The Reactivity of Hydroxyl Radicals toward Boric Acid as a Function of pH

Boric acid and its counter-base, borate, are a commonly used buffer pair in many systems where hydroxyl radicals are generated. Boric acid is also used in light water-cooled nuclear reactors to control the excess reactivity of the nuclear fuel. Hydroxyl radicals are generated within the cooling water of the reactor because of intense radiation. The reactivity of the hydroxyl radical toward boric acid has previously been studied, but to the best of our knowledge, only upper limits of the rate constants are available in the literature. In this study, the rate constants for the reaction between the hydroxyl radical and boric acid and its counter-base including several polyborates that form at high boron concentration are determined. The rate constants were determined from competition kinetics using steady-state γ radiolysis and coumarin-3-carboxylic acid as the competing reactant. By varying the pH and accounting for boron speciation, it was possible to determine the rate constant for the different boron species using multilinear regression. The rate constants for boric acid and the counter-base were determined to be 3.6 × 10⁴ and 1.1 × 10⁶ M^-1·s^-1, respectively, which is very close to the previously determined upper limits of the rate constants. For the polyborate species diborate and tetraborate, the rate constant was determined to be 6.4 × 10⁶ and 6.8 × 10⁶ M^-1·s^-1, respectively.