NATURAL-CONVECTION AT MICROELECTRODES

Steady-state voltammetric currents for Fe(CN)(6)(4-) oxidation and Fe(CN)(6)(3-) reduction in 0.2 M KCI solutions are shown to be a function of the orientation of Au and Pt microdisk electrodes (radius = 6.4-25 mu m) with respect to the gravitational field. The observed angular dependencies for the Fe(CN)(6)(3-/4-) couple are in agreement with theoretical expectations based on electrochemically generated fluid density gradients within the depletion layer, The results demonstrate that steady-state voltammetric currents measured at microelectrodes result from both diffusional and natural convection mass transfer fluxes. At 25-mu m-radius microdisks, natural convection accounts for between 2 and 15% of the total current for redox concentrations between 10 and 500 mM, respectively, The contribution of mass transfer due to natural convection is shown to decrease rapidly with decreasing electrode size.