Speciation of tin(II) in aqueous solution: thermodynamic and spectroscopic study of simple and mixed hydroxocarboxylate complexes

his contribution reports the results of potentiometric and Mossbauer investigations on the formation, stability, and structure of binary and ternary mono- and binuclear complexes of Sn2+ with three hydroxocarboxylic ligands (namely L = tartrate, malate, and citrate) and chloride at T = 298.15 K in different ionic media and ionic strengths (0.15 and 1.00 mol dm(-3) in NaCl(aq) and 1.00 mol dm(-3) in NaNO3(aq)). The stability constants of various simple Sn (i) H (j) L (k) ((2i+j-kz)) and mixed Sn (i) H (j) L (k) Cl (l) ((2i+j-kz-l)) species obtained in the different experimental conditions are reported, and various speciation diagrams of the simple and mixed systems are shown in different conditions. The sequestering ability of the three ligands toward Sn2+ was assessed by means of the calculation of the pL(0.5) parameter at different ionic strengths and in the different ionic media. The effect of the chloride anion on the stability of various species and on the sequestering ability of the investigated ligands toward Sn2+ was also evaluated. The structural results obtained by Mossbauer spectroscopy supported the speciation schemes obtained by the potentiometric investigations and indicated the possible trigonal bipyramidal arrangement of various Sn2+/ligand species. Measurements at different temperatures were also performed in the case of citrate, in order to estimate the temperature dependence of the stability of these species. A modified version of the computer program STACO is briefly presented here for the first time and allows the simultaneous determination of the stability constants and the enthalpy changes of various species from potentiometric titrations at different temperatures.