Impurities in ionic liquids

From our point of view, this is exactly what commercial ionic liquid production is about. Commercial producers try to make ionic liquids in the highest quality that can be achieved at reasonable cost. For some ionic liquids they can guarantee a purity greater than 99 %, for others perhaps only 95 %. If, however, customers are offered products with stated natures and amounts of impurities, they can then decide what kind of purity grade they need, given that they do have the opportunity to purify the commercial material further themselves. Since trace analysis of impurities in ionic liquids is still a field of ongoing fundamental research, we think that anybody who really needs (or believes that they need) a purity of greater than 99.99 % should synthesize or purify the ionic liquid themselves. Moreover, they may still need to develop the methods to specify this purity. 

In this context it is important to note that the detection of this land of alkali cation impurity in ionic liquids is not easy with traditional methods for reaction monitoring in ionic liquid synthesis (such as conventional NMR spectroscopy). More specialized procedures are required to quantify the amount of alkali ions in the ionic liquid or the quantitative ratio of organic cation to anion. Quantitative ion chromatography is probably the most powerful tool for this kind of quality analysis. 

Among the potential impurities in ionic liquids water, halide ions and organic starting material are of great importance for transition metal chemistry while the colour of an ionic liquid is not a critical parameter in most applications. 

Berthier, D., Varenne, A., Gareil, R, Digne, M., Lienemann, C.-P, Magnac, L., and Olivier-Bourbigouc, H., Capillary electrophoresis monitoring of halide impurities in ionic liquids. Analyst, 129,1257-1261, 2004. 

Our clear recommendation is to have a critical look at any ionic liquid delivered by any of the suppliers. Glassy carbon is a bad substrate to probe inorganic impurities in ionic liquids. In our experience noble metal electrodes like gold or platinum are better suited to detect low amounts of impurities. We have further examples in Clausthal where we saw clearly, with the in situ STM, metal deposition in apparently ultrapure liquids. In several liquids we found, with XPS and/or EDX, considerable amounts of potassium. Upon our insistence, the supplier finally told us that the synthesis route had been changed. For technical experiments such low amounts of... 

In order to obtain a more complete picture of the organic impurities in ionic liquids the leading academic and industrial players have now established a comprehensive set of state-of-the-art analytical methods. Today, there is much agreement that GC-headspace - in combination with H PLC - techniques are the analytical methods of choice to quantify organic impurities, such as e.g. the content of methylimidazole impurities, down to the ppm range. 

Fig. 2.2-2 Ion chromatography-the method of choice for the quantification of ionic impurities in ionic liquids.

Halide impurities are probably the most studied of the four general categories of impurities common to ionic liquids and, besides electrochemical analysis, two methods are currently being used to determine the level of residual halide impurities in ionic liquids [12]. The titration of the ionic hquid vnth AgN O3 is still widely used but suffers from a certain solubility of AgQ in the ionic hquid under investigation. This method can be enhanced by the Vollhard method for chlorine determination where the chloride is first precipitated with excess AgNO3 followed by back-titration of the mother liquor with aqueous potassium thiocyanate [13]. This method uses a visual endpoint through the formation of a complex between thiocyanate and an iron (III) nitrate indicator. 

Water is one of the typical impurities in ionic liquids, because these low... 

A well-known effect of water impurities in ionic liquids is the viscosity decrease. " However, also ionic liquids were reported, in which water impurities induces gelation. Spohr and Patey have shown with ionic liquid model systems that water tends to replace the counter ions from the ion solvation shell in ionic liquids with small ion size disparity, leading to a faster diffusion of the lighter ion-water clusters. However, water can increase viscosity of ionic liquids if the ion size disparity is too large, or if strong directional ion pairs are found. Spohr and Patey attributed this behavior to extended water-anion chains and strongly bound water-anion-cation clusters. A classical molecular dynamics study by Raju and Balasubramanian observed that the anion diffuse faster than the cation in water ionic liquid mixtures in contrast to neat ionic liquids. The larger... 

Chloride and fluoride were detected above the threshold in each sample. This technique detected 1.25 ppm chloride and 2.23 ppm fluoride in the non-chromatographed sample and 0.265 ppm chloride and 0.401 ppm fluoride in the chromatographed sample. This result is noteworthy in that chloride is well-established as an undesirable impurity in ionic liquids... 

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