Region of Film Diffusion Control

On a film-covered rotating disk electrode, for example, the concentration of a mass-transfer-limited species is given by  

concentration profiles within the coating and the bulk region can be seen to be controlled by the coating porosity e and by the ratio of diffusion lengths Sj j/Sf. 

For a given film thickness Sf, the important parameter is the concentration at the film surface. If this concentration tends toward zero, the film has a negligible influence on mass transfer. Otherwise, the influence of the film cannot be neglected. 

The concentration at the interface between the film and the bulk electrolyte indicates whether the diffusion impedance of the system can be assumed to be that of the stationary film. The interface concentration, given by 

---