ABSTRACT:The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry isThe rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given byAt an ionic strength of 2.0 M and at 25 • C, k u = (3.9 ± 0.1) × 10 3 L mol −1 s −1 and k c = (6 ± 1) × 10 4 L mol −1 s −1 . The formation constant for the acetatoiron(II) complex, K f , at an ionic strength of 2.0 M and 25 • C was found to be (4.8 ± 0.8) × 10 −2 L mol −1 . The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of H ‡ and S ‡ are 64 ± 3 kJ mol −1 and +40 ± 10

The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5-8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl -, NH + 4 , and N 2 O. The rate law is given byAt an ionic strength of 1.2 M, at 25 • C, and under pseudo-first-order conditions, k + = (1.03 ± 0.06) ×10 3 L · mol −1 · s −1 and k 0 = 91 ± 15 L · mol −1 · s −1 . Isotopic studies demonstrate that both nitrogen atoms in the N 2 O come from the NH 2 OH/NH 3 OH + . Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be H ‡ = 36 ± 3 kJ · mol -1 and S ‡ = −66 ± 9 J · K −1 · mol −1 , and H ‡ = 12 ± 2 kJ · mol −1 and S ‡ = −168 ± 6 J · K −1 · mol −1 , respectively, and confirm that the transition states are significantly different for the two reaction pathways. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: [124][125][126][127][128][129][130][131][132][133][134][135] 2006