On the mechanism of anodic chlorate formation in concentrated NaCl solutions

Abstract

The mechanism of anodic chlorate formation in concentrated NaCI solutions is different from that in the dilute solutions studied previously. In concentrated NaCI solutions the chlorine hydrolysis near the anode is restricted by equilibrium unless the pH in the otherwise acid anodic diffusion layer is increased by the action of a buffer. Experiments perfomed by electrolysing NaCI solutions containing various amounts of carbonate comfirmed this. In the absence of a buffer, hypochlorite has to build up in the bulk before anodic chlorate formation can set in at appreciable rate; the hypochlorite itself acts now as a buffer in the anodic diffusion layer. The proposed model is consistent with the fact that in concentrated NaCI solution the anodic chlorate-formation rates were smaller than those predicted for a purely diffusion-controlled process.