EHD AND STEADY-STATE MEASUREMENTS FOR CHARACTERIZATION OF TRANSPORT-PROPERTIES IN A GEL LAYER

Mass-transfer to a rotating disk electrode has been studied in the cas e of an interface covered by porous inert layers of thickness delta(f) through which species are transported by molecular diffusion with a d iffusivity D-f. In this study, the porous layer consists of a gel depo sited on a gold electrode and the species used as tracers are Fe(CN)(6 )(3-) and oxygen. The method is based on the analysis of tracer reduct ion current under total mass transport control (diffusion plateau of t he species). The tracer concentration gradient is distributed between the porous layer where mass transport is only determined by molecular diffusion and the electrolyte solution where mass transport is governe d by convective diffusion. Two techniques are used here namely electro hydrodynamical impedance (EHD) and a steady flow technique. From the p roposed physical model, the diffusion time (delta(f)(2)/D-f) is obtain ed by regression of the impedance data while the steady state current data yields the diffusion rate (D-f/delta(f)) through the porous layer . By combining the EHD and steady state data, a separate determination of delta(f) and D-f is available and allows the calculation of the po rosity of the film. A comparison between the two tracers on the same l ayer has been made.