Purification nonaqueous solvents

While water has been used as a solvent more than any other media, nonaqueous solvents [e.g., acetonitrile, propylene carbonate, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or methanol] have also frequently been used. Mixed solvents may also be considered for certain applications. Double-distilled water is adequate for most work in aqueous media. Triple-distilled water is often required when trace (stripping) analysis is concerned. Organic solvents often require drying or purification procedures. These and other solvent-related considerations have been reviewed by Mann (3). 

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. 

The reversible dissociation of antigen-antibody bonds by nonaqueous solvents such as dioxane may prove of considerable practical use in procedures for the isolation and purification of specific antibodies (cf. Singer et al., 1960). 

Table 1 lists the most common nonaqueous solvents that have previously been utilized for the electrochemistry of metalloporphyrins. Purification procedures, potential limits and physical characteristics of these solvents are given in the literature [17-20, 49, 50]. 

The selection of a specific nonaqueous solvent from the list in Table 1 was based in many cases only on the habit of the individual laboratory, but in others it very much depended on the requirements of the individual experiment. The first requirement is, of course, solubihty and the electrochemical parameters that one is investigating. One must also consider ease of purification, the chemical reactivity of the solvent, its ability to stabilize rr-anion or tt-cation radicals, and its overall potential range for both oxidation and reduction, the latter of which will depend in part on the type of electrode material (Hg for example caimot be used for oxidations, while Ag and Au both have a Kmited positive range in solvents containing some anions.). Other practical factors include the cost of the solvent, its toxicity, and its general ease of handling. 

Wherever possible aqueous solutions are preferred, but substances of low solubility prevent the use of water as a solvent and consequently the solvents normally used are aqueous alcohols, dioxane-water or Cellosolve-water mixtures. Sometimes even nonaqueous solutions are used (e.g. glacial acetic acid, N,N -dimethylformamide, benzene-methanol mixtures etc.). Usually it is necessary to purify the solvent in order to prevent contamination by substances which give waves in the same potential range as the substance under examination. Often procedures developed for the purification of solvents for spectrophotometric purposes have proved useful. 

In Fry and Britton s handy review of solvents and electrolytes, acetonitrile, ethanol, methanol, and methylene chloride are recommended as good oxidative (anodic) electrochemical solvents, while acetonitrile, DMF, and dimethyl sulfoxide (DMSO) are suggested for reductive (cathodic) electrochemistry. Acetonitrile is suggested as the best overall nonaqueous solvent on the basis of its electrochemical properties and its relative nontoxicity. The review of Fry and Britton also is a good place to start when looking for purification methods. 

Recovery and Purification. AH processes for the recovery and refining of maleic anhydride must deal with the efficient separation of maleic anhydride from the large amount of water produced in the reaction process. Recovery systems can be separated into two general categories aqueous- and nonaqueous-based absorption systems. Solvent-based systems have a higher recovery of maleic anhydride and are more energy efficient than water-based systems. 

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox... 

One of the problems in electrocatalysis is that electrochemical reactions are generally carried out in aqueous or nonaqueous solution. Thus, the solvent may intervene in the over-all reaction. In addition, it is necessary to carry out the reaction under highly purified conditions. Otherwise, impurities in the solution may affect the kinetics of the reaction concerned, so that mechanism studies become difficult. For gas phase reactions, though impurity concentrations are generally lower than in electrochemical reactions, one uses high-vacuum techniques for purification. Electrochemical purification techniques— pre-electrolysis or adsorption of impurities near the potential of maximum adsorption—are often simpler. The activation of a poissoned catalyst is often difficult or impossible. An electrocatalyst can often be reactivated in situ, by pulse techniques (cf. Section VII,D). 

Systems In nonaqueous systems [87], the amount of water is very limited, coming from trace water that remains after purification of these solvents or from water intentionally added to the solvent. The extensive hydrogen bonding of bulk water is broken, resulting in vq-h bands of greater resolution than observed in aqueous media. Fig. 41 gives selected SERS spectra of water in different solvents using LiBr as the electrolyte. 

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