Binding Ability of Sodium Catechol Disulfonate (Tiron) toward Hg2+, CH3Hg+, (CH3)3Sn+, and (CH3)2Sn2+ Cations

A potentiometric study on the interactions between catechol disulfonate (disodium 4,5-dihydroxybenzene-1,3-disulfonate, tiron) and Hg2+, CH3Hg+, (CH3)(2)Sn2+, and (CH3)(3)Sn+ cations is reported in aqueous NaCl solution with ionic strength in the range (0 <= I <= I) mol.L-1 and at a temperature of 298.15 K. In the presence of tiron, CH3Hg+ and (CH3)(3)Sn+ form only the two species ML and MLH; Hg2+ forms ML, MLH, and ML(OH) species; and (CH3)(2)Sn2+ forms ML, MLH, and ML2 species. The interactions of tiron with Hg2+ and (CH3)(2)Sn2+ are stronger than those with CH3H+ and (CH3)(3)Sn2+. As an example, at the ionic strength I = 0.1 mol-L-1, equilibrium constants for the formation of ML species are log beta = 18.87, 17.076, 10.39, and 7.55, for M = Hg2+, (CH3)(2)Sn2+, CH3Hg+, and (CH3)(3)Sn+, respectively. On the basis of the speciation models proposed, the sequestering ability of tiron toward Hg2+, CH3Hg+, (CH3)(2)Sn2+, and (CH3)(3)Sn+ was quantitatively evaluated by determining the concentration of the ligand able to complex half of the metal ion fraction. In the range 5 <= pH <= 8, the sequestering ability is high toward Hg2+ and (CH3)(2)Sn2+ (at the (10(-11) to 10(-6)) mol.L-1 level), fairly low toward CH3Hg+ (at the 10(-3) mol.L-1 level), and very low toward (CH3)(3)Sn+ (at the 10(-1) mol.L-1 level).