Ionic Mass Transfer and Concentration Polarization at Rotating Electrodes

Abstract

Rates of ionic mass transfer at nickel electrodes rotating about their axes in the center of stationary electrodes were studied using the ferri‐ferrocyanide couple in alkaline solutions. A general mass transfer correlation was found to apply equally well to dissolution rates of rotating solids and to rates of ionic mass transfer at rotating electrodes. This correlation takes into account physical properties of the system as well as geometric and hydrodynamic factors. The correlation allows prediction of limiting currents and concentration polarization at rotating electrodes under a wide range of conditions. The nature of polarization involved in reduction of and oxidation of was also investigated. Polarization was found to depend strongly on the presence of electrode poisons. With freshly prepared solutions, under exclusion of light, and with cathodically treated nickel electrodes, relatively small chemical polarizations were determined. For rotational speeds not exceeding Reynolds number 11,000, chemical polarization was found to be negligible in comparison with concentration polarization. Under such conditions, the ferro‐ferricyanide couple can be conveniently used to obtain mass transfer rates for various hydrodynamic conditions, or conversely, to verify the validity of mass transfer equations by a comparison of experimental and calculated values of limiting currents and concentration polarization. The rotating electrode model was found to be most suitable for studying the nature of electrolytic polarization phenomena because of uniformity of the current distribution and the hydrodynamic diffusion layer at the electrode surface.