Kinetic and mechanistic study of the reaction of 2,6-dichlorophenol-indophenol and cysteine

In this work the reaction of cysteine (H(2)Q) with 2,6-dichlorophenolindophenol (D) is studied kinetically in the pH range 2.5-9.0. Taking into consideration the distribution diagrams for the species H(3)Q(+), H(2)Q, HQ(-), Q(2-) for cysteine and DH(+)(2), DH, D(-) for 2,6-dichlorophenolindophenol the reaction rate constants k(i) for all possible combinations of the reacting species were determined. The maximum reactivity appears at pH 6.88 with an overall reaction constant k = 306 1.mole(-1).sec(-1) at 22 degrees . The effect of the concentrations of the reagents and the ionic strength on the reaction rate is also given. From Arrhenius plots an activation energy E(a) = 8.1 kcal/mole was calculated. Working curves for the determination of cysteine in aqueous solutions are also presented applying the reaction rate method. Finally the paper includes important analytical information for the calculation of the errors due to interference of cysteine by the kinetic determination of ascorbic acid, using its reaction with 2,6-dichlorophenolindophenol.