KINETICS OF REDUCTION OF IODINE BY OXALATE AND FORMATE IONS IN AN AQUEOUS-SOLUTION - DIFFERENCE IN MECHANISMS OF THE REACTIONS OF OXALATE AND FORMATE IONS WITH IODINE

Kinetics of the reduction of iodine by oxalate and formate ions have been studied in an aqueous solution. The reduction of iodine by the oxalate ion did not occur at all in the dark but proceeded greatly by irradiation with visible light of solution. The photosensitized reaction was inhibited by the addition of acrylonitrile and acrylamide as radical scavengers. The rate of reaction was of half-order in both the concentrations of iodine and the intensity of light irradiated. The rate also increased with increasing pH at pH 3-5 and was proportional to concentrations of sodium oxalate added up to 0.1 mol dm-3. Iodide ions inhibited the reaction. These results show that the photoreaction of iodine with oxalate proceeds by the dissociation of iodine to iodine atom by light. On the contrary, iodine was reduced by the formate ion in the dark, being indifferent to the intensity of light irradiated. The reduction of iodine by formate was not inhibited at all by the radical scavengers of acrylonitrile and acrylamide. The rate was proportional to the concentrations of sodium formate but was inversely proportional to those of iodide. The reactive species are not I3- and HCOOH but are I2 and HCOO-. The mechanisms of reaction of formate are completely different from those of oxalate: the former is a thermal and bimolecular reaction, while the latter is a light-induced and radical reaction containing CO2·- and I2·-. © 1990 American Chemical Society.