Quantitative study on the interaction of Sn 2+ and Zn 2+ with some phosphate ligands, in aqueous solution at different ionic strengths Rosalia Maria Cigala, Francesco Crea, Concetta De Stefano, Gabriele Lando, Giuseppe Manfredi, Silvio Sammartano ⁎ Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Università di Messina, viale Ferdinando Stagno d'Alcontres, 31, I-98166 Messina (Vill. S. Agata), Italy abstract article info Article history: Received 28 June 2011 Received in revised form 24 October 2011 Accepted 9 November 2011 Available online 19 November 2011 Keywords: Tin(II) Phosphates Formation constants Ionic strength Speciation Sequestration Tin(II) interaction with different phosphate ligands, namely phosphate (PO 4 ), pyrophosphate (PP), tripolypho- sphate (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 NaNO 3 . We also compared our results with those experimentally determined for the Zn/PO 4 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 K ML (Sn) = 3.01·log K ML (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 ~ PO 4 >> 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 pL 0.5 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 = 0.15 mol L -1 , the sequestration trend was: PO 4 > PP ~ MFP >> TPP > ATP. The dependence of pL 0.5 values on pH and ionic strength was modeled by means of two empirical relationships. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Natural waters and biological fluids are multi-component systems containing a variety of metal or organo-metal cations and ligands both inorganic and organic in nature [1–4]. It is widely accepted that the knowledge and the understanding of the speciation of a chemical element or compound (e.g. metal cation) are of fundamental impor- tance for the determination of the quality of a generic “solution” (i.e. natural and biological fluids). The toxicity and the biogeochemical cycling of a heavy metal in the aquatic environment depend on the physicochemical form in which the metal occurs in the dissolved state [5]. Speciation studies put their attention on this topic. For this reason we considered very important the knowledge of the tin(II) behavior in aqueous solutions, although it is not considered a micronutrient neither a pollutant agent, since its concentration is high in the earth crust. The corrosion of tin plated food cans by acidic foods and beverages has caused several intoxications by the soluble tin com- pounds. This led, for example, the Food Standard Agency in the UK to propose upper limits of 200 mg kg -1 for solid food and 150 mg kg -1 for beverages [6]. Among the different radionuclides, tin is of interest because of the presence in radioactive waste of one of its isotopes, 126 Sn, coming from fission; its half-life value is close to 10 5 years [7]. Tin can also exist as organic compounds (e.g., tributyltin, triphenyltin) that are commonly used in various industrial sectors because of their biocide properties [8]. While the chemistry of organotin compounds has been studied extensively because of their high toxicity toward numerous living organisms, less work has been performed on the inor- ganic forms that are generally considered to be non hazardous [9]. However, the presence of 126 Sn in the radioactive waste requires a better knowledge of the chemistry of inorganic tin to understand and model its behavior after disposal in a deep repository. In addition inorganic tin(II) can easily be organicated by ubiquitous methylating and transmethylating bacteria. Phosphates are very important molecules in nature for different rea- sons. Phosphate is a macronutrient, for both plants and seawater organ- isms, its concentration in soils and water is very high and is related to the concentration of phytate (see ref [10] and references therein). As an example, the profile of phosphate concentration in seawater is very different with respect to other components, its concentration in surface is close to zero, as phytoplankton and other organisms decompose, the phosphate is regenerated in the water column, reaching the maximum Journal of Molecular Liquids 165 (2012) 143–153 ⁎ Corresponding author. Tel.: + 39 090 393659; fax: + 39 090 392827. E-mail addresses: rmcigala@unime.it (R.M. Cigala), fcrea@unime.it (F. Crea), cdestefano@unime.it (C. De Stefano), glando@unime.it (G. Lando), gmanfredi@unime.it (G. Manfredi), ssammartano@unime.it (S. Sammartano). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.11.002 Contents lists available at SciVerse ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq