Methylmercury(ii)-sulfur containing ligand interactions: a potentiometric, calorimetric and 1H-NMR study in aqueous solution

Abstract

The interactions between CH₃Hg⁺ and four sulfur containing ligands [2-mercaptopropanoic or thiolactic acid (tla), 3-mercaptopropanoic acid (mpa), 2-mercaptosuccinic or thiomalic acid (tma) and L-cysteine (cys)] are studied in aqueous solution by potentiometry, calorimetry and ¹H-NMR spectroscopy. Equilibria are studied at T = 25 °C and I = 0.1 mol L⁻¹ using NaNO₃ as an ionic medium in the presence, for potentiometric measurements, of iodide (NaI) as a competitive ligand. Speciation models obtained for the different systems are quite similar, with the formation of ML^1−z and MLH^2−z species for all systems, and M(tla)(OH)²⁻, M(mpa)(OH)²⁻ and M(cys)H₂⁺ species. Formation constant values show a very high stability (for ML^1−z species, log β ranges from 16.57 to 17.90). For the CH₃Hg⁺-tla and -cys systems, ¹H-NMR spectroscopy fully supported the chemical model proposed, providing fairly similar formation constant values to those obtained by potentiometry. As expected for typically soft–soft interactions, the enthalpy values are strongly exothermic, and the contribution to the complexation free energy is mainly of enthalpic origin. Speciation diagrams show that, in a wide pH range, most of the metal fraction is present as complex species.