Temperature Dependence of the Acid Dissociation Constant of the Hydroxyl Radical

The acid dissociation constant of the hydroxyl radical in aqueous solution was determined by pulse radiolysis from the [OH - ] dependence of the rate constant for reaction of the • OH/O •- radical with benzoate ions. The rate constant and the p K a ( • OH) values were determined over the temperature range 284−343 K. At 298 K, p K a 298 ( • OH) = 11.54 ± 0.04 and Δ ion G ° 298 ( • OH) = (65.9 ± 0.3) kJ mol -1 . From the temperature dependence of K a ( • OH), we calculate Δ ion H ° 298 ( • OH) = (24.85 ± 0.5) kJ mol -1 and Δ ion S ° 298 ( • OH) = (−139 ± 2) J mol -1 K -1 . The activation energy for the reaction of • OH with C 6 H 5 CO 2 - was found to be (8.8 ± 0.6) kJ mol -1 and the Arrhenius preexponential factor (2.4 ± 0.6) × 10 11 L mol -1 s -1 .