Ligand exchange spectrophotometric method for the determination of mole ratio in metal complexes

Abstract

An innovative ligand exchange method involving the displacement of Fe(III) or Cu(II) ion by a ligand such as EDTA from the corresponding metal-salicylate complex has been developed to determine the mole ratio of the metal-ligand complex. The validity of the method is ascertained as the overlay of the plot showing the decrease in the absorbance of 1.0mM Fe(III)-salicylate upon the addition of serial concentrations of EDTA (0.1–0.6mM) with the absorbance calibration curve of Fe(III)-salicylate. At the intersection point of the two lines, the ratio of the two parts of the x-axis to the left and to the right of the intersection point is equal to 1:1, the mole ratio of Fe(III)-EDTA. Two other different concentrations (0.9 and 1.1mM) of Fe(III)-salicylate are used to confirm our method using the same concentration range of EDTA with each. The analysis of variance (ANOVA) is applied to compare the three slopes where no significant difference is observed with confidence limit of 95%. This method for the determination of metal complex mole ratios is compared with the reported methods known as Job, Mole Ratio, Bent-French, Asmus, and Nash. The mole ratios for Fe(III)-citrate, Cu(II)-citrate, and Cu(II)-EDTA complexes are also determined. Application to many transition metals which also complex with salicylate or related aromatic ligands is envisioned.