Kinetics and mechanism of autocatalyzed reaction between phenyl hydrazine and toluidine blue in aqueous solution

The kinetics and mechanism of reduction of aqueous toluidine blue (TB+) by phenyl hydrazine (Pz), which exhibits nonlinear behavior, is studied spectr ophotometrically at 630 nm. Typical kinetic curves exhibited autocatalytic characteristics. The role of H+ as an autocatalyst is established. Rate con stants for the uncatalyzed and acid catalyzed reactions are determined. The forward rate constants for the uncatalyzed and acid catalyzed reactions we re 1.4 x 10(-2) M-1 s(-l) and 60 M-l s(-1). Reaction products are toluidine white, phenol, and an azo dye. From the stoichiometric ratios, the major r eaction is Pz + 2 TB+ + H2O = PhOH + 2 TBH + 2 H+ + N-2. The rate expressio n and a detailed 12-step reaction mechanism supported by simulations are pr oposed. (C) 1999 John Wiley & Sons, Inc.