Electrosynthesis of Sodium Hydrosulfite II. The Effect of Cathode Material

Sodium hydrosulfite is a major commodity reagent currently manufactured mainly by chemical means. A new, lower cost, electrochemical pathway is being used in some cases. One of the key determinants in the marketability of sodium hydrosulfite is the level of sodium thiosulfate in the product. At present, the level of thiosulfate from the electrochemical process is higher than that from the chemical process. Here a critical evaluation is presented of cathode materials for the electrochemical manufacture of sodium hydrosulfite nearly devoid of thiosulfate. It is demonstrated that the yield, selec-tivity, and efficiency of hydrosulfite and thiosulfate production depends upon the applied potential, pH of the feedstream,buffering capacity, and cathode material. Several planar cathodes for the electrosynthesis of hydrosulfite have been crit-ically evaluated. We demonstrate that five main process parameters contribute to high hydrosulfite to thiosülfate ratios; NaHSO3 reactant concentration, feedstream pH, buffer capacity, cathode material, and applied potential. By control of these parameters, we demonstrate that hydrosulfite solutions devoid of thiosulfate can be electrochemically produced. Infroduction Sodium hydrosulfite is an important commodity chemi-cal with global production of over 180 million kg/yr. In the United States alone, annual production exceeds 50 million kg. This powerful reducing agent has many industria