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 degrees C by potentiometry ISE-H+ glass electrode) in NaNO3aq at different ionic strengths (0.1 <= 1/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 <= Cu-2 :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 Cup chloride, previously investigated. In the experimental conditions adopted, the formation of three Cu(i)H(j)Phy((12-2i-j)-) species is observed: the mononuclear CuH(4)Phy(6-) and CuH(5)Phy(5-), and the dinuclear Cu(2)H(5)Phy(3-). 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-Hijckel) and SIT (Specific ion Interaction Theory) equations. The sequestering ability of phytate toward copper(II) has been evaluated by the calculation of pL(50) (the total ligand concentration, as - log C-L, 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. (c) 2007 Elsevier B.V All rights reserved.