Complex formation between phytic acid and divalent metal ions: a solution equilibria and solid state investigation

Abstract.

An investigation on the complex formation equilibria between divalent metal ions Me (with Me=Mn, Co, Ni, Cu, Cd, and Pb) and phytic acid (H₁₂L) is presented. Experiments were performed through a potentiometric methodology by measuring, at 25 °C, the proton and, in some cases (Cu²⁺, Cd²⁺, and Pb²⁺), also the metal ion activity at equilibrium in solutions containing, besides the metal and the ligand, 3 M NaClO₄ as the ionic medium. Unhydrolyzed solutions of the metal ion at millimolar concentration levels were titrated with solutions of about 10 mM sodium phytate, until the formation of a solid phase took place (always at pH ≈2.5, except in the case of Cu²⁺, which formed soluble complexes up to pH ≈3.3). Coulometry was employed to produce very dilute solutions of either Cu²⁺, Cd²⁺, or Pb²⁺ of accurately known composition. The emf data were explained by assuming, in the acidity interval explored, the formation of the complexes of general stoichiometry MeH₅L^5– and Me₂H₃L^5–. Coordination compounds in the solid state were also synthesized and characterized by elemental analysis, thermal analysis, and ICP spectroscopy. The solids had a general stoichiometry Me₆H _t LCl _t ·x H₂O, with the following t and x values for each metal investigated: Me (t; x) = Mn (4; 2); Co (4; 2); Ni (4; 2); Cu (2; 2.5); Zn (2; 1); Sn (6; 6).