Speciation of phytate ion in aqueous solution. Sequestration of magnesium and calcium by phytate at different temperatures and ionic strengths, in NaClaq

The formation and stability of Mg2+ and Ca2+-phytate complexes was studied potentiometrically using an ISE-H+ electrode. Measurements were performed at 10 °C and 25 °C in NaClaq in the ionic strength range 0.1 ≤ I ≤ 0.75 mol L− 1. For both magnesium and calcium systems, the formation of ten MiPhyHj(12−2i−j)− species was observed in the range 3 ≤ pH ≤ 7 with i = 1, 2, 3 and j = 3, 4, 5 (and i = 3, j = 2). These species are quite stable; here we report for example some quantitative data for the species CaiPhyH3(9−2i)−, i = 1, 2, 3 (equilibrium iCa2+ + HjPhy(12−j)− = CaiPhyHj(12−j−2i)−: Kij) at I = 0.25 mol L− 1 and t = 25 °C: logK13 = 3.42, logK23 = 6.47 and logK33 = 9.41. The speciation of the Ca2+-phytate system was also checked by ISE-Ca2+ measurements. Dependence on ionic strength was modeled using a simple Debye-Hückel type equation and formation constants were calculated at infinite dilution. The stability constants of complexes formed at pH > 7 were estimated using an empirical predictive equation. The sequestering ability of phytate towards Mg2+ and Ca2+ was calculated in different experimental conditions and compared with those of other chelating agents.