Pronounced Pressure Effects on Reversible Electrode Reactions in Supercritical Water

摘要

An alumina electrochemical cell containing an ultra- microelectrode was used to make precise voltammetric measurements in supercritical water. The effect of pressure on the redox potential of the I2/I(-) couple was measured from 230 to 300 bar at 385 degrees C. The partial molar volume change for the reduction of I2 to I(-), Delta nu max, is pronounced, mostly because of the interplay between strong electrostatic forces and the large isothermal compressibility of the fluid. A modified Born model together with a perturbed hard sphere equation of state predicts the data accurately above 265 bar with no adjustable parameters. At lower pressures ion paring is thought to reduce the magnitude of Delta nu max. These large pressure effects on solvation free energies may be used to manipulate reaction equilibria and reaction mechanisms in supercritical water. Keywords: Electrochemistry; Reprints. (jhd)