Modeling of Sn2+ speciation in aqueous solution, with particular reference to natural fluids

In contrast with organotin(IV) compounds, inorganic tin forms are less considered and
are not generally envisaged as biologically and/or environmentally relevant, though their
biological activity and their importance from the environmental point of view is documented.
Moreover, the formers are commonly formed by (bio)alkylation of inorganic tin in the
environment, and the extent of this (and other) process(es) is strictly dependent on the
speciation of the latter. Despite this, very few studies are reported on the speciation of
inorganic tin forms in aqueous solution and less are those referred to natural fluids. The
speciation studies in such systems are particularly difficult, since the knowledge of a lot of
thermodynamic parameters is requested for all the components involved in the formation
equilibria in those conditions. Moreover, inorganic tin forms are already characterized by
some particular chemico-physical solution properties that make their study more difficult than
other cations. The 2+ and 4+ oxidation states of tin are both fairly stable and both undergo
strong hydrolysis low pH (pH ~ 2 for Sn2+), and the precipitation of sparingly soluble species
already occurs in the acidic pH ranges even at millimolar concentrations. In the case of tin(II),
it can be also oxidized to tin(IV) by atmospheric oxygen, so that particular attention must be
paid during experiments for this cation. These facts are probably the most important causes of
the lack of reliable information on the solution chemistry of inorganic tin forms (and Sn2+ in
particular) and its interactions with various ligands. For this reason, recently our group has
undertaken a systematic study of the speciation of Sn2+ in aqueous solution, in the presence of
inorganic and organic ligands of biological and environmental importance [1-3]. Owing to the
objective impossibility of measuring all interactions of tin(II) with all components of real
systems, modeling studies are very useful. In this light, tin(II) behavior in natural waters and
biological fluids can be assessed by modeling its reactivity towards the main classes of
ligands usually present in natural systems (carboxylates, amines, S-donor ligands, phosphates,
etc.), of both natural origin or derived from anthropogenic activities, so that particular
attention was paid by our group to the study of the interactions of this cation with these
ligands, by means of potentiometric, voltammetric and calorimetric techniques. In this
contribution, several empirical relationships are presented, for a rough but immediate
estimation of various thermodynamic parameters (stability constants / free energies, formation
enthalpy changes, quantitative parameters for the sequestering ability, etc.) as a function of
different variables, including the structure of ligands and the nature of their functional groups.
References
[1] Cigala, R.M.; Crea, F.; De Stefano, C.; Lando, G.; Manfredi, G.; Sammartano, S.,
Quantitative study on the interaction of Sn2+ and Zn2+ with some phosphate ligands, in
aqueous solution at different ionic strengths. J. Mol. Liquids 2012, 165, 143-153, and
refs. therein.
[2] Cigala, R.M.; Crea, F.; De Stefano, C.; Lando, G.; Milea, D.; Sammartano, S.,
Thermodynamics of binary and ternary interactions in the tin(II)/phytate system in
aqueous solutions, in the presence of Cl- or F-. J. Chem. Thermodyn. 2012, 51, 88-96,
and refs. therein.
[3] Cigala, R.M.; Crea, F.; De Stefano, C.; Lando, G.; Sammartano, S., The Inorganic
Speciation of Tin(II) in Aqueous Solution. Geochim. Cosmochim. Acta 2012,
http://dx.doi.org/10.1016/j.gca.2012.03.029, and refs. therein.