KINETICS AND MECHANISM OF THE OXIDATION OF FERROUS ION BY IODATE ION IN STRONG PERCHLORIC-ACID, AQUEOUS-MEDIA

The kinetics and mechanism of the oxidation of Fe(II) to Fe(III) by IO3- in strongly acid, aqueous media have been investigated. The stoichiometry of this reaction is given by 5Fe(II) + IO3- + 6H+ → 5Fe(III) + 1/2I2 + 3H2O (1). All experiments were carried out at a temperature of 30.0 ± 0.2°C and at pH near zero. The ionic strength was maintained at 1.5 M, and in all experiments ClO4- was the only anion present besides IO3-. Reaction 1 is catalyzed by I2, one of its products. Thus experiments falling into two general classes were carried out. In the first class of experiments I2 was allowed to accumulate, while in the second class of experiments I2 was continuously removed by extraction into CCl4. On the basis of these experiments, we propose here a complex mechanism for reaction 1 which is composed of eight reactions (three reversible) and which involves seven reactive intermediates: HIO3, IO2̇, HIO2, IȮ, HOI, I2, and I-. Numerical simulation techniques are used to demonstrate that the proposed mechanism is able to quantitatively rationalize all experimental data obtained. Furthermore, an analytical rate expression is derived which reproduces the data nearly as well as the exact numerical calculation and which also expresses the essence of the proposed mechanism. This expression indicates that there are two parallel paths by which the overall stoichiometry of reaction 1 may be achieved. The first path is a sequence of reactions initiated by the rate-determining, direct interaction of Fe(II) with HIO3. The second path is associated with the I2 catalysis and is initiated by hydrolysis of I2. In the course of this work values were assigned to seven rate constants for which values were previously unknown. Some of these rate constant values are estimated here by various techniques and others are derived from the experimental data reported here. The reliability of these rate constant values is discussed. © 1979 American Chemical Society.