Homogeneous and heterogeneous decomposition ofhypochlorite - A study of the oxygen evolving sidereaction using mass spectrometry

Abstract: Oxygen evolution from homogenous and heterogenous decomposition of hypochlorite is a small but nonetheless important side reaction in the electrolytic production of chlorate. In this diploma work, a method using mass spectrometry for analyzing the amount of oxygen formed in a hypochlorite containing electrolyte has been developed, and some preliminary experiments have been made. The method works satisfactory for initial screening, but for use in further studies, needs to be developed to include measurement of concentrations in the electrolyte and the ability to maintain a constant pH during experiments. Based on results from the limited experiments made, some preliminary conclusions can be drawn. The amount of oxygen evolved was measured with the initial pH of 7, 8, and 9, and three dierent types of aqueous electrolytes at initial pH 7; NaOCl(0.19M), NaCl(1.8M/2.7M)+ NaOCl(0.19M), and NaClO 4(1.8M) + NaOCl(0.19M). DSA (Dimensionally Stable Anode) particles, two types of cerium salts, and a cobalt salt were tried as catalysts, the concentration of the salts were 0.018 mM in all cases. The DSA particles and the cobalt used in this study catalyze the oxygen evolution reaction, while cerium does not. Both hypochlorous acid and hypochlorite ion seem to decompose separately into oxygen in the presence of catalyst, while the uncatalyzed decomposition mechanism require the presence of both species as no oxygen is detected outside of the pH range where they are both present (approximately 6<pH<10). The rate of oxygen formation has a maximum around neutral pH for both catalyzed and uncatalyzed decomposition, and the rate increases with a decrease in pH in the approximate interval 7<pH<10, below which it decreases. No clear eects of ionic medium or ionic strength were noticed in this study.