Homogeneous versus heterogeneous electron-transfer processes in solution: A 'fifty percent rule' for volumes of activation

Ac voltammetry at pressures up to 200 MPa shows that volumes of activation for heterogeneous electron transfer in three aqueous couples {[Fe(CN)(6)](3-/4-), [Co(sep)](3+/2+) and [Co(en)(3)](3+/2+)} are numerically about 50% of those for homogeneous (bimolecular) electron transfer in these same couples, with the same algebraic sign, as predicted by an extension of Marcus theory.