Kinetic study of the reaction between Fe2+-oxalate and 2,6-dichlorophenolindophenol

✍ Scribed by M. I. Karayannis; S. M. Tzouwara-Karayanni; S. R. Crouch

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 168 KB
• Volume: 30
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/(sici)1097-4601(1998)30:12<941::aid-kin9>3.0.co;2-#

✦ Synopsis

Ascorbic acid present in natural products is often analyzed by its reducing effect on 2,6-dichlorophenolindophenol (DCPI). In case of coexistence of Fe(II) in natural samples and if oxalic acid is used for their extraction and stabilization, a significant interference is observed. The reaction of Fe 2ϩ with 2,6-dichlorophenolindophenol (DCPI) in acidic solutions in the presence or absence of oxalate ions was investigated. The reaction was studied with a Stopped-Flow Spectrophotometer (S.F.S) equiped with a data acquisition system, which allows the determination of initial rates and reaction rate constants. Results for the effect of the concentrations of all the reagents of the reaction are presented. During the experimental work all concentrations of the reagents were kept in such levels to make the reaction first-order in respect of DCPI. An interesting finding is, that, although Fe 2ϩ and C 2 O 4 2Ϫ ions do not react separately with DCPI (or react very slowly), they react very rapidly in mixtures and show a strong synergetic effect. The experimental data fit with the mechanism of the formation of intermediates FeC 2 O 4 and [Fe(C 2 O 4 ) 2 ] 2Ϫ . The proposed rate law agrees with the above mechanism and the experimental results. The value was calculated 5 Ϫ1 Ϫ1 k ϭ (1.6 Ϯ 0.2) ϫ 10 M •s c for the second-order rate constant of the reaction of FeC 2 O 4 and DCPI.