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Thin-layer electrochemical reactors are analytical reactors where mass transfers are generally assumed not to affect the overall reaction rate. The validity of this hypothesis was tested for systems involving a second-order or an enzymatic homogeneous reaction that consumed the product generated by the electrochemical heterogeneous reaction. The experiments were p e~o r m e d with the oxidation of hexacyanofen-ate 11 into hexacyanofen-ate III, coupled with the consumption of hexacyanoferrate 111 by the reduced form of nicotinamide adenine dinucleotide, catalyzed or not by a diaphorase. The tools commonly used for gas -liquid inte$aces were adapted to the description of mass transfer in a closed thin film. The theoretical model agreed well with the experimental data and served as the basis for testing the thin-layer hypothesis. The validity of the hypothesis was plotted as a function of the Hatta and Damkohler numbers. The influence of concentration and difision parameters are discussed with focus on the validity criterion. IntroductionDescription of mass transfers with chemical reaction is a widely investigated topic in the field of gas-liquid interfaces. Astarita (1967) and Danckwerts (1970) proposed complete studies in this area at a very early date. Other authors have broadened the scope of the investigations (Cornelisse et al., 1980; Trambouze, 1981;Charpentier, 1981). Electrochemical engineering can benefit to a large extent from these works because most of the methods and results can be applied directly to the description of mass transfers with chemical reaction at electrochemical interfaces. A great similarity exists between the two kinds of interfaces: in both cases, the essential phenomena occur in the liquid layer adjacent to the interface, where the concentrations are monitored by equilibrium conditions. In gas-liquid systems the interface concentrations are determined by the solubility of the gaseous species; for electrochemical interfaces they are generally determined by the Nernstian equilibrium. As a consequence, the concepts of surface renewal (Tzedakis et al., 1989), Hatta number (Weise et al., 1986), or van Krevelen's representation (Labrune and Bergel, 1992) are, for instance, commonly encountered in electrochemical engineering works.Nevertheless the case of thin-layer electrochemical reacCorrespondence concerning this article should be addressed to R. Devaux tors seems to have no similarity in the field of gas-liquid interfaces. They are analytical reactors with a very small volume of liquid and a large electrode surface area, resulting in a thickness of approximately the same order of magnitude as a diffusive layer. In the theoretical representation the boundary conditions of the mass balance equations at the side opposite to the electrode indicate that the mass fluxes of all the species are nil at the wall of the reactor. There is no similar case for gas-liquid interfaces because mass transfers are always occurring between the interface layer and the bulk of the solution. The boundary condit...

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