Coordination study of chitosan and Fe3+

The coordination of Fe3+ with chitosan was studied, considering the type of acid for dissolving the ligand, the pH for the medium, and the metal-biopolymer ratio. Potentiometric and ultraviolet-visible titrations, infrared spectroscopy, thermal analysis and X-ray diffraction were employed. The polymer coordinates with the metal cation through the amino and hydroxyl groups in the entire pH range studied, with pH values around 3-6 being the most important region. The logarithms of the overall binding constants for the equilibria were: A: Chit-Fe, beta(A) 16.06 +/- 0.07; B: Chit(2)-FeH, beta(B) 32.64 +/- 0.07; C: Chit(3)-Fe, beta(C) 35.6 +/- 0.1; D: Chit(3)-FeH, beta(D) 49.0 +/- 0.2, respectively. The coordination is made either by the amino or/and the hydroxyl groups with water molecules or/and chloride ions completing the coordination sphere for the metal giving rise to among other possible, di- to hexacoordinated complexed species [FeNO2Cl3], [FeN2O2Cl2], [FeN3O3] and [FeN2O4]. (C) 2007 Elsevier B.V. All rights reserved.