Kinetics of reactions in solutions containing monochloramine and bromide

Abstract

The reactions of monochloramine in saline solutions containing bromide have been investigated. Monochloramine oxidizes the bromide ion via NH 3Cl + formation. The rate constant for the second-order reaction was found to be 6 ± 1 × 10 4 mol −1 dm 3 s −1 at 25°C. Further rapid reactions follow, resulting predominantly in the formation of NHBr 2 NHBr 2 − and NClBr − at pH ∼ 7. The distribution of these species appears to be thermodynamically controlled and dependent upon the pH, the ionic strength and chloride/bromide ratio. The equilibrium constants at 25°C for NBr 2 − and NClBr − formation with respect to NHBr 2 were estimated to be H +NBr 2 −/[NHBr 2] = 1.3 ± 0.1 × 10 −7 mol dm −3 and H + NClBr − Br −/[NHBr 2] Cl − = 6.1 ± 0.2 × 10 −10moldm −3, respectively. These species decay through a series of reaction steps resulting in nitrogen formation. In unbuffered solutions, these reactions can lower the pH sufficiently for Br 3 − formation to occur.