Conductivity and voltammetry in liquid and supercritical halogenated solvents

A previous study of the voltammetry of ferrocene in liquid and supercritical chlorodifluoromethane revealed electrochemically reversible behavior. However, shifts in the half-wave potential as a function of fluid conditions were observed which were tentatively attributed to ohmic distortion due to changes in fluid resistance. To more completely understand the voltammetry in this fluid, conductivity measurements have been made for a range of fluid conditions. Additionally, a second reference couple, cobaltocenium hexafluorophosphate, has been introduced, and the difference in half-wave potentials between the two redox couples has been examined as a function of fluid conditions. In the liquid, the difference in the half-wave potentials of the two couples corrected for ohmic distortion decreases as the fluid temperature increases (25-85 degrees C) at constant pressure (5.2 MPa). In the supercritical fluid at constant temperature (115 degrees C), the difference in the half-wave potentials corrected for ohmic distortion is constant at 1.276 +/- 0.005 V over a considerable range of fluid pressure (10-30 MPa). Ion aggregation in the supercritical fluid is indicated both by the conductivity measurements and by the rather large hydrodynamic radius of cobaltocenium computed from the voltammograms. Preliminary voltammetry in supercritical trifluoromethane is also presented.