The Effect of Chromate Addition on Cathodic Reduction of Hypochlorite in Hydroxide and Chlorate Solutions

The cathodic reduction of chromate and its effect on the reduction of hypochlorite have been studied by cyclic voltammetry, using a rotating disk electrode of platinum in 1 M NaOH solution as well as in an electrolyte with a composition and temperature which were typical for industrial chlorate synthesis. It was found that in both electrolytes a thin film of presumably Cr ( OH ) 3 , with a thickness of only one or two molecular layers, is formed. After formation of this film in a cathodic sweep of the potential, the hypochlorite reduction reaction is almost completely inhibited in the reversed sweep, up to the potential region for dissolution of the formed film. From the experimental results and a theoretical analysis of the mass transfer rate of hypochlorite ions to the cathode surface, it is concluded that the addition of chromate to the electrolyte in the industrial chlorate process leads to the formation of a thin film on the cathode surface, which hinders electron transfer at the reduction of hypochlorite.