Supporting Electrolyte purification

If we measure a residual current-potential curve by adding an appropriate supporting electrolyte to the purified solvent, we can detect and determine the electroactive impurities contained in the solution. In Fig. 10.2, the peroxide fonned after the purification of HMPA was detected by polarography. Polarography and voltammetry are also used to determine the applicable potential ranges and how they are influenced by impurities (see Fig. 10.1). These methods are the most straightforward for testing solvents to be used in electrochemical measurements. 

One of the most comprehensive reviews on solvent use and purification was prepared by C.K. Mann [13]. Many solvents are discussed, and for each particular solvent, suitable supporting electrolytes and reference electrodes are presented. Lund and Iversen [65], among others [66], have also surveyed the use, purification, and properties of solvents. There are overlaps among all of these reviews and the present one, but each has its own special flavor and the different viewpoints make for instructive comparisons. There are also a number of articles devoted to specific solvents, such as DMSO [62,67], DMF [68], methylene chloride [69], and pyridine [70]. 

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. 

Meites, L. Purification of Supporting Electrolytes for Polarographic Trace Analysis by Controlled Potential Elektrolysis at Mercury Cathode. Anal. Chem. 27, 416 (1955). 

Solvents and supporting electrolytes of the highest commercial quality may often be used without further purification. However, it is recommended that DMF be distilled at reduced pressure prior to application. 

Polarographic methods of analysis have been applied to samples of foods containing saccharin (1+1-1+1+ ). In a procedure (1+1+) saccharin is extracted into organic solvents in an acidic medium. Further purification is achieved by column chromatography. The residue obtained is dissolved in 0.1 N NaOH and an aliquot is polarographed in a supporting electrolyte of 0.1 N HC1, 0.1 N KC1 and 0.1 Bu N Br. 

A number of companies supply reagent or spectroquality solvents that have been purified to remove UV-absorbing impurities. Some of them, particularly dimethyl sulfoxide, may be suitable for general electrochemical use as purchased. However, small quantities of electroactive impurities (particularly water) often are present in spectroquality solvents. Therefore, a particular batch of solvent always should be tested by measurement of the residual current with an appropriate supporting electrolyte and a platinum, gold, or carbon electrode (to test the anodic limits) and a platinum electrode (to test the cathodic limits). The voltage window or domain of electroactivity is a sensitive measure of the adequacy of the purification procedures. 

As we pointed out in previous chapters, the quality and purity of the solvent and supporting electrolyte used is important in electrochemical measurements. For most measurements in aprotic solvents it is necessary to keep water levels as low as possible. Earlier in this chapter procedures were described for purifying solvents and supporting electrolytes. However, it is tedious work, which requires time and energy. Moreover, it is not possible to obtain as low water levels as those available from specialized companies. From our own experience the solvents purified by Burdich Jackson ( distilled in glass grade ), a division of Baxter, can be used in electrochemical measurements without further purification (most attempts to improve their materials result in higher H20 levels). Table 7.12 lists maximum water contents and dielectric constants for several Burdick Jackson solvents that are frequently used by electrochemists. However, the actual water level in most cases is much lower. 

Several fine chemicals companies sell salts that can be used as supporting electrolytes without further purification. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

CV is usually done with substrate concentrations in the mM range. The redox process that occurs at the working electrode often generates species - radicals, ions, or both - that are reactive and short lived. Traces of impurities in the solvent or supporting electrolyte (at a typical concentration of 0.1 M and above) can significantly shorten the lifetimes of these species. The supporting electrolyte and the solvent are both present in a large excess compared to the substrate procedures for the prerequisite solvent and electrolyte purification have been summarized. ... 

The solutions of Cd in 1 M NaCl were prepared using Cd(N03)2 -4H20 (MERCK, pro analysi), NaCl (MERCK, pro analysi) and high-purity water which was obtained by passing distilled water through a Milli-Q water purification system. A supporting electrolyte of 1 M NaCl was used in all the experiments. 

An important example of this type of separation process is the purification of the supporting electrolyte by controlled potential electrolysis. Thus by electrol5reis at —2-35 V, the alkali metals (and the lower amines) can be separated from 0-1 M tetraethyl-ammonium hydroxide in 50 per cent ethanol after 45 min.h5)... 

Currently, low-temperature CO oxidation over Au catalysts is practically important in connection with air quality control (CO removal from air) and the purification of hydrogen produced by steam reforming of methanol or hydrocarbons for polymer electrolyte fuel cells (CO removal from H2). Moreover, reaction mechanisms for CO oxidation have been studied most extensively and intensively throughout the history of catalysis research. Many reviews [4,19-28] and highlight articles [12, 29, 30] have been published on CO oxidation over catalysts. This chapter summarizes of the state of art of low temperature CO oxidation in air and in H2 over supported Au NPs. The objective is also to overview of mechanisms of CO oxidation catalyzed by Au. 

Solvent Extraction. A modified, one-cycle PUREX process is used at Rocky Flats to recover plutonium from miscellaneous Pu-U residues (11). The process utilizes the extraction of uranium (VI) into tributyl phosphate (TBP), leaving plutonium (III) in the raffinate. The plutonium is then sent to ion exchange for purification. An extraction chromatography method is being studied as a possible substitute for the liquid-liquid extraction process (12) TBP is sorbed on an inert support so ion exchange column equipment can be used. Electrolytic valence adjustment could significantly improve this process. 

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