Published: March 22, 2011 r2011 American Chemical Society 1995 dx.doi.org/10.1021/je101007n | J. Chem. Eng. Data 2011, 56, 1995–2004 ARTICLE pubs.acs.org/jced Potentiometric, Calorimetric, and 1 H NMR Investigation on Hg 2þ - Mercaptocarboxylate Interaction in Aqueous Solution Paola Cardiano, Daniela Cucinotta, Claudia Foti, Ottavia Giuffr  e,* and Silvio Sammartano Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universit  a di Messina, Viale Ferdinando Stagno D'Alcontres 31, 98166 Messina, Italy b S Supporting Information ABSTRACT: The interaction between Hg 2þ and three different mercaptocarboxylic acids (thiolactic, 3-mercaptopropanoic, and thiomalic acids) was studied in aqueous solution by potentiometry, calorimetry, and for thiolactic acid 1 H NMR spectroscopy. Potentiometric measurements were performed in NaNO 3 in the presence of a small amount of iodide (NaI) as a competitive ligand. For all systems, the formation of the MLH, ML, and ML 2 species was found, with very high values of formation constants (for the ML species, log β ranges from 32.10 to 35.10 at I = 0.1 mol 3 L À1 ). For the system containing thiolactic acid, the 1 H 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 Hg 2þ -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 softÀsoft interactions, where the contribution to the Gibbs energy of complexation is mainly enthalpic in nature. The sequestering ability of Hg 2þ is very high even at physiological pH, and it was analyzed and compared at different ionic strengths, pH, and temperatures. ’ INTRODUCTION Sustained interest in the coordination chemistry of Hg 2þ arises from its inherent toxicity. Mercury is extremely toxic to living organisms and, owing to its bioaccumulation in the food chain, is dangerous to higher organisms. 1 For humans its toxicity is mainly directed toward the central nervous system and the kidney 2,3 and derives from the affinity of Hg 2þ compounds for cysteinyl sulfur residues. Clinical chelation therapy of mercury poisoning generally uses thiol compounds such as dimercapto- succinic acid and dimercaptopropane sulfonic acid. 4,5 These ligands are of special interest as sequestering agents for metal ions, since they have two mercapto groups available for chelate ring formation. 6 The very high affinity of Hg 2þ for sulfur donors is well-known, also suggesting the use of sulfur-containing ligands for Hg removal from wastewaters. Usually, in natural waters, mercury has a low abundance in geological formations, and its concentration is less than 0.1 μg 3 L À1 . Ocean waters have an average mercury concentration of 0.03 μg 3 L À1 . Most waters with greater than 0.1 μg 3 L À1 mercury have mercury pollution. 7 One promising technique for the mercuric ion remediation from groundwater is to trap it using complexing ligands, such as thiols. As coordinating agents for mercury, they are covalently linked to a high surface area support. 8,9 Thiols are naturally occurring ligands because they can be formed via a variety of pathways. Their concentration in seawater and marine sediment interstitial waters is commonly reported to range from nanomolar to millimolar. 10 The main contribution of analytical chemistry to the fields of heavy metal intoxication treatment and mercury remediation of natural waters is the determination of the forma- tion constants of the species between the metal ions and chelating agents and the definition of speciation models of these systems, to compare the strength and characteristics of the complexes formed. 1 The literature reports only a few data concerning Hg 2þ - mercaptocarboxylate systems. This shortage of data is certainly attributable to the considerable difficulty in determining experi- mentally too high values of formation constants. Because of the very high stability of species, potentiometric measurements require the presence of an auxiliary ligand able to compete for the metal ion. In this paper we tested the use of iodide as the competitive ligand, and for one system we used two different instrumental techniques to confirm the speciation model, the magnitude of the formation constants obtained by potentiome- try, and therefore the method of the use of iodide as a competitive ligand in the potentiometric measurements. This work represents the continuation of our speciation study on Hg 2þ -organic ligand systems. In the first part of this short series the ligand classes considered were: (i) O-donor ligands, such as polycarboxylates and polyelectrolytes; (ii) N-donor ligands, such as polyamines and polyelectrolytes; and (iii) amino acids. 11 The missing link is constituted by S-donor ligands. 2-Mercaptopropanoic, 3-mercaptopropanoic, and 2-mercapto- succinic acids (Chart 1) were chosen because they are S-donor ligands, also containing O-donor groups. For this reason, in this work thermodynamic formation parameters and speciation models in aqueous solution over a wide pH range, between Hg 2þ and three mercaptocarboxylic acids, are discussed on the basis of potentiometric and calorimetric results. Equilibria are studied at I = 0.1 mol 3 L À1 , using as ionic medium NaNO 3 in the Received: October 7, 2010 Accepted: March 7, 2011