Bromate production in chlorinated waters: reaction of monochloramine and hypobromite

Mixed solutions of monochloramine and hypobromite produced bromate, rather than the NHBrCl that might have been expected. Bromate was identified by u.v. spectrophotometry and polarography. The stoichiometric ratio for the reaction showed that, in excess monochloramine, 1 mol of hypobromite reacted with 2 mol of monochloramine, while in excess hypobromite, 2 mol of hypobromite reacted with 1 mol of monochloramine. The rate of loss of hypobromite was followed voltammetrically at a rotating platinum electrode and was found to be first-order with respect to hypobromite and hydroxide ion concentrations. It is established that ion-pair formation between hypobromite and sodium (β <10 mol^-1 dm^-3) retards the reaction. Retardation by bromide (but not by other anions) indicates that bromite is an intermediate step in its formation appears to be rate-determining. The rate in 0.1 mol dm^-3 NaClO₄ medium is - d dt[OBr^-]= exp{25.1±0.5-(624±0.2)×10³/T} [OH^-][OBr^-mol dm^-3s^-1 The general rate equation at 25°C is - d dt[OBr^-]= 38 γ² {OH^-} T_OBr 1+{H}γ/K_a+β[Na⁺]γ moldm^-3s^-1 where γ is the univalent ion activity coefficient, k₃ the acid dissociation constant of HOBr and T_OBr is the total Br(I) concentration as HOBr, OBr^- and NaOBr. © 1990.