Photolysis of Dilute Hydrogen Peroxide Solution in the Presence of Dissolved Hydrogen and Oxygen: Evidence Relating to the Nature of the Hydroxyl Radical and the Hydrogen Atom Produced in the Radiolysis of Water

Abstract

The purpose of this study was to characterize more exactly the hydroxyl radical and the hydrogen atom produced in the radiolysis of water. This was done by comparing properties of the hydroxyl radical produced by photolysis of hydrogen peroxide, and of the hydrogen atom produced by reaction of H2 with this hydroxyl radical, with the corresponding properties, previously determined, of the species produced in the radiolysis of water. The comparison was made by accurate evaluation of relative rate constants for reaction of H202 and H2 with the hydroxyl radicals, and for reaction of H202 and 02 with the hydrogen atoms produced in the two systems. The effect of pH on these ratios was also studied. There is little or no direct information based on physical measurements concerning the reactive intermediates produced in the radiolysis of liquid water. The identity of these species is inferred from observations on water vapor and ice, and from chemical kinetic studies of various solutions as a function of concentration, total radiation dose, dose rate, LET (linear energy transfer), pH, temperature, isotopic content, etc. (1, 2). Chemical studies indicate radiation decomposition of liquid water to OH radicals and H atoms or their chemical equivalents. Evidence, based mostly on pH studies, suggests more than one form of each, but the actual identity and molecular constitution of these species is not known. It is also not known whether only one form of each is produced in the initial processes of the radiation decomposition of water. For example, very recent results of Czapski and Allen (3) indicate that only one form of hydrogen atom is produced initially, whereas other results (4, 5) suggest about 20 % of a second form. It is conceivable that hydroxyl