Electrochemical Quantification of Hygroscopicity of Ionic Liquids with Solution-Dissolved Potassium Ferricyanide as the Redox Probe

Quantification of hygroscopicity of ionic liquids (ILs) is of great importance in both fundamental studies and practical applications of ILs. This study demonstrates an electrochemical method for effectively quantifying the hygroscopicity of ILs through electrochemically monitoring water contents absorbed into ILs. The measurements of water content absorbed into the ILs are performed with square wave voltammetry (SWV) based on the water-induced enhancement of diffusion of solution-dissolved potassium ferricyanide (K 3 Fe(CN) 6 ) redox probe. For demonstration, two kinds of ILs with different hygroscopicity (i.e., hydrophilic Bmim + Gly − and hydrophobic Bmim + PF 6 − ) are employed in this study. The dissolution of K 3 Fe(CN) 6 redox probe into ILs is found to have little effect on the hygroscopicity of ILs. The hygroscopicity of ILs is thus able to be quantified by monitoring water content absorbed into ILs as a function of time when ILs are stored at room temperature and standard atmospheric pressure under 55 % relative humidity (RH). Under the conditions employed in this study, the hygroscopicity of Bmim + Gly − and Bmim + PF 6 − is determined to be 1.33 M per hour and 0.05 M per hour, respectively, which are almost consistent with those measured with Karl Fischer titration under the same conditions. The electrochemical method demonstrated in this study is experimentally simply and environmentally benign and may be potentially extended for general quantification of hygroscopicity of ILs.