Acoustical Techniques for the Study of Electrochemical Processes

Abstract

At relatively low frequencies, sound waves produce an alternating component in the potential of a reversible electrode in accord with the thermodynamic dependence of the potential on pressure and temperature. With increasing frequency, the ac response of the electrode decreases until at sufficiently high frequencies only the alternating components associated with the variation of the electrode-solution interface capacity and the ionic vibration potentials remain. The departure of the response from thermodynamic expectations occurs because the electrode processes responsible for the establishment of the potential are too slow for instantaneous equilibrium to be maintained in the sound field. The situation is analogous to the relaxation effects associated with the excess sound absorption in polyvalent electrolytes such as magnesium sulfate. From the frequency dependence of the alternating potentials at various electrolyte concentrations and temperature, information can be obtained as to the orders, rate constants, and energy barriers for various electrode processes, e.g., electron transfer from a metallic electrode to an ion. Since most electrochemical systems involve several consecutive steps, a spectrum of relaxation frequencies is anticipated. The measurement of these acoustically produced alternating potentials offers considerable promise for the study of electrode processes which are too rapid for conventional electrochemical techniques. (This paper is based on research sponsored by the Office of Naval Research.)