Temperature and pressure effects on the kinetics of the Bromate ion‐iodide ion‐L‐ascorbic acid clock reaction

The kinetics of the bromate ion‐iodide ion‐L‐ascorbic acid clock reaction was investigated as a function of temperature and pressure using stopped‐flow techniques. Kinetic results were obtained for the uncatalyzed as well as for the Mo(VI) and V(V) catalyzed reactions. While molybdenum catalyzes the BrO   3 − ‐I − reaction, vanadium catalyzes the direct oxidation of ascorbic acid by bromate ion. The corresponding rate laws and kinetic parameters are as follows. Uncatalyzed reaction: r 2 = k 2 [BrO   3 − ] [I − ][H + ] 2 , k 2 = 38.6 ± 2.0 dm 9 mol −3 s −1 , Δ H ‡ = 41.3 ± 4.2 kJmol −1 , Δ S ‡ = −75.9 ± 11.4 Jmol −1 K −1 , Δ V ‡ = −14.2 ± 2.9 cm 3 mol −1 . Molybdenum‐catalyzed reaction: r ′ 2 = k 2 [BrO   3 − ] [I − ] [H + ] 2 + k Mo [BrO   3 − ] [I − ] [ H + ] 2 [M 0 (VI)], k Mo = (2.9 ± 0.3)10 6 dm 12 mol −4 s −1 , Δ H ‡ = 27.2 ± 2.5 kJmol −1 , Δ S ‡ = −30.1 ± 4.5 Jmol −1 K −1 , Δ V ‡ = 14.2 ± 2.1 cm 3 mol −1 . Vanadium‐catalyzed reaction: r ′ 1 = k V [BrO   3 − ] [V(V)], k V = 9.1 ± 0.6 dm 3 mol −1 s −1 , Δ H ‡ = 61.4 ± 5.4 kJmol −1 , Δ S ‡ = −20.7 ± 3.1 Jmol −1 K −1 , Δ V ‡ = 5.2 ± 1.5 cm 3 mol −1 . On the basis of the results, mechanistic details of the BrO   3 − ‐I − reaction and the catalytic oxidation of ascorbic acid by BrO   3 −are elaborated. © 1995 John Wiley & Sons, Inc.