Effect of ionic equilibria on redox potentials in supercritical difluoromethane

The equilibrium constants for the dissociation of ferrocene carboxylic acid and the tetrabutyl ammonium salt have been determined in supercritical (sc) difluoromethane (HFC32). It is shown that the redox potential of ferrocene is largely unaffected by changes in pressure whereas the redox properties of ferrocence derivatives vary significantly. The changes in the oxidation potential are related to the dissociation constants of the electroactive species using classical equilibrium thermodynamics. Deviations from Nernstian redox behaviour are interpreted in terms of a decrease in the activity coefficient of the ferrocene derivatives at high pressure. The results also suggest that ionic atmosphere effects are insignificant in sc fluids like difluoromethane at lower pressures due to low ionic strengths and high ionic mobilities.