Sequestering Ability of Phytate toward Biologically and Environmentally Relevant Trivalent Metal Cations
2012
Bretti, Clemente | Cigala, Rosalia Maria | Lando, Gabriele | Milea, Demetrio | Sammartano, Silvio
Quantitative parameters for the interactions between phytate (Phy) and Al³⁺, Fe³⁺, and Cr³⁺ were determined potentiometrically in NaNO₃ aqueous solutions at I = 0.10 mol L–¹ and T = 298.15 K. Different complex species were found in a wide pH range. The various species are partially protonated, depending on the pH in which they are formed, and are indicated with the general formula MHqPhy (with 0 ≤ q ≤ 6). In all cases, the stability of the FeHqPhy species is several log K units higher than that of the analogous AlHqPhy and CrHqPhy species. For example, for the MH₂Phy species, the stability trend is log K₂ = 15.81, 20.61, and 16.70 for Al³⁺, Fe³⁺, and Cr³⁺, respectively. The sequestering ability of phytate toward the considered metal cations was evaluated by calculating the pL₀.₅ values (i.e., the total ligand concentration necessary to bind 50% of the cation present in trace in solution) at different pH values. In general, phytate results in a quite good sequestering agent toward all three cations in the whole investigated pH range, but the order of pL₀.₅ depends on it. For example, at pH 5.0 it is pL₀.₅ = 5.33, 5.44, and 5.75 for Fe³⁺, Cr³⁺, and Al³⁺, respectively (Fe³⁺ < Cr³⁺ < Al³⁺); at pH 7.4 it is pL₀.₅ = 9.94, 9.23, and 8.71 (Al³⁺ < Cr³⁺ < Fe³⁺), whereas at pH 9.0 it is pL₀.₅ = 10.42, 10.87, and 8.34 (Al³⁺ < Fe³⁺ < Cr³⁺). All of the pL₀.₅ values, and therefore the sequestering ability, regularly increase with increasing pH, and the dependence of pL₀.₅ on pH was modeled using some empirical equations.
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