Liquid Junction Potential Between Hydrophobic Ionic Liquid and Aqueous Electrolyte Solution

Abstract

The liquid junction potential (LJP) across the two phases is very important for the basic science and real application of electrochemistry. In the case of the reference electrode the KCl salt-bridges are widely used, because the diffusion potential can be neglected in some cases by the almost same mobility of potassium ion and chloride ion in aqueous solution. However, if the difference of the concentration of solute between the two phases is great the LJP becomes several ten mV. And in the KCl salt-bridge KCl solution flow from reference electrode to the sample solution is essentially inevitable. To overcome these shortcomings of KCl salt bridge Kakiuchi et al. introduced the ionic liquid salt bridge (ILSB) concept and found that LJP for ILSB is always constant and independent on the concentration of electrolyte in aqueous phase. The LJP for ILSB is distribution of cation an anion of IL to aqueous solution.[1] In the present study we have measured the electric conductivity of lithium bis(pentafluoroehtanesulfonyl)amide (LiC2C2N) and bis(pentafluoroethanesulfonyl)amide acid(HC2C2N) in aqueous solution. C2C2N anion is the ionic liquid component of ILSB and the cation is tributyl(2-methoxyethyl)phosphonium (TBMOEP). Form the conductivity measurements we have found that the molar conductivity at infinite dilution of C2C2N anion is 28.1 ± 0.7 S cm2 mol-1 and mobility is 2.91 × 10-4 cm2 V-1 s-1. Sakaida et al. reported the solubility of TBMOEP cation in aqueous solution is 2.64 x 10-4 cm2 V-1 s-1.[2] From these results we can estimate the diffusion potential across TBMOEPC2C2N | HCl solution using the Henderson equation. Even in the case of 0.5 mmol dm-3 the magnitude of the diffusion potential is 60 microvolt and the we can conclude that the diffusion potential is negligible in the TBMOEPC2C2N ILSB. We are now investigating the coupled Nernst-Plank and Poisson-Boltzmann equation to see the detail of the potential and the concentration profile. From the conductivity measurements we also found that HC2C2N is strong acid and C2C2N anion in aqueous phase is completely dissociated.