MEASUREMENT OF REDOX REACTION VOLUMES FOR IRON(III II) COMPLEXES USING HIGH-PRESSURE CYCLIC STAIRCASE VOLTAMMETRY - HALF-CELL CONTRIBUTIONSTO REDOX REACTION VOLUMES/

Redox reaction volumes for a series of iron complexes of the type Fe(L -L)(3-0-5),(CN)(x), where L-L is either 1, 10-phenanthroline or 2,2'-b ipyridine and x = 0, 2, 4, or 6, have been measured using high-pressur e cyclic staircase voltammetry. For these Fe(III/II) systems, where th ere are no significant bond length changes associated with the change in oxidation state, changes in the intrinsic volume for the complex io ns are minimal and changes in the reaction volume therefore primarily reflect electrostrictive changes associated with the Fe(III/II) reduct ion. Estimates of the volume changes for individual Fe(III/II) half-ce ll reductions require an assignment of the molar volume change associa ted with the reference half-cell. From the charge dependence of the ce ll reaction volume for a series of Fe(III/II) reactions using a consta nt Ag/AgNO3 reference half-cell, we have estimated the reference half- cell volume change for the oxidation of Ag(s) to Ag+(aq) at 298 K and an ionic strength of 1.0 M to be -11.9 +/- 0.5 cm(3) mol(-1). The assi gnment is independent of any previously determined molar volumes for t he reactants. Addition of supporting electrolyte, necessary for the el ectrochemical measurement and normal for most kinetic and mechanistic studies, has some effect on the volume changes.