Speciation of phytate ion in aqueous solution. Cadmium(II) interactions in aqueous NaCl at different ionic strengths

Interactions between myo-inositol 1,2,3,4,5,6-hexakis(dihydrogen phosphate) (phytic acid) and cadmium (II) were studied by using potentiometry (at 25 degrees C with the ISE-H(+) glass electrode) in different metal to ligand (Phy) ratios (1:1 <= Cd(2+):Phy <= 4:1) in NaCl(aq) at different ionic strengths (0.1 <= I/mol L(-1)<= 1). Nine Cd(i)H(j)Phy((12-2i-j)-) species are formed with i=1 and 2 and 4 <= j <= 7; and trinuclear Cd(3)H(4)Phy(2-). Dependence of complex formation constants on ionic strength was modeled by using Specific ion Interaction Theory (SIT) equations. Phytate and cadmium speciation are also dependent on the metal to ligand ratio. Stability of Cd(i)H(j)Phy((12-2i-j)-) species was modeled as a function of both the ligand protonation step (j) and the number of metal cations bound to phytate (i), and relationships found were used for the prediction of species other than those experimentally determined (mainly di- and tri-protonated complexes), allowing the possibility of modeling Phy and Cd(II) behavior in natural waters and biological fluids. A critical evaluation of phytate sequestering ability toward cadmium(II) has been made under several experimental conditions, and the determination of an empirical parameter has been proposed for an objective "quantification" of this ability. A thorough analysis of literature data on phytate-cadmium(II) complexes has been performed.