Magnetic field effects on the mass transport at small electrodes studied by voltammetry and magnetohydrodynamic impedance measurements

The effect of moderate magnetic fields (up to 0.75 T) on the redox reaction of the ferro–ferricyanide couple at small disk electrodes (radii 12.5, 50 and 800 μm) has been investigated using voltammetric techniques and the magnetohydrodynamic (MHD) transfer function analysis. A special feature of the experimental setup is that it works in the time domain and therefore does not need a lock-in amplifier. The magnetic field was applied parallel to the electrode surface and perpendicular to the gravity force. Special focus was given to the influence of the electrode size and the concentrations of the redox species on the MHD effect. In accordance with literature it was found that the MHD effect decreases with decreasing electrode size and concentration of the electroactive species. Furthermore it was found that the characteristic time constant of the MHD effect increases with decreasing concentration of the electroactive species. Combining the information from the voltammetric and the MHD transfer function data we could estimate the magnetically induced flow velocities which were in a reasonable order of magnitude. The implications of our results for MHD stirring in small feature sizes will be discussed shortly.