Mn(II)-catalysed S(IV) oxidation and its inhibition by acetic acid in acidic aqueous solutions

The kinetics of the S(IV) oxidation by oxygen in the presence of Mn(II) ions and acetic acid has been studied. Experiments were carried out at 25°C, 3.5≤pH≤5.0, [S(IV)]≈1×10^-3 mol/dm³, 1×10^-6 mol/dm³≤[Mn(II)]≤1×10 ^-5 mol/dm³, 1×10^-6 mol/dm ³≤[CH₃COOH]≤1×10^-4 mol/dm ³. Based on the experimental results, rate constants and orders of the reactions were determined. Depending on the reaction conditions, the observed rate constants for the Mn(II)-catalysed S(IV) oxidation ranged between 3.91×10^-8 and 8.89×10^-7 (mol/dm³) s^-1, and in the presence of acetic acid they ranged between 2.95×10^-8 and 7.45×10^-7 (mol/dm³) s^-1. The reaction order in S(IV) was zero for both reactions. The effect of Mn(II) ion and acetic acid concentrations as well as an initial pH of the solution on the S(IV) oxidation rate was discussed. It was found that the rate of the S(IV) oxidation depends on the initial pH of the solution but it is independent of the pH change during the reaction. Acetic acid has a weak inhibiting effect on the Mn(II)-catalysed S(IV) oxidation. Under the experimental conditions the S(IV) oxidation rate decreased no more than twice.