MECHANISM OF THE OXIDATION OF L-ASCORBIC-ACID BY THE PENTAAMMINEAQUACOBALT(III) ION IN AQUEOUS-SOLUTION

The kinetics of oxidation of L-ascorbic acid (H(2)A) by the mononuclea r complex [Co(NH3)(5)(OH2)] (ClO3)(3) have been studied spectrophotome trically at 490 nm. The mechanism involves single one-electron transfe rs involving the pentaamminehydroxo complex and the ascorbate anions ( HA(-) and A(2-)), subsequent formation of ascorbate radicals and > 90% Co-II. The appropriate rate law in the 7.00 less than or equal to pH less than or equal to 8.40 and 0.005 less than or equal to [A(T)] less than or equal to 0.05 mol dm(-3) ranges, has been established as: - d [complex]/dt = {(k(3)[H+] + k(4)K(a1))K-2[A](T)[complex](T)}/ {([H+ K-a1)([H+] + K-2)} The rate is slower at lower pH as the less importan t reaction k(1) (ROH(2)(3+) + HA(-) --> products) becomes dominant, wh ere R is (NH3)(5)Co-III. The more significant rate constant k(4) (ROH( 2+) + A(2-) --> products) was calculated at 25.7 degrees C as (2.7 +/- 0.5) x 10(2) dm(-3) mol(-1) s(-1), with Delta H# = 105 +/- 11 kJ mol( -1) and Delta S# = 93 +/- 45 J mol(-1) K-1.