The inorganic speciation of tin(II) in aqueous solution

This paper reports new voltammetric measurements on the interactions between tin(II) and the most important natural inorganic ligands, OH⁻, Cl⁻, F⁻, CO₃ ²⁻, SO₄ ²⁻ and PO₄ ³⁻. For a better understanding of tin(II) speciation, an analysis is also given of prior data on the same systems from the literature. The formation constants were determined at t=25°C in different ionic media and at different ionic strengths, specifically the following: Sn(OH)q (0.1⩽I/molL⁻¹⩽1.0 in NaNO₃), SnClᵣ and Sn(OH)Cl (0.1⩽I/molL⁻¹⩽2.3 in Na(NO₃, Cl)), Sn(SO₄)ᵣ (0.1⩽I/molL⁻¹⩽1.6 in Na(NO₃, SO₄)), SnHqCO₃ and SnHqPO₄ (0.15⩽I/molL⁻¹⩽1.0 in NaNO₃), where the subscripts r and q represent the stoichiometric coefficients. Concerning the SnFᵣ species, reliable literature values were considered (0.15⩽I/molL⁻¹⩽1.0 in NaClO₄). Fifteen voltammetric measurements were performed in synthetic seawater; the total seawater binding ability was evaluated by a model in which synthetic seawater is expressed as a single salt, BA. The formation of species between tin(II) and the anion of the marine salt (A) was also proposed, and the corresponding stability constants at different salinities (5⩽S⩽50) were reported. In addition, studies on the solubility of Sn(OH)₂₍ₛ₎ were carried out using voltammetry and light scattering measurements. The “extra-stability” of the mixed species with respect to the parent species was evaluated, in particular for Sn(OH)Cl and the corresponding species involving the anion of the marine salt (A). The dependence of the formation constants on ionic strength was analysed using extended Debye-Hückel and Specific ion Interaction Theory (SIT) type equations. Tin(II) speciation was also evaluated in different natural fluid conditions, where, in all cases, carbonate complexation was predominant, hampering the formation of hydrolytic species throughout the investigated pH range. Moreover, some formation enthalpy changes were calculated for the Sn(OH)⁺, Sn(OH)₂₍ₐq₎, Sn(OH)₂₍ₛ₎, Sn(OH)₃ ⁻, Sn₂(OH)₂ ²⁺, Sn₃(OH)₄ ²⁺, Sn(OH)Cl, SnCl⁺, SnCl₂ and SnCl₃ ⁻ species on the basis of the available literature stability constant values at different temperatures and using the empirical relationships reported in the literature. The ΔH values at t=25°C were positive in all cases except for the Sn(OH)₂₍ₛ₎ and Sn(OH)₃ ⁻ species, indicating an increase in the stability constant values with increasing temperature. This work represents an advance in the knowledge, understanding and modelling of the inorganic speciation of tin(II) in natural fluids, particularly for solutions containing chloride, fluoride, sulphate, carbonate and phosphate anions.