Kinetic Investigation of the Fe2+/Fe3+ Reaction in Frozen and Liquid HClO4·5.5H2O

The Fe2+/Fe3+ redox reaction on platinum is investigated in liquid and frozen HClO4 · 5.5 H2O over a temperature range from 130 — 300 K. The exchange current density follows an Arrhenius law in the liquid and in the solid state. Both the energy of activation and the preexponential factor are higher in the solid state; the exchange current density shows a discontinuous rise as one approaches the freezing point from higher temperatures. In the liquid state, mass transport to the electrode occurs through diffusion of the ions; in the solid state mass transfer is much slower is probably due to electron hopping rather than ion transport.