Impurities such as O 2 and water natively present in the IL, require different techniques to those presented above. The presence of O 2 can be determined by electrochemical methods, which is often demonstrated in studies where ILs are part of O 2 sensors. The common approach to determine the water content is Karl-Fischer titration. The removal of impurities, including oxygen and water, is, especially for electrochemical measurements, detrimental, as they can have a significant impact on the electrochemical properties and interfere with other processes studied on the electrodes. Typically, the amount of water and volatile impurities, in general, is reduced by drying the IL under vacuum conditions and elevated temperatures. Applying this approach to batch 1 reduces the water content from 75 ppm to 50 ppm. The value can even be reduced to less than 15 ppm, depending on the experimental setup used for drying (see Experimental section). The CVs for Au(111) recorded in the heat-treated and vacuum-dried IL (batch 1) and the as-received IL are shown in Figure a by solid and dashed lines, respectively. Overall, there are only minor differences between the CVs, except for a slight decrease of peak C1 after drying the IL. As shown elsewhere and described below, we suggest that peak C1 is unrelated to water but to O 2 (which is also related to peak A1). Hence, removing water apparently does not affect the electrochemical properties in pure ILs.