Technical Note pubs.acs.org/ac
Ionic Liquid Salt Bridge with Low Solubility of Water and Stable Liquid Junction Potential Based on a Mixture of a PotentialDetermining Salt and a Highly Hydrophobic Ionic Liquid Limin Zhang, Takahiro Miyazawa, Yuki Kitazumi, and Takashi Kakiuchi* Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan S Supporting Information *
ABSTRACT: A new type of ionic liquid salt bridge(ILSB) based on a mixture of pentyltripropylammonium bis(pentafluoroethanesulfonyl)amide, [N3335+][C2C2N−], and heptadecafluorodecyltrioctylphosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [TOPH+][TFPB−], shows a W stable phase-boundary potential (ΔIL 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 W excursion in 1 h of ±0.3 mV. The reproducibility of ΔIL 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.
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t has been demonstrated1−7 that ionic liquid salt bridge(ILSB) has distinct advantages over KCl-based salt bridges that have been used over 100 years as an indispensable component in an electrochemical cell for potentiometry and other electroanalytical methodologies.8−11 ILSBs reported so far consist of a moderately hydrophobic ionic liquid (IL) whose solubility in water is on the order of mM (M = mol dm−3) to a tenth mM, such as 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide [C8mim+][C1C1N−]1 and tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide([TBMOEP+][C2C2N−]).4 The new ILSB we herein report consists of a mixture of a very hydrophobic IL and a moderately hydrophobic IL. The latter IL functions as the potential-determining salt (PDS) that dictates the phase-boundary potential between the IL and a sample W solution (ΔIL ϕ) by partitioning of the IL-constituent ions between the ILSB and a sample solution, while the former works as an ion-conductive matrix medium without disturbing W ΔIL ϕ determined by the PDS because of much lower solubility in water than that of the PDS. The new ILSB has distinct advantages over previously reported single component ILSBs. First, the two salts form a eutectic mixture with a lower melting point.12−14 Second, the PDS is not necessarily a room temperature ionic liquid, as long as it forms a eutectic mixture with the matrix IL. This is nontrivial because a majority of moderately hydrophobic ILs have melting points higher than room-temperature or above 0 °C, which in fact makes these salts unsuitable to ILSBs and narrows our choice of a salt for single-component ILSBs. Third, the highly hydrophobic nature of the matrix IL suppresses the partitioning of water into ILSB and also of interfering ions into the ILSB, both of which are detrimental to the stability, © 2012 American Chemical Society
reproducibility, and durability of ILSBs. We will show that a mixture of pentyltripropylammonium bis(pentafluoroethanesulfonyl)amide, [N3335+][C2C2N−], as a PDS and heptadecafluorodecyltrioctylphosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [TOPH+][TFPB−], as W ϕ. a matrix salt gives a very stable ΔIL
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EXPERIMENTAL SECTION Reagents. Tripropylpentylammonium bromide (N3335Br) was synthesized from tripropylamine and 1-bromopentane.15 N3335Br was then converted to the chloride salt, N3335Cl, by use of an ion-exchange resin (IRA900JCl, Organo Co.). [N3335+][C2C2N−] was prepared from N3335Cl and hydrogen bis(pentafluoroethanesulfonyl)imide(HC2C2N) (Central Glass Co. Ltd.).16 Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate(NaTFPB) was synthesized with a procedure similar to that previously reported by Nishida et al.17 Trioctylphosphineheptadecafluorodecyl iodide (TOPHI) was synthesized from trioctylphosphine (Tokyo Chem. Ind. Co.) and heptadecafluorodecyl iodide.18 [TOPH+][TFPB−] was prepared by mixing a methanol solution of NaTFPB with an equimolar amount of trioctylphosphineheptadecafluorodecyl chloride (TOPHCl) obtained through ion exchange from TOPHI.16 Other chemicals were used without further purification. Mixtures of [TOPH+][TFPB−] and [N3335+][C2C2N−] were prepared by dissolving weighed amounts of these salts in acetone. The composition of the solution was typically 1:6:20 Received: December 22, 2011 Accepted: March 5, 2012 Published: March 5, 2012 3461
dx.doi.org/10.1021/ac203425u | Anal. Chem. 2012, 84, 3461−3464
Analytical Chemistry
Technical Note
by weight for [N3335+][C2C2N−], [TOPH+][TFPB−], and acetone, respectively. After stirring the mixture, the solution was dried at 80 °C under vacuum overnight to remove acetone. The removal of acetone in the mixture was confirmed by 1H NMR. The mixture of [N3335+][C2C2N−] and [TOPH+][TFPB−] was a viscous liquid at room temperature and was in a liquid state for 1 month in a refrigerator. Methods. Electrical conductivity (k), melting point (Tm), and the solubility of water in the saturated IL (SW/IL) were measured with a conductometer (CM-117, Kyoto Electronics), a micro melting point apparatus (MP-S3, Yanako), and a Karl Fischer moisture meter (CA-21, Dia Chemical), respectively. The solubility of [N3335+][C2C2N−], [TOPH+][TFPB−], and [TBMOEP+][C2C2N−] in water (SIL/W) were estimated by visual inspection of aqueous solutions of ionic liquids at different concentrations. A small amount of an IL, about 2 mg each time, was added into 0.5 dm3 water in a glass flask under stirring and the IL droplet was observed. When the droplet became invisible, another IL droplet was added to the solution. This procedure was repeated until the IL cannot be further dissolved. Then, the solution was kept stirring for another 5 days to ensure no further dissolution of the IL. Cyclic voltammograms recorded at the IL|W interface in the absence and presence of [N3335+][C2C2N−] are given in the Supporting Information. The composition of the electrochemical cell used for potentiometric measurements is represented as
Table 1. Physicochemical Properties (Melting Point (Tm), Conductivity (k), Solubility of the ILs in Water (SIL/W), and Solubility of Water in the IL (SW/IL) for IL Saturated with Water
a
IL
Tm (°C)
[N3335+][C2C2N−] [TOPH+][TFPB−] mixture [C8mim+][C1C1N−] [TBMOEP+][C2C2N−]
45 33
