Speciation of Phytate Ion in Aqueous Solution. Thermodynamic Parameters for Zinc(II) Sequestration at Different Ionic Strengths and Temperatures

Results of an investigation on phytate interactions with zinc(II) cation in NaNO_3aq at different ionic strengths (0.1≤I/mol⋅L⁻¹≤1.0) are reported. Stability constants of various Zn _i H _j Phy^{(12−2i−j)−} species were determined by potentiometry (ISE-H⁺ glass electrode) and the corresponding formation enthalpies by direct calorimetric titrations. Data obtained were used to provide an exhaustive speciation scheme of zinc(II) in the presence of phytate, as well as a comprehensive representation of the binding ability of phytate toward zinc(II) in different conditions. Different pL₅₀ values [an empirical parameter already proposed, expressed as the −log ₁₀ C _Phy, where C _Phy is the total phytate concentration necessary to bind 50% zinc(II)] were calculated in several conditions, and equations were formulated to model its dependence on different variables, such as ionic strength, temperature and pH. Other empirical predictive relationships are also proposed. Previous contributions to this series reviewed in ref. [1].