Nonaqueous solvents, electrochemistry

The electrochemistry of oxo-bridged manganese complexes in aqueous solution is characterized by coupled electron and proton-transfer reactions. The cyclic voltammetric behavior of [Mn2 02(phen)4] + in aqueous pH 4.5 phosphate buffer is illustrated in Fig. 12 [97]. It is of interest to compare this result with that obtained for the same complex dissolved in CH3CN (Fig. 9). Two one-electron reactions are observed in each case. However, these correspond to Mn(IV,IV) Mn(IV,III) and Mn(IV,III) Mn(III,III) reductions in the nonaqueous solvent and to Mn(IV,III) Mn(III,III) and Mn(III,III) Mn(III,II) reductions in... 

Although the entire discussion of electrochemistry thus far has been in terms of aqueous solutions, the same principles apply equaly well to nonaqueous solvents. As a result of differences in solvation energies, electrode potentials may vary considerably from those found in aqueous solution. In addition the oxidation and reduction potentials characteristic of the solvent vary with the chemical behavior of the solvent. as a result of these two effects, it is often possible to carry out reactions in a nonaqueous solvent that would be impossible in water. For example, both sodium and beryllium are too reactive to be electroplated from aqueous solution, but beryllium can be electroplated from liquid ammonia and sodium from solutions in pyridine. 0 Unfortunately, the thermodynamic data necessary to construct complete tables of standard potential values are lacking for most solvents other than water. Jolly 1 has compiled such a table for liquid ammonia. The hydrogen electrode is used as the reference point to establish the scale as in water ... 

In nonaqueous solvents, nonprotonated species can be generated. Consequently, many electrochemical studies of organic compounds employ nonaqueous solvents such as acetonitrile, dimethylformamide, and dimethyl sulfoxide [53]. Electrochemistry in nonaqueous solvents is addressed in Chapters 15-18. 

This immediately leads to a question How small must these excursions be in order for the predictions to be valid Theoretically, the answer is zero millivolts, a clever but uninteresting answer. Practically the answer usually found in the literature is between 8/n and 12/n mV where n is the number of electrons transferred in the electrochemical reaction. These numbers are arrived at by estimating what kind of deviation from theoretical behavior can be detected experimentally. For purposes of this discussion we will use 10 mV. At this point it is useful to remember that the exponential terms are of the form anF(E - E°)RT, where T is the absolute temperature and a is either a or 1 - a. The 10/n mV figure is based on an a of 0.5 at 25 °C. Any change in these parameters from their nominal value would influence this limit (particularly in the case of low-temperature electrochemistry in nonaqueous solvents). This leads to the obvious next question What happens if you exceed this limit The answer is that the response begins to deviate noticeably from the ideal, theoretical model. How great the deviation is depends upon how far one exceeds... 

Comprehensive reviews describing the preparation, purification, and physical and electrochemical properties of these melts have been published [17-20]. The most popular systems are mixtures of A1C13 with either l-(l-butyl)pyridinium chloride (BupyCl) or 1 -methyl-3-ethylimidazolium chloride (MeEtimCl). These systems are very versatile solvents for electrochemistry because they are stable over a wide temperature range. In many ways they can be considered to be a link between conventional nonaqueous solvent/supporting electrolyte systems and conventional high-temperature molten salts. 

Table 18.1 Drying Agents for Nonaqueous Solvents Used in Electrochemistry...

For discussion and references, see Zahradnik, R., Parkanyi, C. Talanta 12, 1289 (1965) For an extensive review on nonaqueous solvents in electrochemistry, see Mann,... 

Nonaqueous Solvents. Many organic compounds are not soluble in water, and the investigator who desires to study their electrochemistry must resort to organic solvents. The solvents most often used are the so-called dipolar aptotic solvents that belong to Class 5a in the classification scheme of Table 7.5. These are solvents with moderately large dielectric constants and low proton availability. This aptotic character tends to simplify the electrochemical reactions often the primary product is a stable radical cation or anion that is produced by removal or addition of an electron. 

Scheme 1 (a and b) The most important organic polar aprotic solvents in electrochemistry. (c) The most important inorganic nonaqueous solvents in electrochemistry. 

Electrical conductivity is a critical issue in nonaqueous electrochemistry, since the use of nonaqueous solvents, which are usually less polar than water, means worse electrolyte dissolution, worse charge separation, and, hence, worse electrical conductivity compared with aqueous solutions. In this section, a short course on electrical conductivity in liquid solutions is given, followed by several useful tables summarizing representative data on solution conductivity and conductivity parameters. 

Dimethyl sulfoxide is an important solvent in nonaqueous electrochemistry due to its high polarity (dielectric constant of 47), its high donor number (29.8), and a relatively wide electrochemical window. The limiting cathodic voltages in which this solvent can be used depend on the cation used (as expected from the discussion on the cation effects on the reduction processes of the above nonaqueous solvents). Using salts of alkali metals (Li, Na, K), the cathodic limit obtained was around -1.8 — -2 V versus SCE [49], whereas with tetrabutyl ammonium, the cathodic limit was as low as -2.7 — -3 V versus SCE [49], There is evidence that in the presence of Na ions, DMSO reduction produces CH4 and H2 on plati-... 

Although these nonaqueous solvents are highly polar and thus may be attractive as media for nonaqueous electrochemistry, their electrochemical window is very narrow, since their cathodic potential limit is high (2.5-4 V versus Li/ Li+). Hence, their major importance remains as cathodic active materials in primary, high energy density batteries based on active metal (Li, Mg, Ca) anodes. 

From Mann s review, it is clear that the anion and the electrode material have a pronounced effect on the oxidation potentials of the nonaqueous systems. The metals to which the highest potentials can be applied in nonaqueous systems are obviously the noble metals (Pt, Au). The limiting reaction when the anions are halides (Cl-, Br, I ) was found to be their oxidation to the elemental form. When the anion is C104 , its oxidation onset at potentials above 1.5 versus Ag/ Ag+ may promote further intensive solvent degradation, as was found with ACN. It is important to note that using BF4 instead of C104- in ACN (which is an important and useful nonaqueous solvent in electrochemistry) extended its anodic stability limit by 2 V. 

Nonaqueous solvents can form electrolyte solutions, using the appropriate electrolytes. The evaluation of nonaqueous solvents for electrochemical use is based on factors such as -> dielectric constant, -> dipole moment, - donor and acceptor number. Nonaqueous electrochemistry became an important subject in modern electrochemistry during the last three decades due to accelerated development in the field of Li and Li ion - batteries. Solutions based on ethers, esters, and alkyl carbonates with salts such as LiPF6, LiAsly, LiN(S02CF3)2, LiSOjCFs are apparently stable with lithium, its alloys, lithiated carbons, and lithiated transition metal oxides with red-ox activity up to 5 V (vs. Li/Li+). Thereby, they are widely used in Li and Li-ion batteries. Nonaqueous solvents (mostly ethers) are important in connection with other battery systems, such as magnesium batteries (see also -> nonaqueous electrochemistry). 

Bedford College (see Chap. 4) on magnetochemistry. With the arrival of the balance, French began a collaboration with D. Harrison of QMC. French also undertook extensive research into the electrochemistry of nonaqueous solvents and on the preparative side of chemistry, she synthesised novel boron compounds. During her years at QMC, 23 students undertook their graduate research under her supervision, resulting in close to 50 publications. 

Several problems are encountered when potentials in different solvents are sought compared to the potential scale in water. A variety of approaches [186-194] have been followed to attack this problem usually the approach has been to introduce some kind of nonthermodynamic assumption, such as the supposition that certain large, monovalent ions (Rb, CS ) [191] or redox systems [186-188] of the charge type n/n + 1 (preferably 0/ + 1 [186,187]) have a nearly equal free energy of solvation in the two solvents so that the free energy of a transfer of the reference ion is small. The redox couples [194] ferro-cenium/ferrocene and bis(biphenyl)chromium(I)/bis(biphenyl)chromium(0) (BCr" "/BCr) have been recommended as reference redox systems for measurements in nonaqueous solvents however, an investigation concluded [195] that the electrochemistry of ferrocene in MeCN at microelectrodes was far from ideal, as some film formation may occur. 

---