Rate of hypobromite formation in chlorinated seawater

The factors affecting the rate of formation of hypobromite ion in chlorinated seawater have been investigated. Apart from the expected oxidation of bromide by HOCl, the rate constant of which was found to be k_H = 1.57 × 10⁶ exp(-1.62 × 10³/T) mol^-1 dm³ s^-1, the hypochlorite ion-pairs formed with the major cations were found to provide another reaction pathway. The ratios k′_m = k_m K_m mol^-2 dm⁶ s^-1, where k_m is the rate constant and K_m the ion-pair formation constant, were as follows: CaOCl⁺, 1.7 × 10⁴; MgOCl⁺, 4.2 × 10³; NaOCl°, 12; KOCl°, 3 and these enabled the effect of the presence of the metal ions on the overall rate to be calculated. In natural seawater the ion-pairs provide about 3.5% of the hypobromite production, but at higher pH, as in many laboratory studies, this proportion would increase. Transition metal ions (Fe³⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) at the low concentrations (5 × 10^-6 mol dm^-3) that could be tested had no catalytic effect but Fe²⁺ and Mn²⁺ ions were themselves oxidized. © 1986.