Kinetics and mechanism of the oxidation of iron(II) ion by chlorine dioxide in aqueous solution

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 is 5Fe(2+) + ClO2 + 4H(+) --> 5Fe(3+) + Cl- + 2H(2)O. The 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 by d\Fe3+\/dt = k(u)\Fe2+\\ClO2\ + k(c)\FeOAc+\\ClO2\. At an ionic strength of 2.0 M and at 25degreesC, k, = (3.9 +/- 0.1) x 10(3) L mol(-1) s(-1) and k. = (6 +/- 1) x 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 25degreesC was found to be (4.8 +/- 0.8) x 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 DeltaH(+) and DeltaS(+) are 64 +/- 3 kJ mol(-1) and +40 +/- 10 l K-1 mol(-1) respectively. (C) 2004 Wiley Periodicals, Inc.