Critical Behavior of Iron(III) with a Typical Catecholate Siderophore

Siderophores are ligands that serve as iron scavengers or iron chelators, which hinder the absorption of iron in the body; tannic acid is one of them. Since a tannic acid molecule has 8 gallic acid groups, it can bind 1-4 Fe(III) ions, depending upon ligand concentration. pH 3, 4, 5, 8 and 10 were selected for this study. The \lambda max and M:L molar ratio of the complexes varies with pH, as well as with ligand concentration. For low ligand concentration at pH 3, 4 and 5, the maximum absorbance of the complex was found at 650 nm, while, at pH 8 and 10, 500 nm was the \lambda max. For high ligand concentration, pH 550 nm was the \lambda max. Stability constant values of iron(III)tannic acid complexes were measured spectrophotometrically using the mole ratio method, as well as the slope ratio method, potentiometrically. The experimental results of pH titrations were treated by program ``Best''. The $\beta$ values obtained by these methods were compared. Stability constant values were found to be 10^3, 10^8 and 10^16 for ML, M_2L and M_4L, respectively.