Solvent and Supporting Electrolytes

There is no universal solvent, and even for a given application one rarely finds an ideal system. One must factor some informed guesswork into one s choice of solvent and electrolyte. In order to optimize conditions for an electrode reaction, one must consider how its chemical and electrochemical features, for 

An electrochemical solvent must be able to dissolve a wide range of substances at acceptable concentrations. In general, this means that electrolytes must be 

Solvents and electrolytes should also be inexpensive, nontoxic, and nonflammable. The latter two characteristics are not well satisfied by most organic solvents, but with reasonable safety precautions and reasonable ventilation they can be used routinely without incident. Another solvent property, viscosity, may be of importance on occasion. High viscosities are useful when one wishes to extend the time interval over which mass transport occurs purely by diffusion, such as for potential-step experiments, but a low-viscosity solvent is preferred when efficient mass transport is required, as in preparative electrolyses. 

Many solvents and electrolytes have been used in electrochemical applications. In the preceding sections we suggested a few solvents for general use. It may 

Solvated electrons are readily prepared in hexamethylphosphoramide (HMPA), ammonia, and methylamine [19]. Lithium chloride is the preferred electrolyte for this application. 

The solvent and the supporting electrolyte must be appropriate for each measurement, because they have significant effects on electrode processes. 

The following criteria should be used to select an appropriate solvent (i) the electroactive species under study and the reaction products must dissolve and remain sufficiently stable in the solvent (ii) a polar solvent of weak acidity is suitable for an electrode reaction that occurs at negative potentials or whose measurement is affected by acidic solvents (iii) a polar solvent of weak basicity is suitable for an electrode reaction that occurs at positive potentials or whose measurement is affected by basic solvents (iv) the solvent should be easy to purify, low in toxicity, benign to the environment, reasonable in price, and should dissolve enough supporting electrolyte. Generally, DMF and DMSO (protophilic aprotic) and AN (protophobic aprotic) are used in case (ii), while AN is used most frequently in case (iii). 

Eleefroehemical measurements are commonly carried out in a medium that consists of solvent eontaining a supportmg electrolyte. The choice of the solvent is dictated primarily by the solubility of the analyte and its redox activity, and by solvent properties such as the electrical conductivity, electrochemical activity, and chemical reactivity. The solvent should not react with the analyte (or products) and should not undergo elecffochemical reactions over a wide potential range. 

Wliile water has been used as a solvent more than any other media, nonaqueous solvents [e.g., acetonihile, propylene carbonate, dunethylformamide (DMF), dunetliyl sulfoxide (DMSO), or methanol] have also fi equently been used. Mixed solvents may also be considered for certam applications. Double-distilled water is adequate for most work m aqueous media. Triple-distflled water is often required when trace (shipping) analysis is concerned. Organic solvents often require drying or purification procedures. These and otlier solvent-related considerations have been reviewed by Maim (3). 

Supportmg electi olytes are required m controlled-potential experiments to decrease the resistance of the solution, to elunmate electi-omigration effects, and to maintam a constant ionic stiength (i.e., swampmg out the effect of variable 

While water has been used as a solvent more than any other medium, non-aqueous solvents [e.g., acetonitrile, propylene carbonate, dimethylformamide 

The electrochemical reduction of oxygen usually proceeds via two well-separated two-electron steps. The first step corresponds to the formation of hydrogen peroxide 

The half-wave potentials of these steps are approximately -0.1 and -0.9 V (vs. the saturated calomel electrode). The exact stoichiometry of these steps is dependent on the medium. The large background current accrued from this stepwise oxygen reduction interferes with the measurement of many reducible 

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The reversibility of the carrier was tested by cyclic voltammetry. The scan of the solvent and supporting electrolyte is shown in Fig. 13, with and without dissolved oxygen. The oxygen reduction occurs at about — 0.43 V. (vs. SCE). The scan with the complex added, but the solution free of dissolved oxygen is shown as Fig. 14. The carrier is seen to be reduced at about 0.04 V, well within the window of the solvent and electrolyte, and well before reduction of molecular oxygen. 

Fig. 13. CV scan of solvent and supporting electrolyte, with and without dissolved oxygen.

It needs to be pointed out that E values may also be quite sensitive to the nature of the solvent and supporting electrolyte used for an electrochemical study. Apart from solvation effects of the non-specific type, solvent molecules may occupy coordination sites in either the starting complex or the products and hence influence redox behaviour (Fabbrizzi, 1985). Similarly, the nature of the anion present may also strongly influence the redox potential if it has ligating properties (Zeigerson etal., 1982). Because of such effects, caution needs to be exercised in attempting to compare electrochemical data which have not been obtained under similar conditions. 

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

The third application is the oligomerization of phenol. By selecting solvent and supporting electrolyte, phenol is electro-oxidatively polymerized to yield poly (phenyleneoxide) as a tan-colored powder. 

U sually, there is a significant interdependency between the electrode properties and the electrolyte composition, that is, reactants, products, solvents, and supporting electrolytes, including impurities. 

In addition to the function as reaction medium - as in all chemical reactions - in electrochemical processes, the electrolyte has to provide the transport of ions between the electrodes. An optimal combination of solvent and supporting electrolyte has to be found, considering the reaction conditions and the properties of reactants, products, and electrodes. A short overview of usual electrolytes - and some examples of unconventional electrolytes as thought-provoking impulse for research - is given... 

Rather surprisingly, the differences in half-wave potentials of hydrocarbons from one solvent to another are very small. This constancy in energy values as well as slopes of correlation lines in widely varying solvents and supporting electrolytes implies that solvation energies, provided they are not small, change in the same way from system to system. 

The direct electrochemical oxidation of aliphatic alcohols (1) to carbonyl compounds (2) (Eq. 1) is not a convenient way for synthesis because of the high oxidation potentials of alcohols. The oxidation always competes with the oxidation of a solvent and supporting electrolyte, leading to low current efhdencies and side products. 

Scheme 1) [10]. The choice of solvent and supporting electrolyte was important for the selectivity. 

EGA-catalyzed ring opening of epoxides, (14), is one of the most-studied catalytic EGA reactions. The proper choice of solvent and supporting electrolyte allows selective formation of a ketone, an allylic alcohol, an acetonide, or an a-hydroxy ether. Scheme 7. 

This same solvent and supporting electrolyte dependence effect is observed for C70 and for the higher fullerenes (see the following). 

The solubility of the electrogenerated species also depends upon the choice of solvent and supporting electrolyte [38]. For example, Ceo is insoluble in DMF and TFfF, but all of its anions readily dissolve in these solvents. In acetonitrile, Ceo is... 

Tab. 7 Half-wave reduction potentials (in V vs. Fc/Fc+) of Cg2 and its endohedral metalloderivatives using various solvents and supporting electrolytes...

As in solution phase electrochemistry, selection of solvent and supporting electrolytes, electrode material, and method of electrode modification, electrochemical technique, parameters and data treatment, is required. In general, long-time voltam-metric experiments will be preferred because solid state electrochemical processes involve diffusion and surface reactions whose typical rates are lower than those involved in solution phase electrochemistry. 

Recently most of the polymer studies, not only ionic but radical polymerization, too, have been carried out in organic media. However, polymer chemists engaging in electropolymeiization have come upon many difficult problems when they introduced the electrolytic processes to their own field of chemistry, because there has been little knowledge of the electrochemistry in organic media free from water. The problems were how to choose organic solvents and supporting electrolytes which would not affect the polymerization, electrodes and cells to be used in the electropolymerization. 

This chapter is aimed at the inexperienced researcher who desires to carry out electroanalytical measurements in molten salts and seeks introductory information about the experimental details associated with the use of these solvents. It is intended to complement the chapters appearing elsewhere in this volume that discuss conventional molecular solvents and supporting electrolytes and various electroanalytical techniques. 

The solvents and supporting electrolytes are discussed in Chapter 15. Therefore, only some aspects with regard to electroorganic syntheses are discussed here. For this application, the following conditions have to be fulfilled by the electrolyte ... 

Table I comprises 28 columns and extends across two facing pages. The division between facing pages is very nearly such that the left-hand pages identify the compounds for which data are giver., the technique by which the data were obtained, and the electrodes used, and also describe the solvent and supporting electrolyte, apparatus, and experimental conditions, while the right-hand pages give the data and other information obtained and provide cross-references to additional information contained in other tables.

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