Ionic Vibration Potentials Research Articles

Acoustical Techniques for the Study of Electrochemical Processes

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.)

Ultrasonic Waves and Electrochemistry. II. Colloidal and Ionic Vibration Potentials

When ultrasonic waves are introduced into an ionic solution or a suspension of charged colloidal particles, alternating potential differences are generated between points separated by a finite distance in the direction of propagation (other than an integral multiple of the wavelength). Pulse-modulated ultrasonic waves have been used to study these effects at a carrier frequency of 200 kc/sec. The colloidal vibration potentials have been measured in a number of colloidal silica suspensions of various concentrations, particle sizes, and conductivities. The experimental results are in general agreement with the mathematical treatment of Enderby. Measurements of ionic vibration potentials in potassium chloride solutions indicate a value of 6μv per cm/sec for a 10−3-molar solution. This value corresponds to a difference of 80 in the apparent gram-ionic-masses of the hydrated potassium and chloride ions.

The Measurement of Ionic Vibration Potentials (Debye Effect) with Pulse-Modulated Ultrasonic Waves

If ultrasonic waves are propagated through an electrolytic solution, alternating potentials are produced within the solution as a result of the dynamical reaction of the ions to the material waves. Apparatus has been developed for the measurement of these ionic vibration potentials with pulse-modulated ultrasonic waves at frequencies from 200 to 1000 kc/sec. Preliminary measurements indicate that the amplitude of the alternating potentials is a function of the nature of the electrolyte but is essentially independent of the concentration for dilute solutions as is predicted by theory. A value of 5 microvolts per unit velocity amplitude has been found for 0.005 molar potassium chloride solution at 200 kc/sec. These results are in general agreement with the semiquantitative, relative measurements obtained with a standing wave technique during the first experimental confirmation of the effect. Ionic vibration potentials, particularly their frequency dependence, should prove especially interesting for electrolytes such as magnesium sulfate for which structural relaxation effects have been recently postulated in order to account for the abnormal absorption of sea water.