Thermodynamic parameters for the interaction between etidronic acid and inorganic and organic mercury(II)

Thermodynamic parameters of etidronic acid [(1-Hydroxy-1,1-ethanediyl)bis(phosphonic acid), HEDPA] interaction with Hg2+ and CH3Hg+ were determined in NaCl aqueous solution at different ionic strength values (0.1 I 1 molL1) and at T = 298.15 K. For Hg2+ system, ML2, MLH2 0 , MLH3 + and ML(OH)3 species were obtained. In presence of CH3Hg+, ML3, MLH2, MLH2 , MLH3 0 and the dinuclear M2LH and M2LH2 0 species were determined with formation constant values lower respect to the Hg2+ species. As an example, logb = 7.98 and 15.61 were obtained for (CH3Hg)L3 and HgL2 species, respectively (at I 0.1 molL1 and T = 298.15 K). The dependence of the stability constants on ionic strength, studied through a Debye-Hückel type equation, is also reported. To enrich the thermodynamic study, the enthalpy values of the complex species were obtained by calorimetric titrations at I = 0.1 molL1 in NaCl and T = 298.15 K. Finally, the sequestering ability of HEDPA towards Hg2+ and CH3Hg+ was calculated by means of an empirical parameter, known as pL0.5, under blood (I = 0.1 molL1, pH = 7.4, T = 310.15 K) and sea water (I = 0.7 molL1, pH = 8.1, T = 298.15 K) conditions. pL0.5 represents the total ligand concentration required to sequester the 50% of a metal cation present as traces.