ELECTROSYNTHESIS OF SODIUM HYDROSULFITE - III - POROUS CATHODE MATERIALS AND PROCESS MODEL

The electrolytic manufacture of sodium hydrosulfite is affected by fee dstock composition, electrode morphology, and electrode material. Here we show that porous graphite cathodes can provide the highest one-pas s conversion of sodium bisulfite to hydrosulfite without contamination by sodium thiosulfate. For example, at current densities up to 0.90 k A/m(2), thiosulfate-frec hydrosulfite can be produced at 66% current e fficiency. At higher current densities, both hydrogen and thiosulfate are produced. In acetate-buffered bisulfite, these rise to 1.35 kA/m(2 ) at nearly 100% current efficiency. In unbuffered bisulfite feedstock s, variations in product quality depend solely on cathode composition. That is, the electrode kinetics for each of the reductions are a stro ng function of the electrode material. However, we show that the rate of bisulfite consumption is controlled by diffusional mass transfer in the porous electrode and can be predicted through use of a simple pac ked-bed, plug-flow reactor model.