Correlation of Limiting Currents; Electrodeposition of Copper

Limiting currents have been measured using microelectrolytic cells of the type: Cu/CuSO 4 or CuSO4- ~ H2804 (or Na2SO4)/Cu. Good agreement with existing theory of the correlation of l imiting current densities under free convective conditions was obtained for relat ively thick (5 ram) electrodes confirming that convection does occur in microcells. For thin electrodes a cor-relating factor, which is assumed to be related to "edge effect, " is evaluated. Free or natural convection in an electrochemical cell results from density changes caused by concentration changes in the vicinity of the electrode (1). A spon-taneous laminar flow of fluid sets in due to the density difference between the solution at the electrode/solu-tion interface and the bulk. From a consideration of mass transfer under f ee convection conditions Tobias and co-workers ( la) were able to explain the depend-ence of l imiting currents on electrode dimensions, bulk concentration, etc., in the deposition of copper at vertical cathodes from unstirred copper sulfate-sul-furic acid solutions. Such a correlation, however, has not yet been attempted in the type of microcell as-sembly recently used in optical interferometric studies of electrolysis (2). Any indication of free convection in such a microset-up wil l prove useful in correctly in-terpreting the interference patterns. In view of this, examination of the current-voltage r lationship in the electrolysis of copper sulfate solutions in microcells with fixed copper electrodes of known dimensions has been undertaken. Theory On the basis of the combined effects of diffusion, electrical migration and free convection the average l imiting current density, I was quantitatively expressed as follows ( la) they can all be combined in one constant K resulting in kxoS/41Co 5/