The interfacial concentration cB,e (or simply ce) of a species B is the concentration of that species at the boundary of the electrical double-layer facing the solution, i.e. just outside the region where the departures from the electroneutrality of the solution are significant. This concept is mainly used in the usual case where the thickness of the electric double layer is very small as compared to that of the diffusion layer. It is often calculated from theory or derived from measurements of the limiting current.