Kinetics and mechanism of the initial phase of the bromine-chlorite ion reaction in aqueous solution

The kinetics and mechanism of the chlorine(III)-bromine reaction are studied by the stopped-flow method under acidic conditions in 1.0 M NaClO(4) and at 25.0 degrees C. There are two kinetically well-separated phases in this reaction. A detailed mechanism is proposed for the first phase of the reaction, in which Br(2) oxidizes ClO(2)(-) to chlorine dioxide. It is confirmed that the oxidation occurs via competing parallel reaction steps. The autoinhibition observed in the reaction is attributed to a backward shift in the reversible initial step as the oxidation proceeds. On the basis of simultaneous evaluations of the kinetic traces, the following forward rate constants are obtained for the kinetically significant reaction steps: Br(2) + ClO(2)(-) reversible arrow ClO(2) + Br(2)(-), k(1) = (1.3 +/- 0.2) x 10(3) M(-1) s(-1) (k(-1) = 1.1 x 10(9) M(-1) s(-1)); Br(2)(-) + ClO(2)(-) = ClO(2) + 2Br(-), k(2) = (4.0 +/- 0.1) x 10(6) M(-1) s(-1); Br + ClO(2)(-) = ClO(2)(-) + Br(-), k(8) (2.3 +/- 0.7) x 10(8) M(-1) s(-1); HOBr + HClO(2) = BrClO(2) + H(2)O (BrClO(2) + ClO(2)(-) = Br(-) + 2ClO(2), very fast), k(9) = (1.9 +/- 0.1) x 10(5) M(-1) s(-1). The possible kinetic role of the reactive BrClO(2) intermediate is discussed in detail.