Kinetics and mechanism of the reaction of sodium azide with hypochlorite in aqueous solution

Production of toxic sodium azide (NaN3) surged worldwide over the past two decades to meet the demand for automobile air bag inflator propellant. Industrial activity and the return of millions of inflators to automobile recycling facilities are leading to increasing release of NaN3 to the environment so there is considerable interest in learning more about its environmental fate. Water soluble NaN3 could conceivably be found in drinking water supplies so here we describe the kinetics and mechanism of the reaction of azide with hypochlorite, which is often used in water treatment plants. The reaction stoichiometry is: HOCl + 2N3− = 3N2 + Cl− + OH−, and proceeds by a key intermediate chlorine azide, ClN3, which subsequently decomposes by reaction with a second azide molecule in the rate determining step: ClN3 + N3− → 3N2 + Cl− (k = 0.52 ± 0.04 M−1 s−1, 25 °C, μ = 0.1 M). We estimate that the half-life of azide would be ≈15 s at the point of chlorination in a water treatment plant and ≈24 days at some point downstream where only residual chlorine remains. Hypochlorite is not recommended for treatment of concentrated azide waste due to formation of the toxic chlorine azide intermediate under acidic conditions and the slow kinetics under basic conditions.