MEASUREMENT OF THE RATE CONSTANTS OF THE REACTIONS OH-REVERSIBLE-ARROW-O-+H2O AND OD+OD-REVERSIBLE-ARROW-O-+D2O IN FORWARD AND REVERSE DIRECTIONS - KINETIC DETERMINATION OF THE PKS OF OH AND OD RADICALS(OH)

The rate constants of the reactions OH + OH- reversible arrow O- + H2O (k(1), k(2)) and OD + OD- reversible arrow O- + D2O (k'(1), k'(2)) ha ve been measured by pulse radiolysis in the forward and reverse direct ions. At 22 degrees C, the bimolecular rate constants (in mol(-1) L s( -1)) are k(1) = (6.3 +/- 1.3) x 10(9) and k(2) = (1 +/- 0.2) x 10(6) i n H2O and k'(1) = (5 +/- 1) x 10(9) and k'(2) = (3.5 +/- 0.7) x 10(5) in D2O. The pks of the OH and OD radicals were calculated in nonequili brium conditions by the ratio of the rate constants k(1) and k(2). Exp eriments were also performed using the same chemical system to determi ne the pK when the equilibrium between OH and O- or OD and O- is estab lished. 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. T he 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 re action 2, which is 3 times slower in D2O than in H2O.