Metallocene Voltammetric Reference Related to a Normal Hydrogen Electrode

The potentials of electrochemical processes in ideal aqueous media are related to the potential of a normal hydrogen electrode (NHE). However, in non-ideal media, the potentials of a metallocene redox couple are used as a reference. Such measurements with free metallocene in solution are complicated by adsorption and mass transport phenomena and solvation effects. Herein, a platinum electrode with an anchored ferrocene moiety (Pt,Fc) was used for cyclic voltammetric measurements of the potential of ferrocene/ferrocenium (Fc/Fc⁺) redox transformation in not only non-aqueous but, for the first time, aqueous solutions as well. This enabled us to eliminate the aforementioned problems associated with the application of free metallocene molecules in solution and, thus, to relate the midpoint potential (<i>E</i>_pm) of the Fc/Fc⁺ redox couple to a NHE. After elimination of the liquid junction potential in an aqueous 0.1 M KCl solution at 25 °C, the average intraday <i>E</i>_pm value obtained with freshly prepared Pt,Fc electrodes was found to be 0.312 ± 0.008 V versus the secondary Ag|AgCl electrode. The Pt,Fc electrode can be applied for the standardization of electrochemical measurements and investigation of solvation phenomena at interfaces in non-ideal media.