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-].
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