Buoyancy Effect of Ionic Vacancy on the Change of the Partial Molar Volume in Ferricyanide-Ferrocyanide Redox Reaction under a Vertical Gravity Field

With a gravity electrode (GE) in a vertical gravity field, the buoyancy effect of ionic vacancy on the change of the partial molar volume in the redox reaction between ferricyanide (FERRI) and ferrocyanide (FERRO) ions was examined. The buoyancy force of ionic vacancy takes a positive or negative value, depending on whether the rate-determining step is the production or extinction of the vacancy. Though the upward convection over an upward electrode in the FERRO ion oxidation suggests the contribution of the positive buoyancy force arising from the vacancy production, the partial molar volume of the vacancy was not measured. On the other hand, for the downward convection under a downward electrode in the FERRI ion reduction, it was not completely but partly measured by the contribution of the negative buoyancy force from the vacancy extinction. Since the lifetime of the vacancy is decreased by the collision between ionic vacancies during the convection, the former result was ascribed to the shortened lifetime due to the increasing collision efficiency in the enhanced upward convection over an upward electrode, whereas the latter was thought to arise from the elongated lifetime due to the decreasing collision efficiency by the stagnation under the downward electrode. 1. Introduction In electrochemistry, the density change in the solution during an electrode reaction gives rise to a gravitational convection [1]. Constant currents observed for hanging electrodes often come from the steady-state mass transfer by the gravitational convection under a parallel gravity field. As a force similar to gravitational force, centrifugal force often affects ionic transportation processes, for example, leading to thermodynamic emf generation [2] and bringing about hydrodynamic convection. Gravity electrode (GE) is, as shown in Figure 1, operated in a high gravity field arising from a centrifugal force, which provides a great buoyancy force in the solution, promoting a gravitational convection. As shown in the theoretical analysis in Appendix, it was clarified that the buoyancy force comes from the change in the partial molar volume between product and reactant ions [3]. First, the effective density coefficient in the limiting diffusion in an electrode reaction is measured. Then, from in Appendix, the change in the partial molar volume is calculated. On the other hand, in in Appendix, is defined by the partial molar volumes of the product ion and reactant ion in the form of？？ . Reflecting the measurement of the diffusion current, is expressed by in in Appendix, where and