A New Salt Bridge Based on the Hydrophobic Room-Temperature Molten Salt

Abstract

A conceptually new salt bridge based on a hydrophobic room-temperature molten salt (ionic liquid) has been demonstrated to be a promising alternative to traditional salt bridges based on a concentrated aqueous KCl phase. 1-Methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide (C 8 mimC 1 C 1 N), which is immiscible with water and forms a liquid-liquid two-phase system, exhibits a stable phase-boundary potential when sandwiched by two aqueous electrolyte solutions. The phase-boundary potential between the interface is determined by the partition of C 8 mim þ and C 1 C 1 N À ions in the aqueous phase (W) and is little affected by the type of electrolytes and their concentrations in W, provided that ions dissolved in W are hydrophilic enough. The gelation of the molten salt phase does not affect the phase-boundary potential. The new salt bridge is free from most of the problems inherent to KCl-based salt bridges.

Ionic Liquid Salt Bridge in Dilute Aqueous Solutions

“…2 The configuration of a glass cell for cell (1) was also the same as what we reported previously. 2 The temperature of the cell was maintained at 25 ± 0.5˚C by circulating water through the jacket part of the glass cell.…”

“…2 The configuration of a glass cell for cell (1) was also the same as what we reported previously. 2 The temperature of the cell was maintained at 25 ± 0.5˚C by circulating water through the jacket part of the glass cell. 2 For experiments with gelled RTIL bridges, a RTIL membrane of 0.5 mm thickness was sandwiched by the upper and lower parts of a cylindrical cell containing aqueous solutions and Ag/AgCl electrodes using a clamp.…”

“…We recently proposed a new salt bridge composed of a roomtemperature ionic liquid (RTIL), 1,2 i.e., a room-temperature molten salt. When a RTIL is moderately hydrophobic, so that the solubility of the RTIL in an aqueous solution (W) is on the order of a few mmol dm -3 or submillimolar, the phase-boundary potential across the interface between the RTIL and W is determined by the partition of the cations and anions constituting the RTIL.…”

“…1 The working principle of the RTIL salt bridge is the distribution potential across the RTIL | W interface as the result of a dissolution of RTIL-constituent ions in W. Using 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim + ][C1C1N -]), we showed that the phase-boundary potential across the RTIL | W interface stays constant for a change in the concentration of HCl and LiCl between 1 mmol dm -3 and 0.2 mol dm -3 , and even higher in the cases of NaCl and KCl. 2 One immediate and important field of application of such new RTIL-salt bridges is potentiometry, particularly, pH measurements. It has been known for a long time that potentiometric pH measurements of low ionic strength solutions are problematic, mainly because of ill-behaved KCl-type salt bridges.…”

“…When the concentration of hydrophilic ions in W is higher than the solubility of the RTIL, the diffusion potential due to the imbalance of the mobility in W of the RTIL-constituent cation and anion is negligible. 2 This is because the transference number of the majority ions prevails and the transference number of the minority ions is insignificant. In other words, the contributions of the mobility and the concentration in W of the RTIL-constituting ions are swamped by those of hydrophilic ions.…”