Polyelectrodes: Evidence for and against the Wagner and Traud additivity hypothesis

Wagner and Traud postulated that the behaviour of a mixture of redox couples at an electrode can be quantitatively described by the addition of their individual current-potential curves. It is argued that the most satisfactory tests of this hypothesis are furnished by systems in which the metal electrode is not itself a chemical component of one of the couples, as with the couples Fe(CN) 6 3− /Fe(CN) 6 4− and I 3 − /I − at platinum. Electrochemical and kinetic experiments have therefore been carried out with this system. Agreement with the additivity hypothesis was found to be good for pre-oxidised platinum but poor for reduced platinum. The explanation lies in the fact that a reduced platinum surface preferentially adsorbs iodide ions, and this hinders the electron transfer of the other couple. Attention is drawn to other systems where the additivity hypothesis is likely to break down.