Organometallic potential reference systems

To compare redox potentials of aqueous and nonaqueous systems, a variety of internal references were investigated. In 1984, Gritzner and Kuta recommended two systems for nonaqueous electrolytes that are also accepted by lUPAC [244]. The solvent-independent organometallic redox couples are fer-rocene/ferrocenium (Fc/Fc+) (E = 0.158 V vs saturated calomel electrode (SCE)) and bis(biphenyl)chromium(0)/(l) (BCr/BCr+) (E = —0.82 V vs SGE) [245]. Very stable electrode redox potentials Ei/i vs Ag/Ag+-cryptand electrode of 0.478 V for Fc/Fc+ and —0.616 V for BCr/BCr+ in EMIm tetrafluoroborate were measured [246]. 

The establishment of a solvent-independent reference electrode for the comparison of redox potentials in nonaqueous systems has a long story. It began with the concept of RblRb+ or Rb(Hg)IRb, followed by the proposal of using organometallic redox couples [2], In order to limit the number of redox systems used and then make the comparison easier, lUPAC recommended that the systems ferrocenelferrocenium, Fc° , and bis(biphenyl)chromium(0)lbis(biphenyl)chromium(l), BCi , need to be used as internal reference redox systems in nonaqueous media [2], These two complexes were selected arbitrarily from several published redox systems [2],... 

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