| Summary: | The electrosynthesis of L-cysteine hydrochloride is examined at a range of cathodes. Kinetic studies are possible only at mercury and lead. Both voltammetry and coulometry studies are reported. Based essentially upon electrochemical reaction orders of +1 for L-cystine hydrochloride and protons, and high Tafel slopes of ca. 180 mV/decade, attributed to disulphide adsorption, the mechanism of reduction is, as far as can be ascertained, equivalent at the metals. The RDS is transfer of the first electron to the disulphide molecule. For high disulphide concentrations kf is ca. 7 x 10-1 0 m s - 1 at mercury and ca. 2.5 x 10-1 1 m s-1 at lead. Several techniques yielded values for the diffusion coefficient of L-cystine hydrochloride, the relative merits of which are discussed. Constant current electrolysis is performed in a parallel plate reactor operating in the batch recycle mode and the reactant concentration monitored with time. At mercury plated copper and lead cathodes the reactant depletion is predicted by mathematical models describing a reaction which is initially under charge transfer control followed by an instantaneous change to pure mass transport control. The models do not predict the reactant depletion at the other cathodes, because of an extensive mixed charge transfer-mass transport controlled region. To evaluate the models the average mass transport coefficient in the reactor is determined using the diffusion limited current technique. Additionally, using this technique, the enhancement in mass transport produced by various turbulence promoters is correlated with a specific geometrical characteristic of the promoters. Parametric studies of current density and catholyte flowrate identified the optimum electrosynthesis conditions. High catholyte flowrates and the use of turbulence promoters are particularly important to the process economics. At 500 A m-2 mercury plated copper and lead are the most efficient cathodes but at 2,000 A m-2 titanium compares favourably with these metals. The durability of all three cathodes is, however, inadequate for an industrial process.
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