The formation and stability of Mg2+ and Ca2+-phytate complexes was studied potentiometrically using an ISE-H+ electrode. Measurements were performed at 10 degrees C and 25 degrees 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 M(i)PhyH(j)((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 Ca(i)PhyH(3)((9-2i)-), i = 1, 2, 3 (equilibrium iCa(2+) + H(j)Phy((12-j)-) = CaiPhyHj(12-j-2i)- : K-ij) at I = 0.25 mol L-1 and t = 25 degrees C: logK(13) = 3.42, logK(23) = 6.47 and logK(33) = 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-Hilckel 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. (c) 2006 Elsevier B.V. All rights reserved.
