Interactions of iron(II) and iron(III) with gallic acid and its homologues: a potentiometric and spectrophotometric study

The reactions between iron(III) and the phenols gallic acid, catechol, protocatechuic acid and pyrogallol have been studied in aqueous solution at 25oC (I 0.10 M KCl). From spectrophotometric and pH titration data it was established that (i) complexes with 1 : 1, 1 : 2 and 1 : 3 (M : L) stoichiometry are formed in oxygenated solution, (ii) the redox reaction given at low pH by these ligands Fe3+ + ½phenol → Fe2+ + ½quinone + H+ is not reversed by complexing of Fe3+ at higher pH, and (iii) the percentage composition of iron(II) and iron(III) in solutions of these ligands is a function of ER (ligand), the pH and age of solutions and the partial pressure of dissolved oxygen.