Etude de l'influence de la viscosité de milieu sur la diffion de charge . cas de couple ferrocéne/ferrocinum substitué

Abstract

The obtained diffusion coefficients of ferrocene/ferricenium, ferrocenyl methyl aniline/ferricenium and N-(ferrocenyl methyl)-2-nitro aniline/ferricenium couples in aqueous ethanol at different percentages on two different indicator electrode (GC, Pt) materials elucidate that the material of the indicator electrode has an effect on the diffusion coefficients as well as the voltammetry techniques used in the analysis of the studied molecules behavior; cyclic, rotating disc electrode. The diffusion coefficients obtained on glassy carbon indicator electrode are best than those obtained on platinum indicator electrode, because of its broad potential window and low background current in comparison with platinum electrode. The diffusion coefficients obtained using rotating disc electrode voltammetry are best than those obtained using cyclic voltammetry. The fluid is replenished by a flow normal to the surface. The RDE can be viewed as a pump that draws a fresh solution up from the bulk solution which keeps the diffusion layer thickness invariant. The diffusion layer thicknesses on rotating disc electrodes are smaller than those obtained on fixed indicator electrodes. The kinematic viscosity of the medium has not a significant effect on the diffusion coefficients of the studied electroactive molecules in aqueous ethanol at different percentages. The nearly properties of the ethanol and of the distilled water make the variation of the kinematic viscosity from medium to medium not significant. The separation of the electrode surface /solution interface where the kinematic viscosity has its effect on phenomena of transfer (mass, heat and momentum quantity) and the diffusion layer (Nernst layer) by the Helmholtz layer (inner and outer Helmholtz layer), decrease the effect of the kinematic viscosity of the medium on the diffusion coefficients of the studied electroactive molecules