Speciation of phytate ion in aqueous solution. Sequestration of magnesium and calcium by phytate at different teperatures 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.