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

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.