The formation of mixed ligand complexes of Fe(III) with phosphoric and citric acids in 0.5 M NaNO3 aqueous solutions

Abstract

Several acid–base potentiometric titrations have been carried out to estimate a model of mixed complexation among Fe (III), orthophosphate and citrate ions. The precipitation-free pH range for stable and reproducible experimental data was between 2 and 5. The HYPERQUAD program was used to fit the experimental data. As a result of the numerical treatment, a model composed of three mixed complexes of formulas: Fe(OH)(H2PO4)2(H–citrate), Fe(H2PO4)2(H–citrate)2 and Fe(OH)(H2PO4)(H2–citrate) were obtained. The formation constants of these species are log β−6,1,2,1=21.31, log β−6,1,2,2=33.67 and log β−3,1,1,1=23.45, respectively. The formation of these mixed complexes avoids the precipitation of iron until pH>7 when the ratio [citric acid]/[Fe(III)]=1.5 and until pH>9 for higher values.