Determination of Water in 1-Ethyl-3-methylimidazolium tetrafluoroborate

Abstract

Determination of water in 1-ethyl-3-methylimidazolium tetrafluoroborate has been carried out using both electrochemical and infrared spectroscopic methods. At platinum electrodes, voltammetric responses for water oxidation and reduction have been used to prepare calibration curves. It has been found that the water oxidation current at +2.4V vs Ag/AgCl is suitable for water determination to at least 40 mM. Infrared spectroscopy of water in quartz cells in the OH stretching region revealed an IR feature at 3550 cm -1 for water additions to 20 mM, with another feature at 3650 cm -1 appearing at higher water concentrations. For the lowfrequency feature, a calibration curve is linear to 14 mM. Both electrochemical and IR methods provide a lower limit of detection of approximately 1 mM water in this ionic liquid.

OH Stretching and Libration Bands of Solitary Water in Ionic Liquids and Dipolar Solvents Share a Single Dependence on Solvent Polarity

“…As noted in the Introduction, the seminal ionic liquid study of Cammarata et al attributed the main OH features observed in ionic liquids to symmetric and antisymmetric stretching modes of water molecules donating hydrogen bonds to two anions, i.e., water in A – ···HOH···A – solvates. All subsequent ionic liquid studies have either accepted or provided further support for this assignment. − ,,, However, as seen in Figure , the spectra in most ionic liquids consist of more than the two well-defined peaks implied by this simple description.…”

“…Numerous groups have previously studied the OH stretching spectra of dilute water in both dipolar − ,− ,− ,, and ionic − solvents. Because our interpretation builds upon this prior work, we digress to briefly summarize it here.…”

“…They also performed DFT calculations of one or two ion pairs interacting with a water molecule, which confirmed assignment of the dominant peaks to symmetric and antisymmetric vibrations of “double donor” water molecules hydrogen bonded to two anions, as proposed by Cammarata et al Their calculations led them to propose the flanking peaks to be due to “single donor” water molecules of the sort, A – ···HOH, an assignment that further aligned with interpretations of early work on water in conventional solvents and solvent mixtures. − Ludwig and co-workers also noted a linear correlation between the frequencies of the two primary OH and OD stretching peaks of water and the dielectric constants of conventional solvents, and they proposed the frequencies of dilute water could provide a convenient means of assessing ionic liquid polarities. Subsequent work by multiple groups − expanded the number of ionic liquids in which dilute water has been studied and provided additional confirmation of the assignment of the main peaks to A – ···HOH···A – water. Curiously, none of these later contributions considered the possibility of singly bound water being present in ionic liquids, even though the spectra reported often suggest the presence of more structures than can be accounted for by a single type of water solvate.…”

“…There have already been numerous studies of the vibrational spectra of ionic liquid–water mixtures, including some focused on water in the dilute regime. − The seminal study was reported in 2001 by Cammarata et al, who measured the OH stretching spectra of water in ionic liquids comprised of the 1-butyl-3-methylimidazolium cation (Im 41 + ) combined with eight anions of varying basicity. They monitored spectra after depositing a dry ionic liquid onto the surface of a diamond-ATR cell and observed spectral changes as water was absorbed from the atmosphere.…”

In Situ X-ray Photoelectron Spectroscopic and Electrochemical Studies of the Bromide Anions Dissolved in 1-Ethyl-3-Methyl Imidazolium Tetrafluoroborate

“…This second, very wide negative current maximum is likely caused by the very slow electroreduction of the residual water at the carbon electrode. The electrochemical reduction of the residual water at the glassy carbon electrode was reported by Cheek et al [ 48 ].…”