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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 Mionic 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 KML=6.53), of the monoprotonated complex (log KMHL=3.51) and its acid dissociation constant (pKC2=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)×102M−1s−1), M3+with HL (k2=(6.52±0.04)×104M−1s−1) and MOH2+with HL (k3=(1.60±0.08)×106M−1s−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 SO42−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 5.1×104M−1s−1. This quantity yields a value for the first‐order rate constant of 570 s−1for ligand penetration into the In3+coordination shell, approximately two orders of magnitude less than normal. This finding is interpreted by the hypothesis that SO42−forms a chelate with In3+for which ring closure is the rate‐determining step.

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 KML=6.53), of the monoprotonated complex (log KMHL=3.51) and its acid dissociation constant (pKC2=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)×10² M⁻¹ s⁻¹), M³⁺ with HL (k2=(6.52±0.04)×10⁴ M⁻¹ s⁻¹) and MOH²⁺ with HL (k3=(1.60±0.08)×10⁶ 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 SO4²⁻ 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 5.1×10⁴ M⁻¹ 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 SO4²⁻ forms a chelate with In³⁺ for which ring closure is the rate-determining step. Supporting information (fractions of the free and bound forms of the ligand for different hydrogen ion concentrations in the indium(III)–HQSA system) for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2001/f3164_s.pdf or from the author.

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⁺ (k2 = 1.2 × 10⁶ M^-1 s^-1, k-2 = 1.5 × 10⁵ s^-1) and MOH²⁺ + H3L^- ↔ MH2L⁺ (k3 = 4.4 × 10⁷ M^-1 s^-1, k-3 = 7.4 × 10² s^-1). The rate constant k2 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 ΔH1f^&thermod; = 25.3 kJ mol^-1, ΔS1f^&thermod; = −6.2 J mol^-1 K^-1, ΔH1d^&thermod; = 13.4 kJ mol^-1, ΔS1d^&thermod; = −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 ΔH1⁰ = 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 ΔH2⁰ = 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 Id mechanism, whereas the mode of activation of In(H2O)6³⁺ 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 ΔS1f^&thermod; hints at the possibility that indium(III) undergoes complexation by a concerted mechanism.