KINETIC-STUDIES OF THE REACTION OF NITROUS-ACID WITH IODIDE-ION IN THE PRESENCE OF MOLECULAR-OXYGEN IN AN ACID-SOLUTION

The kinetics of the reaction between nitrous acid and iodide ion were studied in the presence of molecular oxygen (02) in an acid solution (pH 1.9-3.1); much larger amounts of the triiodide ion than expected from the amounts of nitrous acid were formed under conditions of the iodide ion in excess over nitrous acid. The initial rates for the formation of I2 and I3- increased proportionally with increasing both the O2 and nitrous acid contents, and increased greatly with decreasing the pH in the solution. The reaction rate was also decreased by bubbling either air or O2 through the reacting solution, depending on the bubbling velocity. A chain mechanism is proposed to account for the results obtained in the dark: 2HNO2+2I-+2H+-->2NO+I2+2H2O; 2NO+O2-->2NO2; 2NO2 half arrow right over half arrow left N2O4; N2O4+2I-+2H+-2HNO2+I2; I2+I- half arrow right over half arrow left I3-. The rate-determining step was the reaction of NO with O2. The addition of trace amounts of copper(II) or iron(II) ion enhanced the formation Of I2 and I3- due to the occurrence of reactions of I- with the metal ions. Such an acceleration effect by the metal ions, however, could be diminished by the addition of ethylenediaminetetraacetic acid. Although the rate of the reaction of HNO2 With I- was not greatly changed by the irradiation of visible light during the initial period of reaction, it was greatly accelerated after some formation Of I2 and I3-, being dependent on not only the light intensity, but also on the concentrations of H+ and I- ions. The effects of the metal ions as well as visible light are also accounted for by the above-mentioned mechanism.