Binding ability of sodium catechol disulfonate (tiron) towards 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)2Sn2+, and (CH3)3Sn+ cations is reported in aqueous NaCl solution with ionic strength in the range (0 ≤ I ≤ 1) mol·L−1 and at a temperature of 298.15 K. In the presence of tiron, CH3Hg+ and (CH3)3Sn+ form only the two species ML and MLH; Hg2+ forms ML, MLH, and ML(OH) species; and (CH3)2Sn2+ forms ML, MLH, and ML2 species. The interactions of tiron with Hg2+ and (CH3)2Sn2+ are stronger than those with CH3Hg+ and (CH3)3Sn+. As an example, at the ionic strength I = 0.1 mol·L−1, equilibrium constants for the formation of ML species are log β = 18.87, 17.076, 10.39, and 7.55, for M = Hg2+, (CH3)2Sn2+, CH3Hg+, and (CH3)3Sn+, respectively. On the basis of the speciation models proposed, the sequestering ability of tiron toward Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ 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)2Sn2+ (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)3Sn+ (at the 10−1 mol·L−1 level).