Ferrocene, Fc, and cobaltocenium hexafluorophosphate, CcPF6, have been recommended for use as internal reference redox couples in room-temperature ionic liquids (RTILs), as well as in more conventional aprotic solvents. In this study, the electrochemical behavior of Fc and CcPF6 is reported in eight commonly used RTILs; [C2mim][NTf2], [C4mim][NTf2], [C4mim][BF4], [C4mim][PF6], [C4mim][OTf], [C4mim][NO3], [C4mpyrr][NTf2], and [P14,6,6,6][FAP], where [C n mim]+ = 1-butyl-3-methylimidazolium, [NTf2]- = bis(trifluoromethylsulfonyl)imide, [BF4]- = tetrafluoroborate, [PF6]- = hexafluorophosphate, [OTf]- = trifluoromethylsulfonate, [NO3]- = nitrate, [C4mpyrr]+ = N-butyl-N-methylpyrrolidinium, [P14,6,6,6 ]+ = tris(n-hexyl)-tetradecylphosphonium and [FAP]- = trifluorotris(pentafluoroethyl)phosphate, over a range of concentrations and temperatures. Solubilities and diffusion coefficients, D, of both the charged and neutral species were determined using double potential-step chronoamperometry, and CcPF6 (36.5−450.0 mM) was found to be much more soluble than Fc (27.5−101.8 mM). It was observed that classical Stokes−Einstein diffusional behavior applies for Fc and CcPF6 in all eight RTILs. Diffusion coefficients of Fc and CcPF6 were calculated at a range of temperatures, and activation energies calculated. It was also determined that D for Fc and CcPF6 does not change significantly with concentration. This supports the use of both Fc and CcPF6 to provide a well-characterized and model redox couple for use as a voltammetric internal potential reference in RTILs contrary to previous literature reports in the former case.

The recent literature is surveyed to explore the nature of voltammetry in room temperature ionic liquids. The extent of similarities with conventional electrochemical solvents is reported and some surprising differences are noted.

The voltammetry for the reduction of oxygen at a microdisk electrode is reported in six commonly used RTILs: [C(4)mim][NTf(2)], [C(4)mpyrr][NTf(2)], [C(4)dmim][NTf(2)], [C(4)mim][BF(4)], [C(4)mim][PF(6)], and [N(6,2,2,2)][NTf(2)], where [C(4)mim](+) is 1-butyl-3-methylimidazolium, [NTf(2)](-) is bis(trifluoromethanesulfonyl)imide, [C(4)mpyrr](+) is N-butyl-N-methylpyrrolidinium, [C(4)dmim](+) is 1-butyl-2,3-methylimidazolium, [BF(4)](-) is tetrafluoroborate, [PF(6)](-) is hexafluorophosphate, and [N(6,2,2,2)](+) is n-hexyltriethylammonium at varying scan rates (50-4000 mV s(-1)) and temperatures (293-318 K). Diffusion coefficients, D, of oxygen are deduced at each temperature from potential-step chronoamperometry, and diffusional activation energies are calculated. Oxygen solubilities are also reported as a function of temperature. In the six ionic liquids, the Stokes-Einstein relationship (D proportional, variant eta(-1)) was found to apply only very approximately for oxygen. This is considered in relationship to the behavior of other diverse solutes in RTILs.