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

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 H-1-NMR spectroscopy. Equilibria are studied at T = 25 degrees 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 ML1-z and MLH2-z species for all systems, and M(tla)(OH)(2-), M(mpa)(OH)(2-) and M(cys)H-2(+) species. Formation constant values show a very high stability (for ML1-z species, log beta ranges from 16.57 to 17.90). For the CH3Hg+-tla and -cys systems, H-1-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.