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 voltarnmetric 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][Ntf(2)], [emim][Ntf(2)], and [bmim][PF6] (bmpyr = 1-butyl-1-methylpyrrolidinium, emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-methylimidazolium, Ntf(2) = 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 nonidealities of the kind reported for ferrocene not being detected. (c) 2007 Elsevier B.V. All rights reserved.