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, βA 16.06 ± 0.07; B: Chit2–FeH, βB 32.64 ± 0.07; C: Chit3–Fe, βC 35.6 ± 0.1; D: Chit3–FeH, βD 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].