KINETIC-STUDY OF THE REACTION BETWEEN FE2-OXALATE AND 2,6-DICHLOROPHENOLINDOPHENOL()

Ascorbic acid present in natural products is often analyzed by its red ucing effect on 2,6-dichlorophenolindophenol (DCPI). In case of coexis tence of Fe(II) in natural samples and if oxalic acid is used for thei r extraction and stabilization, a significant interference is observed . The reaction of Fe2+ with 2,6-dichlorophenolindophenol (DCPI) in aci dic solutions in the presence or absence of oxalate ions was investiga ted. The reaction was studied with a Stopped-Flow Spectrophotometer (S .F.S) equiped with a data acquisition system, which allows the determi nation of initial rates and reaction rate constants. Results for the e ffect of the concentrations of all the reagents of the reaction are pr esented. During the experimental work all concentrations of the reagen ts were kept in such levels to make the reaction first-order in respec t of DCPI. An interesting finding is, that, although Fe2+ and C2O42- i ons do not react separately with DCPI (or react very slowly), they rea ct very rapidly in mixtures and show a strong synergetic effect. The e xperimental data At with the mechanism of the formation of intermediat es FeC2O4 and [Fe(C2O4)(2)](2-). The proposed rate law agrees with the above mechanism and the experimental results. The value k(c) = (1.6 /- 0.2) X 10(5) M-1.s(-1) was calculated for the second-order rate con stant of the reaction of FeC2O4 and DCPI. (C) 1998 John Wiley & Sons, Inc.