Prospects for a widely applicable reference potential scale in ionic liquids based on ideal reversible reduction of the cobaltocenium cation

Oxidation of ferrocene, Fe(cp)2 or reduction of the cobaltocenium cation, [Co(cp)2]+ represent reversible processes that are widely used to provide a voltammetric potential reference scale. However, the [Fe(cp)2]0/+ process has been reported to exhibit complexities which may restrict its usefulness for this purpose in ionic liquids. In this study, the reduction of [Co(cp)2]+ in the ionic liquids, [bmpyr][Ntf2], [emim][Ntf2], and [bmim][PF6] (bmpyr = 1-butyl-1-methylpyrrolidinium, emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-methylimidazolium, Ntf2 = bis(trifluoromethanesulfonyl)amide) is reported at macro, micro, and rotating disk electrodes. Reversible behaviour, after allowance for ohmic drop, and linear current–concentration relationships are attained over wide concentration ranges for all electrode configurations. Results support the use of the [Co(cp)2]+/0 process for reference potential purposes, with non-idealities of the kind reported for ferrocene not being detected.