Potentiometric Studies of Stability Constants and Speciation of Binary and Ternary Complexes of Metal (II) Ions with Amino Acids and Thiobarbituric Acid

Several chelating agents have been widely used as scavengers to transport metals to or away from vulnerable sites due to their ability to form stable complexes with different metal ions. Many of the chelating agents have been reported to be toxic, non-biodegradable and inflexible for the recovery of bound metal ions. The inherent drawbacks with these chelating agents necessitate a search for their replacement. Therefore, the extent of coordination of L-tyrosine, L-histidine (secondary ligands) and Thiobarbituric acid (primary ligand) with Co(II), Cu(II) and Pb(II) in an aqueous medium at 27°C and 35°C has been examined potentiometrically with ionic strength maintained by 0.02M NaNO3. The potentiometric equilibrium measurements showed that the ligands formed binary and ternary complexes with the metal ions. Ternary complexes were formed by simultaneous mechanisms and they were found to be more stable than the corresponding binary complexes. The order of stability was found to increase with an increase in the covalent index of the metal ions and decrease with increase in temperature. Speciation diagram showed the variation in stability constants of the binary and ternary complexes as the function of pH. The ligands exhibited high coordinating properties and could be used as metal scavengers for transportation of metals to or away from vulnerable sites. The binary metal complexes could be applied as a medium of transporting chemotherapeutic drugs to target sites or detoxifying poisonous substances which possesses donor atoms with chelating capability.