The acid dissociation constant and decay kinetics of the perhydroxyl radical

The acid dissociation constant of the perhydroxyl radical was found to be 4.88 ± 0.10 in a pulse radiolysis study of oxygen-saturated aqueous solutions. The solutions were buffered with formic acid-sodium formate which also served to transform the hydroxyl radicals into perhydroxyl radicals by reaction with the formate and subsequent reaction with oxygen. This pK value is in agreement with one earlier value, but in disagreement with others. The discrepancy is shown to be most likely due to the absence of buffer in most earlier work. The second-order kinetics of recombination of perhydroxyl radicals follow the equation, k = (0.76 × 10⁶ + 8.5 × 10⁷X)/(1 + X)², where X is the ratio of the dissociation constant to the acid concentration, 0.76 × 10⁶ M^-1 sec^-1 is the rate constant for the reaction HO₂ + HO₂, and 8.5 × 10⁷ M^-1 sec^-1 is for the reaction HO₂ + O₂^-. The rate constant for the reaction O₂^- + O₂^- is shown to be less than 100 M^-1 sec^-1. The results suggest that only two forms of the perhydroxyl radical, HO₂ and O₂^-, exist in the pH region 0 to 13.