Sequestering Ability of Phytate toward Biologically and Environmentally Relevant Trivalent Metal Cations

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 MH(q)Phy (with 0 <= q <= 6). In all cases, the stability of the FeH(q)Phy species is several log K units higher than that of the analogous AlH(q)Phy and CrH(q)Phy species. For example, for the MH(2)Phy species, the stability trend is log K-2 = 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 pL(0.5) 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 pL(0.5) depends on it. For example, at pH 5.0 it is pL(0.5) = 5.33, 5.44, and 5.75 for Fe3+, Cr3+, and Al3+, respectively (Fe3+ < Cr3+ < Al3+); at pH 7.4 it is pL(0.5) = 9.94, 9.23, and 8.71 (Al3+ < Cr3+ < Fe3+), whereas at pH 9.0 it is pL(0.5) = 10.42, 10.87, and 8.34 (Al3+ < Fe3+ < Cr3+). All of the pL(0.5) values, and therefore the sequestering ability, regularly increase with increasing pH, and the dependence of pL(0.5) on pH was modeled using some empirical equations.