Kinetics of the Ferrous/Ferric Electrode Reaction in the Absence of Chloride Catalysis

Abstract

The kinetics of the ferrous/ferric redox electrode reaction has been investigated by many workers as a simple, uncomplicated charge transfer reaction which seems ideal for testing experimental techniques and charge transfer theories. However, it has only recently been understood that very small traces of chloride can have a considerable effect on the reaction rate. The relation between the chloride content of the solution and the rate constant of the ferrous/ferric reaction on a gold electrode in perchloric acid solutions is confirmed in this work. The chloride effect free apparent standard rate constant is found to be , which is two to three orders of magnitude smaller than the rate constants normally reported for this reaction if the chloride content of the solution is not scrupulously controlled. Measurements were carried out by using two different in situ methods for cleaning the working electrode surface rather than employing extensive solution purification. In the first method the measuring electrode was potentiostated at sufficiently negative potentials to desorb the chloride from the surface followed by a potential step to the equilibrium potential and a pulse measurement of the kinetics. In the second method chloride ions were removed from the surface before and during the kinetic measurement by continuous oxidation of chromous ions added in small concentration to the test solution. Good agreement was found among the rate constants determined by these methods and a reported rate constant determined in ultraclean solution.