Electrochemical Study on the Stability of Phytate Complexes with Cu2+, Pb2+, Zn2+, and Ni2+: A Comparison of Different Techniques.

The formation constants of various M/Phy systems (M = Cu2+, Zn2+, Pb2+, Ni2+; Phy = Phytate) were determined in NaNO3 solutions at 0.1 ≤ I/mol·L−1 ≤ 1.0 and T = 298.15 K, by potentiometry and voltammetry. The formation constants of the Cu/Phy and Zn/Phy species, already determined, were reanalyzed together with new voltammetric and potentiometric experiments at low concentrations. A new potentiometric and voltammetric study was carried out on the Ni/Phy and Pb/Phy systems. For all of the investigated systems, the dependence on ionic strength was modeled by the Debye−Hückel and the specific interaction theory (SIT) approaches. The sequestering ability of phytate was evaluated toward the considered cations by calculating the pL50 values (i.e., the total ligand concentration necessary to bind 50 % of the cation present in trace) at different ionic strengths and pH. A complete set of “suggested” formation constants was provided. A comparison between the formation constants obtained for the Zn/Phy, Cu/Phy, and Pb/Phy systems reveals that the interactions of phytate with Zn2+, Cu2+, and Pb2+ are quite similar, while the Ni/Phy species showed a weaker complexation. For example, for the MH3Phy species, log K3 = 7.81, 7.51, 7.10, and 5.97 for Zn2+, Cu2+, Pb2+, and Ni2+, respectively. The same trend is observed concerning the pL50. Their dependence on pH and ionic strength was modeled by two empirical equations.