MECHANISM OF COMPLEX-FORMATION - KINETICS AND EQUILIBRIA OF THE GALLIUM(III)-PYROCATECHOL VIOLET SYSTEM

The kinetics and equilibria of the reaction between Ga(III) and pyrocatechol violet (PCV) have been investigated in aqueous solution within an [H+] range between 0.01 and 0.2 M. Absorbance data show that two complexes, respectively 1:1 and 2:1 of Ga(III) to PCV, are formed under the experimental conditions of this work. Formation of the complex GaH2L+ from Ga3+ and PCV in the form H3L- involves loss of one proton whereas formation of Ga2L2+ involves loss of three protons, The values of the first dissociation constant of PCV and the formation constants of the 1:1 and 2:1 complexes have been evaluated by computer analysis of spectral data. The values at 25 °C and I ⋍ 0.2 M are respectively QA = 0.72 ± 0.02 M, Q11 = 88 ± 8, and Q21 = 5.5 ± 0.5 M. The kinetic experiments were carried out by the stopped-flow technique. The results are interpreted in terms of two parallel reaction paths involving reactions of H3L- with Ga3+ and GaOH2+. The rate constant of the pathway via Ga3+ and H3L~ has a much lower constant (klA = 65 ± 9 M-1 s-1) than the rate constant of the pathway via GaOH2+ and H3L- (kIIA = (8.7 ± 0.4) × 103 M-1 s-1), confirming the enhanced reactivity of the hydrolyzed cation. The activation parameters have been determined. The deviations from the Eigen mechanism are discussed. © 1979, American Chemical Society. All rights reserved.