Formation and stability of mixed Mg 2+/Ca 2+/phytate species in synthetic seawater media : Consequences on ligand speciation

Abstract

The results of a potentiometric investigation (by ISE-H +, glass electrode) on the speciation of phytate ion (Phy 12−) in an ionic medium simulating the major components (Na +, K +, Ca 2+, Mg 2+, Cl − and SO 4 2−) of natural seawater, at different salinities and t = 25 °C, are reported. The work was particularly aimed at determining the possible formation of mixed Ca 2 +–Mg 2+–phytate ion pairs, and to establish how including the formation of these mixed species would affect the speciation modeling in seawater media. After testing various speciation models, that considering the formation of the MgCaH 3Phy 5−, MgCaH 4Phy 4−, Mg 2CaH 3Phy 3− and Mg 2CaH 4Phy 2− species was accepted, and corresponding stability constants were determined at two salinities ( S = 5, 10). A discussion is reported both on the choice of the experimental conditions and on the possibility to extend these results to those typical of real seawater. A detailed procedure is also described to demonstrate that the stability of these species is higher than that statistically predicted. As reported in literature, a parameter, namely log X, has been determined in order to quantify this extra stability for the formation of each mixed species at various salinities. For example, at S = 10, log X 113 = 2.67 and log X 114 = 1.37 for MgCaH 3Phy 5− and MgCaH 4Phy 4− (statistical value is log X stat = 0.60), and log X 213 = 6.11 and log X 214 = 2.15 for Mg 2CaH 3Phy 3− and Mg 2CaH 4Phy 2− (log X stat = 1.43), respectively. Results obtained also showed that the formation of these species may occur even in conditions of low salinity (i.e. low concentration of alkaline earth cations) and low pH (i.e., more protonated ligand).