The equilibria and kinetics of the interaction of In(III) with Alizarin Red S (ALZS) and Methyl Thymol Blue (MTB) dyes in aqueous solution were investigated. Spectrophotometric titrations at different pHs (1.8–2.7 range) revealed that, concerning the In-ALZS system, under conditions of metal excess the prevailing complex is ML, while stopped flow experiments have shown that, in the investigated pH range the kinetics are monophasic although the ML complex is formed through different paths involving both In(H2O)63+ and In(H2O)5(OH)2+ as active species reacting with the monoprotonated ligand (HL). The In-MTB system was studied with the same methodology in the pH range 1.0–2.0. The complex (MH3L) prevails although the above metal species react with differently protonated forms of the ligand. The In-MTB system displays a biphasic kinetic behavior involving a network of reaction steps, where fast pre-equilibria related to metal coordination at the carboxylates/nitrogen site, precedes slow coordination at the phenol/phenol or phenol/quinone sites. The key role in the reaction mechanism is played by the ligand features rather than by the solvent/ligand substitution process.
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