Quantitative study on the interaction of Sn2+ and Zn2+ with some phosphate ligands, in aqueous solution at different ionic strengths

Tin(II) interaction with different phosphate ligands, namely phosphate (PO4), pyrophosphate (PP), tripolyphosphate (TPP), monofluorophosphate (MFP) and adenosine-5′-triphosphate (ATP), was studied at T=298.15 K by potentiometry and voltammetry at different ionic strengths (0.15≤I/mol L−1≤1.00) in NaNO3. We also compared our results with those experimentally determined for the Zn/PO4 and Zn/TPP systems. As concerns the Zn/PP, the Zn/ATP and the Zn/MFP systems, we performed a critical literature analysis. In all cases the stability constants observed for the Sn/L species resulted to be higher with respect to the analogous Zn/L ones. The rough correlation (valid for the ML species) log KML (Sn)=3.01·log KML (Zn)−8.13 was obtained from the stability data of the complexes of these cations. In addition, the stability trend found for a given metal cation was: PP~PO4>>TPP>>MFP~ATP. The ionic strength dependence of the stability constants was studied by the extended Debye–Hückel and the SIT (Specific ion Interaction Theory) equations. Speciation and sequestration studies were also performed, and pL0.5 values (i.e., the total ligand concentration necessary to bind 50% of cation present in trace) were calculated for all the systems at different pH and ionic strengths. In this case, as an example at pH=7.0 and I=0.15mol L−1, the sequestration trend was: PO4>PP~MFP>>TPP>ATP. The dependence of pL0.5 values on pH and ionic strength was modeled by means of two empirical relationships.