experiment, 200 g of (33) were oxidized in order to afford a 75% isolated yield of (34). 

Attempts to reduce anthracene with an alkali metal in acetonitrile causes solvent decomposition, whereas controlled-potential electrolysis produces stable anion radicals. Thus the working electrode of a coulometric cell can be considered as a continuously adjustable reagent, capable of producing a wide variety of radical species in diverse solvent systems. The versatility of electrochemical EPR methods is best illustrated by citing a few specific examples from the extensive literature. More complete compilations appear in the reviews listed in Appendix I, but the studies mentioned next provide some appreciation for the techniques. 

Electrode geometry in controlled-potential electrolysis. When fast response and accuracy of potential control are desired, considerable attention must be paid to the design of the cell-potentiostat system, and several papers have discussed the critical parameters and made recommendations for optimum cell design.8"11 In general, to achieve stability and an optimum potentiostat rise time for a fast potential change, the total cell impedance should be as small as possible, and the uncompensated resistance should be adjusted to an optimum (nonzero) value that depends on the characteristics of the cell and potentiostat.9,12 The electrode geometry also should provide for a low-resistance reference electrode and a uniform current distribution over the surface of the... 

A reference electrode [183-185] is a half-cell that defines a potential to which all other measurements are referred. The primary standard electrode is the standard hydrogen electrode (SHE), but as this electrode is inconvenient for practical work, other reference electrodes are used. Such reference electrodes must have a potential that changes very little and is known to within 1 mV or so. In some cases of controlled potential electrolysis, it is sufficient to know the potential of the working electrode during the electrolysis within 10-20 mV, because the potential variations between different points of the electrode are of this magnitude (see earlier), and less precise electrodes may be termed comparison electrodes. In principle, any electrode at the surface of which an electrochemical reaction with a large exchange current can take place may be used as a reference electrode. 

The scope of the reaction type has been explored in particular by Grimshaw and coworkers [276-286]. Since the rate constant for cleavage of the carbon-halogen bond decreases in the order I > Br > Cl > F, yields of coupling products are usually higher using chloro rather than bromo compounds. Most of the studies have been carried out by controlled potential electrolysis —2.1 V for the chloro compounds) in DMF using divided cells and Hg cathodes [276-285]. 

An electrode is inexpensive when compared with most chemical reagents. It is immobile, and thus causes less environmental and solubility problems than most chemical oxidants and reductants. It can change the polarity of reagents by oxidation or reduction ( Redox-Umpolung ) and in this way can shorten synthetic sequences. Controlled potential electrolysis allows the selective conversion of one out of several electrophores in a molecule. A technical scale-up causes in most cases lesser problems than the scale-up of a chemical reaction. These advantages and the wide choice of conversions have made electrolysis today at least for those that take the small effort to assemble an electrolysis cell and connect it to a d.c. power supply - to an attractive alternative and supplement for chemical synthetic methods. 

Controlled potential electrolysis (potentiostatic control) requires a three-electrode cell, so as not to polarize the reference electrode. Controlled potential methods enable one to be very selective in depositing one metal from a mixture of metals. If two components have electrochemical potentials that differ by no more than several hundred millivolts, it may still be possible to shift these potentials by complexing one of the species. One disadvantage of exhaustive electrolysis is the time required for analysis, and faster methods of electrochemical analysis are described. 

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. 

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