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

Abstract

The formation constants of various M/Phy systems (M = Cu 2+, Zn 2+, Pb 2+, Ni 2+; Phy = Phytate) were determined in NaNO 3 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 pL 50 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 Zn 2+, Cu 2+, and Pb 2+ are quite similar, while the Ni/Phy species showed a weaker complexation. For example, for the MH 3 Phy species, log K 3 = 7.81, 7.51, 7.10, and 5.97 for Zn 2+, Cu 2+, Pb 2+, and Ni 2+, respectively. The same trend is observed concerning the pL 50 . Their dependence on pH and ionic strength was modeled by two empirical equations.