Ionic Liquid Salt Bridge with Low Solubility of Water and Stable Liquid Junction Potential Based on a Mixture of a Potential-Determining Salt and a Highly Hydrophobic Ionic Liquid

Abstract

A new type of ionic liquid salt bridge (ILSB) based on a mixture of pentyltripropylammonium bis(pentafluoroethanesulfonyl)amide, [N(3335)(+)][C(2)C(2)N(-)], and heptadecafluorodecyltrioctylphosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [TOPH(+)][TFPB(-)], shows a stable phase-boundary potential (Δ(IL)(W)φ) between the ILSB and an aqueous solution of MCl (M = H(+), Li(+), Na(+), and K(+)) over the concentration range from 0.05 mM to 0.5 M with an averaged excursion in 1 h of ±0.3 mV. The reproducibility of Δ(IL)(W)φ is ±0.6 mV on average (95% confidence interval) in KCl solutions in this concentration range. The mixing of the two different types of salts not only increases the stability of the phase-boundary potential but provides us with more freedom in selecting potential-determining salts to design and customize ILSBs for different purposes.

Solid-Contact Ion-Selective and Reference Electrodes Covalently Attached to Functionalized Poly(ethylene terephthalate)

“…Electrode potentials measured in solutions saturated with ionic liquid varied by a maximum of ∼2 mV between solutions containing only ionic liquid and solutions with added KCl (log a K/Cl = −4.22 to −0.86, see Figure S21). This observation is consistent with reports of other ionic-liquid-based reference electrodes, where the reference potential provided by such electrodes is determined by an ionic liquid distributing across the interface of the sample and the reference membrane and not by the ionic composition of the sample. − Potential stabilities over 1 day were measured to be 180 ± 280 μV/h (see Figure S22). The electrical electrode resistance was measured to be 640 ± 110 kΩ (95% confidence interval), which is of the same magnitude as for nanochannel-type reference electrodes based on the same ionic liquid .…”

“…In practical applications, large or continuously renewed samples are much more common, preventing the ionic liquid from accumulating. Therefore, as common in prior work with reference electrodes comprising ionic-liquid-doped reference membranes, 54 all further tests to assess the stability of solid-contact reference electrodes were performed in solutions saturated with the ionic liquid, avoiding the effect of ionic-liquid accumulation in the sample over time. This way, potential drifts caused by changes in the transmembrane potential or the inner reference electrode are readily recognizable.…”

Single-ion activity: a nonthermodynamically measurable quantity

“…In our recent studies with ILSBs, we have almost exclusively been using tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide (TBMOEP + C 2 C 2 N − ). Although this was among the best we have tried [10,15,28,32], and was recently confirmed by Lindner et al [20], there must be many better alternatives in view of the tuned mobilities of the IL-constituent cation and anion, the solubility in water, the environmentally friendliness, etc. For bridging an aqueous solution and an organic phase, Krossing's group proposed a new IL consisting of cation and anion having nearly equal mobilities and successfully used it to minimize the liquid junction potential across the aqueous-organic boundary [6,26].…”

Recent progress in the development of improved reference electrodes for electrochemistry

“…56 Subsequently, Kakiuchi and co-workers investigated the properties of a wide range of ionic liquid-based reference electrodes against the silver halide reference electrode, demonstrating that they provide stable and concentration independent half-cell potentials. For instance, the phase-boundary potential between an ionic liquid mixture of heptadecafluorodecyltrioctylphosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and pentyltripropylammonium bis(pentafluoroethanesulfonyl)amide when used to measure in 0.5 M KCl solution deviated by 0.3 mV at most from the average over 1 h. [57][58][59] While this variability of the emf over time seems low for many other applications, it is comparatively high in view of long term measurements without recalibration. It appears likely that these reference electrodes could perform much better under more optimized conditions, but that was not the purpose of much of the early work with such reference electrodes.…”