The solvent-supporting electrolyte system

The ideal solvent for electrochemical studies should satisfy a number of requirements. In addition to the properties required for any good solvent for organic chemistry, such as a high solvating power and a low reactivity towards common intermediates, solvents for electrochemical use should be difficult to oxidise or reduce in the potential range of interest. Traditionally, the recommended potential limits are +3 V (versus the SCE) for oxidations and —3 V for reductions. Also, the solvent should have a dielectric constant higher than about 10 in order to ensure that the supporting electrolyte is well dissociated. Commonly used solvents are acetonitrile (MeCN) and dichloromethane for oxidations, and MeCN, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) for reductions. 

Solvents and supporting electrolytes of the highest commercial quality may often be used without further purification. However, it is recommended that DMF be distilled at reduced pressure prior to application. 

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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-... 

ROLE OF THE SOLVENT-SUPPORTING ELECTROLYTE SYSTEM IN ELECTROCHEMISTRY... 

There are several ways in which the solvent-supporting electrolyte system can influence mass transfer, the electrode reaction (electron transfer), and the chemical reactions that are coupled to the electron transfer. The diffusion of an electroactive species will be affected not only by the viscosity of the medium but also by the strength of the solute-solvent interactions that determine the size of the solvation sphere. The solvent also plays a crucial role in proton mobility water and other protic solvents produce a much higher proton mobility because of fast solvent proton exchange, a phenomenon that does not exist in aprotic organic solvents. 

Studies have also been conducted on electrode processes where homogeneous chemical reactions are coupled to heterogeneous electron transfer(s). In this example, the reductive dehalogenations of 3- and 4-bromobenzophenone and of o-bromonitrobenzene (denoted as ArBr) dissolved in N,N-dimethylformamide solution were studied (Compton et al., 1996b). The one-electron reductions of these compounds result in the formation of the corresponding chemically reactive radical anions as shown in (96), where HS denotes the solvent/supporting electrolyte system. 

Quantitative study of ECL systems is a rather difficult task. The combined requirements of reductant and oxidant stability with high fluorescence efficiency of the parent molecule drastically limit the types of compounds suitable for use in the study of the ECL phenomenon. To these, we must add solubility and chemical stability in the presence of electrodes, electrolyte, and solvent. Photochemical stability is an additional requirement. Chemical complications following the initial electron transfer to and from the electrode are still a problem. The chemistry occurring in solution after electrolysis must therefore be examined carefully. Especially, the solvent-supporting electrolyte system should be chosen to prevent lack of reactivity with the electrogenerated species. All of the above complications may lead to misinterpretation of the essentially simple processes of electron transfer excitation. However, in some cases most of these interferences may be removed by... 

Thus the larger the concentration of dissociated ions, the smaller the ohmic drop. This in turn depends on the nature and affinity of the solvent/supporting electrolyte system, as discussed earlier and in Chapter 5. 

In contrast to the direct process, an indirect process is one in which a foreign molecule or ion serves to shuttle electrons between the electrode and the substrate molecules. An indirect oxidation process may also involve the transfer of a hydrogen atom from a suitable substrate to a radical generated electrochemically. Indirect processes are typically observed for saturated aliphatic hydrocarbons and substrates that are more difficult to oxidize than the solvent-supporting electrolyte system. 

SMDE. In most experiments with an SMDE, the residual current is almost entirely of faradaic origin and is often controlled by the purity of the solvent-supporting electrolyte system (Section 7.3.2). 

ECL experiments focused on radical ion annihilation are carried out in fairly conventional electrochemical apparatus, but procedures must be modified to allow the electrogeneration of two reactants, rather than one, as is more commonly true. In addition, one must pay scrupulous attention to the purity of the solvent/supporting electrolyte system. Water and oxygen are particularly harmful to these experiments. Thus, apparatus is constructed to allow transfer of solvent and degassing on a high-vacuum line or in an inert-atmosphere box. Other constraints may be imposed by optical equipment used to monitor the light. 

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