Kinetics of the interaction of indium(III) with 8-quinolinol-5-sulfonic acid and with sulfate

The kinetics and equilibria of indium(iii) binding to 8-quinolinol-5-sulfon ic acid (HQSA) have been investigated in acidic aqueous solution at 0.2m io nic strength and 25 degreesC by stopped-flow, absorption and fluorescence s pectrometric methods. Absorption and fluorescence spectrometry revealed tha t a monoprotonated MHL3+ complex is formed in addition to the ML2+ chelate. The stability constants of the chelate (logK(ML) = 6.53), of the monoproto nated complex (logK(MHL)=3.51) 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+ (k(1) = (3.21 +/-0.04) X 10(2) M-1 S-1), M3+ with HL (k(2) = (6.52 +/-0.04) x 10(4) M-1 s(-1)) an d MOH2+ with HL (k(3)= (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 e xchange experiments. This behaviour has been explained with the assumption that the ligand is distributed between two forms (neutral and zwitterion) o f which only the neutral form is reactive. The rate of complex formation be tween In3+ and SO42- ion has been measured by the temperature-jump method w ith Tropaeolin 00 as the indicator. The second-order rate constant of the b inding process is 5.1 x 10(4)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 no rmal. This finding is interpreted by the hypothesis that SO42- forms a chel ate with In3+ for which ring closure is the rate-determining step.