Kinetics and mechanism of the chromium(vi) catalyzed decomposition of hypochlorous acid at elevated temperature and high ionic strength

Abstract

An important reaction step in the industrial production of NaClO₃ (electrochemical chlorate process) is the thermal decomposition of HOCl/OCl⁻ to yield ClO₃⁻ and Cl⁻. It is widely accepted that this reaction is accelerated by aqueous chromium(VI) species. A detailed kinetic study was conducted under industrially relevant conditions, i.e. at high ionic strength (6.0 M) and elevated temperature (80 °C), to investigate this phenomenon. The decomposition of hypochlorous acid was followed in the presence of Cr(VI) or phosphate (PO₄³⁻) or without any additive. In addition to the beneficial pH buffering effect of Cr(VI), the CrO₄²⁻ form of chromium(VI) was found to slightly catalyze the decomposition of hypochlorous acid. The overall rate of HOCl decomposition can be expressed as −dc_HOCl/dt = k_dec[HOCl]²[OCl⁻] + k_cat[HOCl]²[CrO₄²⁻]. The corresponding rate constants were determined, k_dec = 9.4 ± 0.1 M⁻² s⁻¹ and k_cat = 4.6 ± 0.8 M⁻² s⁻¹, and mechanistic interpretation of the catalytic rate law is given. The contribution of the catalytic path to the overall rate of decomposition changes from ca. 30% at pH = 8 to ca. 70% at pH = 6.