ELECTROCHEMICAL MEASUREMENTS AT HIGH-PRESSURE - SOLVATION AND THERMODYNAMICS OF ELECTRON-TRANSFER REACTIONS

A general methodology for studying effects of pressure on electrochemical systems has been developed. An electrochemical cell, including an Ag/AgCl reference electrode that allows measurements at hydrostatic pressures as high as 10 kbar, has been constructed. With this experimental setup the standard potentials of tris(2,2'-bipyridine)cobalt3+/2+, tris(2,2'-bipyridine)iron3+/2+, and alpha-(hydroxyethyl)ferrocene+/0 in aqueous media have been investigated vs pressure. The potential of the reference electrode has been estimated to be only slightly pressure dependent. Pressure-induced potential shifts in the negative direction are observed for the bipyridyl complexes. From 0 to 10 kbar, potential changes of -200 and -100 mV are observed for the cobalt and the iron compounds, respectively. The differences in these values are explained in terms of the effect of the electronic structure of the metal center on the overall size of the complex molecules and on their ability to interact with the solvent. In contrast, alpha-(hydroxyethyl)FeCp2+/0 exhibits a change in potential of only +30 mV over the full pressure range, indicating weak solvation of both the oxidized and the reduced forms. From the pressure dependences of the standard potentials of the couples, the corresponding volumes of reaction are extracted and reported. The study emphasizes the importance of solute-solvent interactions in the thermodynamics of redox processes.