Measurement of the Rate Constants of the Reactions OH + OH^- ⇄ O^- + H₂O and OD + OD^- ⇄ O^- + D₂O in Forward and Reverse Directions. Kinetic Determination of the p<i>K</i>'s of OH and OD Radicals

Abstract

The rate constants of the reactions OH + OH- ⇄ O- + H2O (k1, k2) and OD + OD- ⇄ O- + D2O (k‘1, k‘2) have been measured by pulse radiolysis in the forward and reverse directions. At 22 °C, the bimolecular rate constants (in mol-1 L s-1) are k1 = (6.3 ± 1.3) × 109 and k2 = (1 ± 0.2) × 106 in H2O and k‘1 = (5 ± 1) × 109 and k‘2 = (3.5 ± 0.7) × 105 in D2O. The pK's of the OH and OD radicals were calculated in nonequilibrium conditions by the ratio of the rate constants k1 and k2. Experiments were also performed using the same chemical system to determine the pK when the equilibrium between OH and O- or OD and O- is established. The two methods give similar results with pK(OH) = 12.0 ± 0.2, in good agreement with the literature, and pK(OD) = 12.6 ± 0.2. The OD radical is a weaker acid in D2O than the OH radical in H2O. The isotope effect on the equilibrium constant is mainly due to reverse reaction 2, which is 3 times slower in D2O than in H2O.