Mise en page 1 - Laboratoire National des Champs Magnétiques ...

MAGNETO-SCIENCE 2009Study of the influence of magnetic forces on the mass transferof paramagnetic particles in electrochemistryThe context of our work is the characterization of the magneticforces which influence the processes of mass transferin electrochemistry. It appears experimentally that in thepresence of a magnetic field in solutions, where a gradientof concentration of paramagnetic ions is present, even if theaction of Lorentz forces is not effective, a driving force actingon the solution will arise. This force, referred to as theconcentration gradient force, is often written,−→ B 2−→ ∇CF m = χ m , (18)2µ 0where χ m (m 3 /mol) is the molar magnetic susceptibility,B(T) the magnetic flux density, C(mol/m 3 ) the bath concentrationand µ 0 (Hm −1 ) the permeability of vacuum. Theexperiment was performed using a 10 MW resistive magnetwhich generates a homogeneous a magnetic field of 6 T ina 286 mm diameter. The experimental device is a rectangularchannel with platinum electrodes on the upper and lowerwalls, between which a voltage drop is applied (figure 152).The channel is closed with insulating walls to eliminate theedge effects. The cell is immersed in an electrolytic bathprepared with an equimolar solution of 0.05 mol/m 3 Ferriferro-cyanide with 0.5 mol/m 3 and K 2 SO 4 as the supportingelectrolyte. The temperature condition was about 17 ◦ Cand an Ag/AgCl reference electrode was used to control theelectrodes potential.of the boundary layer. As a result, an additional mechanismof transport of the solution is generated. Correspondingly,the limiting currents of reactions proceeding in the electrochemicalsystem will become a function of the appliedmagnetic flux density. The influence of magnetic field onthe evolution of the mean limiting current density is plottedusing logarithmic coordinates in figure 153. We can showsthat for the two modes, anodic and cathodic, the limitingcurrent density follows a law in B 2/3 . Note that this lawstarts at 0.5 T for the anodic mode and at 2 T for the cathodicmode due to the paramagnetic forces which drivesa more important flow than the gravity. This phenomenologicalbehaviour was observed by many authors [Waskaas,Acta Chimica 50, 516 (1996)], Rabahand et al., Journal ofElectroanalytical Chemistry 571, 85 (2004)].Figure 152:Experimental configuration.The results were obtained by the polarographic method.When the two electrodes located on the faces of the channelare submitted to a voltage drop, controlled by a potentiostat,an electric current is imposed and in the presence ofan homogeneous magnetic field, the gradient of paramagneticions due to electrode reaction will cause a redistributionof velocities in the bath which then acts on the depthFigure 153: Evolution of the limiting current densities with thework electrode (WE) in the (a) cathodic mode and (b) anodicmode.F. DebrayD. Baaziz, A. Alemany, (EPM-SIMAP Laboratory, Grenoble, France), D. Kalache (Fluids Mechanics Laboratory,University of USTHB, Algiers)108