Hydrogen ion dependence of the oxidation of iron(II) with peroxydisulfate in acid perchlorate solutions

Article abstract of DOI:10.1021/ic50216a027

The title reaction has been studied in the [H⁺] range of 0.01-1.0 M at different ionic strengths and temperatures. Unlike previous reports, the rate is dependent on the concentration of the hydrogen ions, if it is larger than 0.2 M. The rate laws are d[Fe(III)]/dt = 2k₁[S₂O₈^2-][Fe(II)] for [H⁺] < 0.2 M and d[Fe(III)]/dt = (k₁ + k₂K[H⁺])[S₂O₈^2-][Fe(II)] for [H⁺] >0.4 M. In the [H⁺] range of 0.4-1.0 M and at I ≈ 1.0 M, k₁ (M^-1 s^-1) and k₂K (M^-2 s^-1) were found to be 25 ± 1.2 and 83 ± 8, 18 ± 1.2 and 63 ± 4, and 11.5 ± 0.6 and 55 ± 4 at 40, 35, and 30°C, respectively. A plot of log k_obsd (second-order rate constant) vs. I^1/2 for I < 0.02 M yields a straight line with a slope of -3.83. The slope decreases numerically with the increase in the range of ionic strength investigated. The energy of activation (kcal mol^-1) was found to be 14.7 ± 1 and 7.6 ± 2 for the k₁ and k₂ paths, respectively. The value for the k₁ path is dependent on the ionic strength showing ion pairing.