OXIDATION OF BIOLOGICAL SUBSTRATES BY CHROMIUM(VI) .1. MECHANISM OF THE OXIDATION OF L-ASCORBIC-ACID IN AQUEOUS-SOLUTION

The kinetics of oxidation Of L-ascorbic acid (H2A) by potassium chroma te has been studied under aerobic and anaerobic conditions over the ra nge 0.002 less-than-or-equal-to [A]T less-than-or-equal-to 0.040 mol d m-3, 3.50 less-than-or-equal-to pH less-than-or-equal-to 8.70, 17.8 le ss-than-or-equal-to T less-than-or-equal-to 35.1-degrees-C, 0.06 less- than-or-equal-to I less-than-or-equal-to 0.50 mol dm-3 (NaClO4) and 0. 05 less-than-or-equal-to [O2] less-than-or-equal-to 0.1 2 mmol dm-3. T he experimental rate constants obtained in the presence of oxygen are about 10 times smaller than those obtained in its absence. A mechanism involving the formation of a chromium(VI)-ascorbate-oxygen intermedia te is discussed in an attempt to explain this. The disappearance of ch romium(VI) under aerobic conditions follows the rate law (i) where k(f ) = (k1K(a1)[H+] + k2K(a1)K(c) + k3[H+]2)/(K(a1) + [H+])(K(c) + [H+]). -d[Cr(VI)]/dt = k(f)[A]T[Cr(VI)]T (i) At 25-degrees-C and I = 0.50 mo l dm-3 (NaClO4), k1 = 59.4 +/- 0.5, k2 = 0.10 +/- 0.02 and k3 = 115 +/ - 3 dm3 mol-1 s-1.