Complex formation between phytic acid and divalent metal ions: a solution equilibria and solid state investigation
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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 MeH5L5– and Me2H3L5–. 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 Me6HtLClt·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).
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