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.

A temperature-jump apparatus with laser light sources was used to investigate the binding mechanism of indium(III) to pyrocathecol violet (3,3‘,4‘- trihydroxyfuchsone-2-sulfonic acid, whose neutral form is denoted as H4L). Spectrophotometric measurements revealed that for [H+] > 0.015 M, the main bound species are MH3L2+ and MH2L+ whereas for [H+] < 0.015 M, the main bound species is M2L2+. Two relaxation effects were observed. The fast relaxation is associated mainly with the reaction steps M3+ + H3L- ↔ MH3L2+ (k 2 = 1.2 × 106 M-1 s-1, k - 2 = 1.5 × 105 s-1) and MOH2+ + H3L- ↔ MH2L+ (k 3 = 4.4 × 107 M-1 s-1, k - 3 = 7.4 × 102 s-1). The rate constant k 2 decreases, whereas k - 2 increases, with increasing ionic strength. Addition of Na2SO4 results in a large reduction of the fast relaxation time that is ascribed to the binding of indium(III) to sulfate. Analysis of the relaxation time dependence on the sulfate concentration provides the binding constant of the indium−sulfate complex (K = 1.5 × 102 M-1). The activation parameters for the step M3+ + H3L- ↔ MH3L2+ were obtained as ΔH 1f ⧧ = 25.3 kJ mol-1, ΔS 1f ⧧ = −6.2 J mol-1 K-1, ΔH 1d ⧧ = 13.4 kJ mol-1, ΔS 1d ⧧ = −57 J mol-1 K-1. The enthalpy change of the just described reaction step was measured by spectrophotometry at different temperatures and by the relaxation amplitudes, the average value being ΔH 1 0 = 11.9 kJ mol-1. The slow effect is associated with a complexation path leading to formation of M2L+. The amplitude analysis of the slow relaxation gives ΔH 2 0 = 7.6 kJ mol-1. A comparison of this work with other studies on complexation with indium(III) suggests that In(OH)2+ reacts according to the I d mechanism, whereas the mode of activation of In(H2O)6 3+ seems to be less definite because a clear indication about the dependence (or independence) of the rates on the ligand basicity does not emerge. This observation and the slightly negative value of ΔS 1f ⧧ hints at the possibility that indium(III) undergoes complexation by a concerted mechanism.