31P NMR, potentiometric and spectrophotometric studies of phytic acid ionization and complexation properties toward Co2+, Ni2+, Cu2+, Zn2+ and Cd2+

Acido-basic properties of phytic acid as well as its binding properties toward five bivalent metal ions (Co2+, Ni2+, Cu2+, Zn2+ and Cd2+) were investigated. Potentiometric titration curves refined with the program PROTAF allowed the determination of acidity constants of the ligand as well as complexation constants with the considered metal ions for systems containing ligand-to-metal ratios over unity. The chelating ability of phytic acid varies with the metal according to the sequence: Cu2+ > Zn2+ > Cd2+ > Ni2+ approximate to Co2+. P-31 NMR titrations of the Zn2+-phytic acid system suggest that the metal is bound to P2 or P5 in several monocoordinated species for low pH values. On the other hand, near pH=9, bicoordinated species probably occur which mainly involve P2 with pi or P3 as the chelating phosphate groups. Electronic spectra of Co2+-phytic acid complexes lead to similar conclusions: an octahedral geometry for a monocoordinated species in the PH range 2-7 and tetrahedral complexes above pH = 9, which are probably bicoordinated. For metal-to-ligand ratios over unity, a solid compound Co5L . nH(2)O (n approximate to 20) was isolated. (C) 1999 Elsevier Science Inc. All rights reserved.