Microelectrode studies in the absence of deliberately added supporting electrolyte: solvent dependence for a neutral and singly charged species

Steady state voltammetric studies utilizing a 5.6 μm radius platinum inlaid disk microelectrode have been carried out in a series of solvents containing ferrocene (FeCp₂) and/or cobaltocenium hexafluorophosphate ([CoCp₂]⁺ [PF₆]^-) in both the presence and absence of intentionally added supporting electrolyte. The very low dependence of the ferrocene oxidation process (FeCp₂ ⇋ [FeCp₂]⁺ + e^-) on the concentration of electrolyte enables this compound to be used as a convenient internal reference standard for charged compounds. For reduction of the charged [CoCp₂]⁺ ion, ([CoCp₂]⁺ + e^- ⇋ CoCp₂), changes in the migration current as a function of solvent in the absence of electrolyte can be related to the dependence of ionic mobility on solvent viscosity. The ohmic IR drop effects observed in solvents with lower dielectrics are related to the degree of ionic association. Possible reasons for the lack of a simple relation for either migration or IR drop trends with solvent properties are discussed. A steady state analytical solution for mass transport for the charged system is compared to the experimental data. In practical terms, it has been shown that in the absence of deliberately added electrolyte, steady state voltammograms can be obtained for both neutral and singly charged species in solvents as non-polar as tetrahydrofuran with little distortion, whereas with toluene, which has only a slightly lower dielectric constant, the IR drop distortion is extremely large. As expected, the IR distortion for an ionic sample is less than that for a neutral sample. However, whether the IR drop is important or not in non-polar solvents is very dependent on the exact nature of the experimental conditions.