Solvents, nonaqueous leveling effect

Nonaqueous solvents have the advantage of enhanced solubility of organic reactants and products and avoidance of the leveling effects of aqueous solvent. 

I would now like to consider the titration of acidic compounds in nonaqueous solutions. If you wish to titrate an acid in nonaqueous solution, you should choose a solvent that is not acidic and a titrant that is as strong a base as possible. The paper that really aroused people s imagination and created a lot of interest was the one published by Moss, Elliot, and Hall in 1948, in which they introduced ethylenediamine as a solvent. This compound certainly doesn t have any acidic properties and these authors showed that you can titrate phenol, which is normally too weak to titrate as an acid. In recent years, however, the trend has been away from the use of strongly basic solvents because they have a leveling effect on many bases and they are somewhat unpleasant to handle. Solvents now in use are pyridine, which is an inert solvent and a very weak base, acetonitrile, and acetone. Acetone and certain other ketones are surprisingly good. Recently we have done some work with tertiary butyl alcohol, an excellent solvent for certain cases. Sodium or potassium hydroxide can be used as tltrants, but these are not particularly... 

Sulfur is known to be easily reducible in nonaqueous solvents and its reduction products exist at various levels of reduction of polysulfide radical anions (S . ) and dianions (Sm2 ) 173], Recently Be-senhard and co-workers [74] have examined the effect of the addition of polysulfide to LiC104-PC. Lithium is cycled on an Ni substrate with Qc=2.7 C cm 2 and cycling currents of 1 mA cm. The cycling efficiency in PC with polysulfide is higher than that without an additive. The lithium deposition morphology is compact and smooth in PC with added polysulfide, whereas it is dendritic in PC alone. 

The effect of the addition of water and molecular solvents such as propylene carbonate, N-methylformamide, and 1-methylimidazole on the conductivity of [C4Cilm][Br] and [C2Cilm][BF4] was measured at 298 K [211]. The mixture of the IL and the molecular solvent or water showed a maximum on the conductivity/mole fraction IL curves. The maximum for nonaqueous solvents was at the level of approximately 18-30 mScm at low mole fraction of the IL and the maximum for water was at level approximately 92-98 mScm [211]. The conductivity of a mixture of these two ILs depends monotonically on the composition. The temperature dependence of the conductivity obeys the Arrhenius law. Activation energies, determined from the Arrhenius plot, are usually in the range of 10-40 kj mol / The mixtures of two ILs or of an IL with molecular solvents may find practical applications in electrochemical capacitors [212]. 

In reviewing the intrinsic electrochemical behavior of nonaqueous systems, it is important to describe reactions of the most common and unavoidable contaminants. Some contaminants may be introduced by the salts (e.g., HF in solutions of the MFX salts M = P, B, As, etc.). Other possible examples are alcohols, which can contaminate esters, ethers, or alkyl carbonates. We examined the possible effect of alcoholic contaminants such as CH3OH in MF and 1,2-propylenegly-col at concentrations of hundreds of ppm in PC solutions. It appears that the commonly used ester or alkyl carbonate solvents are sufficiently reactive (as described above), and so their intrinsic reactivity dominates the surface chemistry if the concentration of the alcoholic contaminant is at the ppm level. We have no similar comprehensive data for ethereal solutions. However, the most important contaminants that should be dealt with in this section, and which are common to all of these solutions, are the atmospheric ones that include 02, H20, and C02. The reduction of these species depends on the electrode material, the solvent used, and their concentration, although the cation plays the most important role. When the electrolyte is a tetraalkyl ammonium salt, the reduction products of H20, 02 or C02 are soluble. As expected, reduction of water produces OH and... 

However, in the work of Tadjeddine et al. in aqueous and nonaqueous electrolyte (acetonitrile), a lifetime of 1.7 ps was observed with no effect of potential, even over a range of 2 V [107, 108]. The results indicate the CO vibrational Hfe-time is not dependent on electrode potential or solvent molecules or surface orientation [Pt(lll), Pt(llO)] [2, 108]. This discrepancy is small and could be due to an improved signal-to-noise ratio in the later results. Thus, CO vibrational relaxation is presumed to be dominated by the charge transfer-like mechanism because of the interaction between the CO 2n and the Fermi level of the metal. These measurements help in developing molecular models for bonding of molecules at electrode surfaces. 

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