Use of Highly Hydrophobic Ionic Liquids for Ion-selective Electrodes of the Liquid Membrane Type

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The phase-boundary potential at the interface between an aqueous KCl solution (W) and a hydrophobic room-temperature ionic liquid (RTIL), trioctylmethylammonium bis(nonafluorobutylsulfonyl)imide ([TOMA⁺][C₄C₄N^-]), containing dicyclohexano-18-crown-6 (DCH18C6), shows the nernstian response to K⁺ in W within a polarized potential window of 500 mV between [TOMA⁺][C₄C₄N^-] and W, demonstrating that hydrophobic RTILs can be used as a nonvolatile ionic medium for liquid-membrane ion-selective electrodes. The complex formation constant of K⁺ with DCH18C6 in [TOMA⁺][C₄C₄N^-] is estimated to be on the order of 10⁹ from the upper detection limit using a partition equilibrium model in the presence of a neutral ionophore. The response time of the phase-boundary potential is ∼20 min. Gelled [TOMA⁺][C₄C₄N^-] also shows the nernstian response to K⁺, although the upper limit is lower probably due to a change in the solvent properties from the non-gelled [TOMA⁺][C₄C₄N^-]. The response time of the gelled [TOMA⁺][C₄C₄N^-] is ∼5 min, shorter than that of the non-gelled [TOMA⁺][C₄C₄N^-].