Inorganic Chemistry, Vol.33, No.9, 1881-1885, 1994
Mechanistic Study of the Reaction of L-Ascorbic-Acid with Hexacyanometalate(III) Ions of Iron(III), Ruthenium(III), and Osmium(III) in Aqueous Acidic Solution at Elevated Pressures
Oxidation reactions of L-ascorbic acid by three hexacyanometalate(III) ions in aqueous acidic media are studied at elevated pressures. Kinetic parameters characterizing two parallel paths involving ascorbic acid (H2Asc) and ascorbate ion (HAsc-), respectively, at 25-degrees-C are as follows : k1 = 0.366 +/- 0.016 M-1 s-1, k2 = (3.6 +/- 0.2) X 10(2) M-1 s-1, DELTAH-1double-dagger = 34 +/- 2 kJ mol-1, DELTAH-2double-dagger = 17 +/- 1 kJ mol-1, DELTAS1double-dagger = -136 +/- 15 J mol-I K-1, DELTAS2double-dagger = -141 +/- 16 J mol-1 K-1, DELTAV1double-dagger = -40 +/- 68 cm3 mol-1, and DELTAV2double-dagger = -14 +/- 3 cm3 mol-1 for hexacyanoferrate reduction; k, = (6.1 +/- 0.3) X 10(3) M-1 s-1, k2 = (4.26 +/- 0.22) X 10(6) M-1 s-1, DELTAH1double-dagger = 7 +/- 2 kJ mol-1, DELTAH-2double-dagger = 0 +/-2 kJ mol-1, DELTAS1double-dagger = -149 +/- 17 J mol-I K-1, DELTAS2double-dagger = -118 +/- 13 J mol-1 K-1, DELTAV1double-dagger = -26 +/- 18 cm3 mol-1, and DELTAV2double-dagger = -9 +/- 3 cm3 mol-1 for hexacyanoruthenate reduction; k1 = 26.7 +/- 1.3 M-1 s-1, k2 = (4.14 +/- 0.21) X 10(4) M-1 s-1, DELTAH1double-dagger = 30 +/- 2 kJ mol-1, DELTAH-2double-dagger = 9.7 +/- 0.6 kJ mol-1, DELTAS1double-dagger = -117 +/- 13 J mol-1 K-1, DELTAS2double-dagger = -124 +/- 15 J mol-I K-1, DELTAV1double-dagger = -90 +/- 75 cm3 mol-1, and DELTAV2double-dagger = -10 +/- 3 cm3 mol-1 for hexacyanoosmate(III) reduction. The reaction mechanism was discussed on the basis of the volume profiles of the redox reactions between ascorbic acid and hexacyanometalate ions.
Keywords:ELECTRON-TRANSFER REACTIONS;SELF-EXCHANGE REACTION;PERCHLORATE-MEDIA;REACTION VOLUMES;METAL-COMPLEXES;KINETICS;OXIDATION;TRIS(1;10-PHENANTHROLINE)IRON(III);REDUCTION;SOLVENTS