The kinetics of the reactions between iodide and hydrogen peroxide in seawater

The kinetics of the reactions between H2O2 and I− were studied in artificial seawater as a function of temperature (10–30 °C), salinity (0–36), pH (7–9) and the concentrations of molybdate (0–480 μM) at a range of concentrations of H2O2 (5 to 40 mM) and I− (0.1 to 30 mM) by following the changes in the concentrations of the two reactants with time. While the rate of disappearance of H2O2 was accelerated significantly in the presence of I−, the concentration of I− remained constant in all the experiments even when the amount of H2O2 lost had far exceeded the amount of I− present. This catalytic decomposition of H2O2 by I− was consistent with the reaction scheme in which H2O2 first oxidized I− to I2 and then reduced the I2 formed back to I−. The disappearance of H2O2 was first order with respect to both the concentration of I− and H2O2 so that the rate law could be expressed as− d[H2O2] / dt = k[I−] [H2O2] where [I−] and [H2O2] were the concentrations of I− and H2O2 in mM and k, which was equal to 0.0600 mM− 1 h− 1 at a pH of 8.0, a temperature of 20 °C and a salinity of 35.7 in artificial seawater, was the second order reaction rate constant. Within the oceanographic range, k was essentially independent of pH. At a constant concentration of I− of 0.5 mM, the rate constant was linearly related to salinity S so that:kI=0.00012(±0.00001)S+0.0283(±0.0003)wherer kI = k[I−]. Its dependence on temperature, T in K, follows the form of the Arrhenius relationship so that:ln⁡kI=−6865(±84)(1/T)+19.96(±0.02) When these results were extrapolated to the conditions found in surface seawater, the estimated half life of H2O2 was much longer than the observed values, suggesting that the reactions between H2O2 and I− are not an important determining factor of the fate of H2O2 in seawater. The impact of these reactions on the marine geochemistry of iodine will depend on the fate of the I2 formed in the forward reaction. Molecular iodine is a reactive transient and it may participate in other reaction pathways in seawater where forms of iodine other than I− can be formed. Under the experimental conditions used, there was no evidence that the presence of molybdate at the concentrations found in seawater increased the rate of the reactions between H2O2 and I−.