Electrolysis on a porous electrode with a flowing solution (e.p.f.) is a new field in electrochemistry. Several experimental arrangements have been proposed for this type of electrolysis 1-9, which so far has been recommended for analytical applications 2-6 and for the production of radical ions 6'9.
Several mathematical models have been devised for e.p.f. 1, 10-19. They can be divided into two classes : the electrode kinetic and mass transport models. The electrode kinetic models, as developed by Perskaya and Zaidenman, Gurevich and Bagotzky, and Korovin and Chudinov, concentrate on the dependence of the electrolytic current on the applied potential. They neglect however the convective-diffusional limitations of the mass transport rate to the internal surface of the porous electrode (PE), and hence cannot explain the formation of a limiting current smaller in magnitude than the current corresponding to complete electrolysis of the flowing species. The mass transport model, on the other hand, assumes that the potential difference at the interface everywhere in PE is of a sufficient magnitude that the rate of electrolysis is controlled solely by the rate of transport of the electroactive species to the internal surface. This model attempts to calculate the magnitude of a limiting current as a function of the species concentration, the flow rate and the geometric parameters of the electrode 17,18.
The model presented in this paper represents a novel approach. Its purpose is to relate the electrolytic current to both the applied potential and the mass transport limiting factors.
DESCRIPTION OF THE MODEL
A ~chematic drawing of the cell is presented in the Fig. 1. The PE has the form of a right cylinder of base area, a, and of length L. The solution which flows through PE with a uniform flow velocity approaches it at its frontal face, i.e. the one facing the counter-electrode. As before it is supposed that PE can be treated macroscopically as a homogenous, isotropic medium with a uniform distribution of the sources of an electrochemical reaction 13'zยฐ. The microscopic nature of PE is characterised by two effective, statistical parameters: the specific internal surface, s, and the porosity, e. It is further supposed that the material of the electrode has very low ohmic resistance (as for example in the case of a metal), so that the potential drop in the material of the electrode can be practically neglected in comparison with that in the solution. It is also supposed that the solution contains an excess of a base electrolyte, so that migration