Steady-state voltammetry with stationary disk millielectrodes in magnetic fields: Nonlinear dependence of the mass-transfer limited current on the electron balance of the faradaic process

Generally, faradaic current passing through an electrolytic cell placed in a magnetic field causes stirring of the electrolytic solution and current-voltage characteristics similar to those obtained with rotating disk electrodes. It is reported herein that the intensity of the hydrodynamic convection generated by conventional disk millielectrodes in magnetic fields is intimately related to the nature of the faradaic process, and that the mass-transfer limited current, i(1), is proportional to n(3/2) where n is the number of electrons involved in the heterogeneous electron transfer. That finding has been justified on the basis of a feedback mechanism that relies on the dependence of the faradaic current on the hydrodynamic velocity profile within the electrolytic conductor, and of the hydrodynamic velocity profile on the current. The implications of the nonlinear dependence of i(1) on n have been discussed in terms of a moving-boundnry diffusion-layer model which is introduced into digital simulations and reproduces the main features of magnetic field voltammograms. Combination of the new findings with our previous results lends to the following expression for disk millielectrodes in transverse magnetic fields at room temperature: i(1) = 4.31 x 10(2) n(3/2) F A(3/4) \B\(1/3) D v(-1/4) C-bulk(4/3), where A is the electrode area, F the Faraday constant, \B\ the magnetic field strength, D the diffusion coefficient, C-bulk the bulk concentration of the redox-active species, v the kinematic viscosity of the electrolyte, and when the numerical constant has units of cm T-1/3 s(-1/4) mol(-1/3).