title: Kinetics of the interaction of indium(III) with 8-quinolinol-5-sulfonic acid and with sulfate.
abstract: The kinetics and equilibria of indium(III) binding to 8-quinolinol-5-sulfonic acid (HQSA) have been investigated in acidic aqueous solution at 0.2 M ionic strength and 25 C by stopped-flow, absorption and fluorescence spectrometric methods. Absorption and fluorescence spectrometry revealed that a monoprotonated MHL3+ complex is formed in addition to the ML2+ chelate. The stability constants of the chelate (log K(ML) 6.53). of the monoprotonated complex (logK(MHL = 3.5l) and its acid dissociation constant (pK(C2) = 1.4) have been determined. Stopped-flow measurements indicate three reaction paths that involve the interaction of M3+ with H2L+ (k1 =(3.21 +/- 0.04) x 10(2) m(-1) s(-1)), M3+ with HL (k2=(6.52 +/- 0.04) x 10(4) M(-1)s(-1)) and MOH2+ with HL (k3 = (1.60 +/- 0.08) x 10(6) M(-1) s(-1)), respectively. The reactivity of In3+ toward the uncharged form of HQSA has been found to be approximately two orders of magnitude less than expected based upon water exchange experiments. This behaviour has been explained with the assumption that the ligand is distributed between two forms (neutral and zwitterion) of which only the neutral form is reactive. The rate of complex formation between In3+ and SO4(2-) ion has been measured by the temperature-jump method with Tropaeolin 00 as the indicator. The second-order rate constant of the binding process is 5.1 x 10(4) M s(-1). This quantity yields a value for the first-order rate constant of 570 s(-1) for ligand penetration into the In3+ coordination shell, approximately two orders of magnitude less than normal. This finding is interpreted by the hypothesis that SO4(2-) forms a chelate with In3+ for which ring closure is the rate-determining step.
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Ricciu
authors.forename: F
authors.initials: F
authors.lastname: Secco
authors.forename: M
authors.initials: M
authors.lastname: Venturini
authors.forename: B
authors.initials: B
authors.lastname: García
authors.forename: J M
authors.initials: JM
authors.lastname: Leali