Tetraalkylammonium salt electrolytes solubilities

Many electrochemical reactions, especially of organic compounds, are better carried out in non-aqueous solvents and may not even proceed in water. The following requirements should be met by these solvents [73-77] sufficient solubility of the compounds to be examined and, of necessity, of the supporting electrolyte as well (usually tetraalkylammonium salts), chemical inertness towards the electrolyte and the reactive intermediates formed [e.g. the frequently formed radical anions would immediately be pro-tonated by protic solvents), and as high a relative permittivity as possible (usually fir > 10). The latter will increase the electrical conductivity by favoring the dissociation of the electrolyte and hence decreasing the electrical resistance of the solution. Nevertheless, even solvents of low relative permittivity (sr < 5) can be used for electrochemical... 

Tetraalkylammonium salts are the most common electrolytes for the non-aqueous solvent systems and high molecular weight ammoniums have been used for the ion-pair extractant. The thermodynamic properties of the salts such as partition equilibria, solubility, ion-pair formation etc. have been studied in a variety of solvents. The detailed equilibria or structure of ion pair, however, are not fully elucidated. 

Quite a few electrolytes are soluble in methanol, such as NH4CI, LiCl, HCl, KOH, KOMe, NaC104, and tetraalkylammonium salts. For reductions a solution of HCl in MeOH is convenient as it has a high conductance and both components are easy to get rid of during the isolation of the product. 

Many salts are soluble in DMSO, so the choice of supporting electrolyte is less restricted than in most other nonaqueous solvents. In general, perchlorates, even KCIO4, nitrates, and halides, are soluble, whereas fluorides, cyanides, sulfates, and carbonates are not thus not only NaC104, LiCl, NaNO, and tetraalkylammonium salts can be used but also such salts as NH4PF6 and NH4SCN. The ability of DMSO to solvate ions is also of importance in the indirect electrolytic hydrodimerization of, for example, acrylonitrile using Na(Hg) [388]. 

In aqueous solution a very wide range of electrolytes may be used. Electrochemical studies in non-aqueous solvents [12,13] are, however, very dependent on the use of tetraalkylammonium salts, the large cation making many such salts soluble in organic media. Indeed, it was the development of simple methods of preparation of (C4H9)4N C10 and similar salts which led to the widespread development of non-aqueous electrochemistry (note R4N " XT , X = CIO, BF or PF are made simply by precipitation on mixing aqueous solutions of R4NHSO4 and NaX). 

The electrolyte, added to enhance conductivity and to minimize double-layer and migration current effects, is chosen on the basis of solubility in a given solvent as well as inertness toward the electroactive substance and its electrolysis products. There are of course many choices of electrolyte for use in aqueous solution. The tetraalkylammonium salts are the most commonly used non-aqueous electrolytes. Tetrabutylammonium tetrafluoroborate (TBATFB) and tetrabutylammonium hexafluorophosphate (TBAHFP) are recommended by Fry and Britton, who note that TBAHFP in acetonitrile has a particularly large useful potential range of +3.4 to —2.9 V (vs. SCE). 

The same precautions versus the presence of water traces have to be taken for the used electrolyte. In most of the experiments, the electrolyte concentration was 0.1 moLL". Potassium, lithium, sodium, and cesium iodides were used (because of their large solubilities). Tetraalkylammonium salts (puriss grade) were also used with purity >99.7%. It is required that all salts be dried by conventional methods before use [40]. 

Tetraalkylammonium tosylates [74] and trifluoromethanesulfonates [72] are also excellent electrolytes. Although tetraalkylammonium ions are favored as the cations for supporting electrolytes because of their wide potential range, other cations are sometimes used for special applications—for example, methyltri-phenyl phosphonium, whose tosylate is freely soluble in methylene chloride, and other fairly nonpolar solvents [74] or metal ions (lithium salts tend to have the best solubility in organic solvents) where undesirable reactions of the tetraalkylammonium ion might occur [13,75]. The properties of many electrolytes suitable for nonaqueous use have been surveyed [76]. 

One of the most often overlooked variables in electrochemistry is the supporting electrolyte which is often selected on the basis of availability, cost, solubility, or perhaps just precedent (my electrochemical colleagues use a given salt, why shouldn t I ). The most commonly utilized salts in nonaqueous media have generally been tetraalkylammonium perchlorates, tetrafluoroborates or hexafluorophosphates and these are usually utilized in 0.05 to 0.2 M concentrations. Chemistry is sometimes a consideration in selection of the supporting electrolyte but in most cases the... 

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