Thermodynamics of binary and ternary interactions in the tin(II)/phytate system in aqueous solutions, in the presence of Cl− or F−

An extensive study of the tin(II)/phytate (Phy) system was carried out in NaNO3(aq), at different ionic strengths (0.10 ⩽ I/mol · L−1 ⩽ 1.00) and temperatures (278.15 ⩽ T/K ⩽ 328.15), by potentiometric and voltammetric techniques. The stability and formation enthalpy changes of six SnHqPhy species were determined. To better characterise this system, some potentiometric titrations were also carried out in mixed ionic media (NaNO3(aq) + NaCl(aq) and NaNO3(aq) + NaF(aq)) at total ionic strength I = 1.00 mol · L−1. The formation of some ternary mixed SnHqPhyCl and SnHqPhyF species (charges omitted for simplicity) was found. The formation enthalpies of the complex species were calculated, at I = 0.40 mol · L−1 in NaNO3(aq), by the dependence of stability constants on temperature obtained by potentiometric titrations, in the range 278.15 ⩽ T/K ⩽ 328.15. The complex formation process is endothermic, and the main contribution to tin(II) complexation by phytate is entropic in nature. For example, for the SnPhy species we have, at T = 298.15 K and I = 0.40 mol · L−1 in NaNO3(aq): ΔH = 57.7 ± 2.8 kJ mol · L−1, ΔG = −99.9 ± 1.7 kJ mol · L−1, and TΔS = 158 ± 3 kJ mol · L−1. The ionic strength dependence of the formation constants of the simple tin(II)/phytate species, was modelled by the Debye–Hückel and the SIT approaches. The sequestering ability of phytate towards tin(II) was evaluated by calculating the pL0.5 values (i.e., the total ligand concentration necessary to bind 50% of cation present in trace) at different ionic strengths, ionic media, and pH. The sequestering ability increases with increasing the pH, whilst it decreases with increasing the ionic strength (the same behaviour shown by the stability constants). Moreover, taking into account the different sequestering ability of phytate towards tin(II) in the different ionic media, the trend: pL0.5 = 5.70 (in NaNO3(aq) + NaF(aq)) > pL0.5 = 5.16 (in NaNO3(aq) + NaCl(aq)) > pL0.5 = 4.86 (in NaNO3(aq)) was observed at pH 8.1 and I = 1.00 mol · L−1. This is due to the presence of a second ligand (Cl− or F−) that stabilizes the complex species with the formation of ternary complex species. Some empirical relationships were also found.

Graphical abstractSequestration diagrams of the tin(II)/phytate system, at pH 8.1 and I = 1.00 mol · L−1 for the three considered ionic media. As expected, the pL0.5 in the mixed NaNO3(aq) + NaF(aq) is higher than the corresponding value in NaNO3(aq) + NaCl(aq), in turn higher than the value in NaNO3(aq).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The Sn2+/phytate complexes were studied with potentiometry and voltammetry. ► The enthalpy changes were determined from temperature dependence. ► In the complexation process, the entropic contribution is predominant. ► The formation of SnHqPhyX (X = Cl−, F−) species was found in NaCl and NaF media. ► The sequestering ability of phytate vs. Sn2+ is higher in NaF than in other media.