The use of the reference electrode equipped with an ionic liquid salt bridge in electrochemistry of ionic liquids: A convenient way to align the formal potentials of redox reactions in ionic liquids based on the standard hydrogen electrode scale

Abstract

A hydrophobic ionic liquid composed of a cationic and anionic species having similar magnitudes of hydrophobicity and mobilities can work as a salt bridge separating two electrolyte solutions, that is, the sample solution and the inner solution of the reference electrode. A few examples of superiority of this ionic liquid salt bridge (ILSB) over the traditional salt bridges made of a concentrated aqueous KCl solution have been demonstrated. The present study is a further extension of the use of ILSB to voltammtery of the redox reactions of ferrocene/ferrocenium and cobaltocene/cobaltocenium couples in an ionic liquid, tributyl(2-methoxyethyl) phosphonium bis(pentafluoroethanesulfonyl) amide ([TBMOEP][C2C2N]), which is also used as the ILSB. The obtained mid-point potentials (Ems) are converted straightforwardly to those referred to the standard hydrogen electrode (SHE). A comparison of Em (SHE) values of the two redox couples in [TBMOEP][C2C2N] with the corresponding values in molecular solvents suggests that the environment given by [TBMOEP][C2C2N] to the two redox reactions is macroscopically similar to that of methanol.