Spectrophotometric and pH-metric studies of the aqueous binary and mixed ligand complexes of Pb(II), Cd(II), Al(III) and Cu(II) were carried out with ascorbic acid and paracetamol. The study was conducted at an ionic strength of 0.15 mol dm -3 and a temperature of 37.0 ± 0.2 o C. The protonation constants of the free ligands and the stability constants of the binary and mixed ligand complexes were determined using pH titration data. Proton dissociation of free ligands and complex formation were also qualitatively confirmed with UV data. The stabilities of mixed ligand complexes were quantitatively compared with those of the corresponding single ligand complexes in terms of ∆log K. Aluminium formed the most stable binary complex; Al (paracetamol) 2 , with a high overall stability constant (log β 2 = 16.74). The stable mixed ligand complexes; Pb (ascorbic acid) (paracetamol), Cd (ascorbic acid)(paracetamol) and Cu (ascorbic acid) (paracetamol) were observed with positive ∆log K values showing the ability of using them in less harmful chelation therapy of these metals. Further, the results revealed that the bioavailability of both ligands will be lower for a metal intoxicated person.

Lead is the most distributed toxic metal in the world and chelation therapy is a remedy for removing it from the body, if a person is intoxicated. Stability constants of both binary and mixed ligand complexes of Pb(II) with salicylic acid, ascorbic acid, vitamin B 1 (thiamine), vitamin B 3 (niacin) and Na 2 EDTA were potentiometrically studied, in aqueous media at the temperature of 37.0 ± 0.2 ⁰ C and the ionic strength of 0.15 mol dm -3 in order to identify better mixed ligand systems suitable for chelation therapy of lead. Both metal-ligand and proton-ligand formation constants were calculated using IrvingRossotti equations. The Formation of ternary complexes was further confirmed theoretically by plotting composite curves. Δ log K values calculated revealed that all mixed complexes were more stable than the complexes of corresponding binary systems. EDTA-Vitamin B 1 and EDTA-Ascorbic mixed complexes with Pb(II) disclosed very high stability constants, which were two times greater than that of EDTA binary complex. Vitamin B 1 revealed a remarkably high ability in forming mixed ligand complexes with all other four ligands. Results of this work clearly indicate that (EDTA-Vitamin B 1 ), (EDTA-Ascorbic), (Vitamin B 1 -Ascorbic), (Vitamin B 1 -Salicylic) and (Vitamin B 1 -Vitamin B 3 ) combinations can be used for detoxification of lead more effectively using dietary ligands.