Speciation of phytate ion in aqueous solution. Protonation constants and copper(II) interactions in NaNO3aq at different ionic strengths Original Research Article

The acid base behavior of phytate has been studied (at t = 25 °C by potentiometry, ISE-H+ glass electrode) in NaNO3aq at different ionic strengths (0.1 ≤ I/mol L− 1 ≤ 1.0). The interactions with copper(II) were investigated in the same experimental conditions in different metal to ligand (Phy) ratios (1:1 ≤ Cu2+:Phy ≤ 4:1), by using both ISE-H+ and ISE-Cu2+ electrodes. Phytate acid base behavior in sodium nitrate is very similar to that in sodium chloride, previously investigated. In the experimental conditions adopted, the formation of three CuiHjPhy(12−2i−j)− species is observed: the mononuclear CuH4Phy6− and CuH5Phy5−, and the dinuclear Cu2H5Phy3−. Analysis of complex formation constants at different ionic strengths reveals that both ISE-H+ and ISE-Cu2+ electrodes gave, within the experimental error, analogous values. Dependence of complex formation constants on ionic strength was modeled by EDH (Extended Debye–Hückel) and SIT (Specific ion Interaction Theory) equations. The sequestering ability of phytate toward copper(II) has been evaluated by the calculation of pL50 (the total ligand concentration, as − log CL, able to bind 50% of metal cation), an empirical parameter already proposed for an objective “quantification” of this ability. A thorough analysis of literature data on phytate–copper(II) complexes has been performed.