REACTION OF [FE-2(CN)(10)](4-) WITH L-ASCORBIC-ACID

The oxidation of L-ascorbic acid by the iron(III) dimeric complex [Fe- 2(CN)(10)](4-) in acidic aqueous solution at an ionic strength of 1.0 mol dm(-3) (NaClO4) has been studied by stopped-flow spectrophotometry . The overall reaction, which entails reduction of both iron(lll) cent res followed by aquation resulting in the formation of 2 mol of [Fe(CN )(5)(OH2)](3-), takes place in three stages. The first is a one-electr on reduction to form the mixed-valence iron(III, II) dimer followed by a second one-electron reduction of the other iron(III) centre to form the iron(II) dimer. The third stage which occurs with a very small ab sorbance change was not studied. The mechanisms for both stages of the reaction are similar and involve formation of an ion triplet [Fe-2(CN )(10)](n-). M(+). A(p-) where n = 4 or 5 and A(p-) is the reacting asc orbate species with p = 1 or 2. The rate law was found to be of the fo rm (i) where [A](T) is the total concentration of added ascorbate, K-1 and K-2 the acid dissociation constants Rate = k'K-1[H+] + k''K1K2/K1 K2 + K-1[H+] + [H+](2) [A](T)[complex] (i) of ascorbic acid and k' and k'' the pseudo-second-order rate constants. They were found to be 6.4 4 x 10(3). 2.48 x 10(9) and 1.25 x 10(3), 2.98 x 10(8) dm(3) mol(-1) s (-1) at 19 degrees C for HA(-). and A(2-) for the first and second sta ges respectively. The reaction is affected by alkali-metal cations. in creasing rate with increasing size of the metal ions: Li+ < Na+ < K+.