Platinum Metals Rev., 1963, 7, (3), 93
Polarisation of Platinum Electrodes
Impedance in Chlorine-Chloride Systems
The reactions at platinum electrodes in aqueous solutions of halides when an alternating potential is imposed are complex and have been a continuing source of interest for many years to J. Llopis and his collaborators at the Instituto de Química Física “Rocasolano” in Madrid.
In their most recent paper (Electrochemica Acta, 1963, 8, 163-174), Llopis and M. Vazquez report the results of a study of the changes in electrode impedance in aqueous solutions prepared from mixtures of 0.5 M perchloric acid saturated with chlorine, 0.5 M perchloric acid, and hydrochloric acid. The working electrodes were platinum wires carefully arranged in the axis of a cylindrical ancillary electrode of platinised platinum to ensure regular distribution of the current flow.
Measurements of impedance were made of a special hermetically-closed cell having a very small gas volume at frequencies ranging from 20 to 20,000 cycles/sec and the results were analysed in an attempt to determine which of a series of equivalent resistance-capacity networks (the components of which are shown to correspond to different types of electrode reactions) best approximated to the observed results.
One clear conclusion was that in the system Cl2/Cl– surface oxidation of the platinum electrode hinders establishment of the equilibrium potential. It is still not clear, however, whether this effect is due simply to loss of active adsorption sites through the blocking of part of the surface by oxidation, or whether the values of the free energy barriers which control the electron-transfer process at the electrode interface are changed.