Potentiometric, Calorimetric, and 1H NMR Investigation on Hg2+-Mercaptocarboxylate Interaction in Aqueous Solution

The interaction between Hg2+ and three different mercaptocarboxylic acids (thiolactic, 3-mercaptopropanoic, and thiomalic acids) was studied in aqueous solution by potentiometry, calorimetry, and for thiolactic acid H-1 NMR spectroscopy. Potentiometric measurements were performed in NaNO3 in the presence of a small amount of iodide (NaI) as a competitive ligand. For all systems, the formation of the MLH, ML, and ML2 species was found, with very high values of formation constants (for the ML species, log beta ranges from 32.10 to 35.10 at I = 0.1 mol . L-1). For the system containing thiolactic acid, the H-1 NMR spectroscopy fully supported the chemical model proposed, providing fairly similar formation constant values to those obtained by potentiometry. In confirmation of the very high stabilities of Hg2+-mercaptocarboxylate species, the speciation diagrams show that the metal fractions of the complex species are very high over a wide pH range, suppressing almost completely the hydrolysis of the cation. All enthalpy values are strongly exothermic, as typical for soft soft interactions, where the contribution to the Gibbs energy of complexation is mainly enthalpic in nature. The sequestering ability of He2+ is very high even at physiological pH, and it was analyzed and compared at different ionic strengths, pH, and temperatures.