Binary and ternary metal complexes of Congo red with amino acids

The stability constants of the 1:1 binary complexes of Ag+, Ni2+, Co2+, Y3+ metal ions with glutamic acid (Glu), leucine (Leu) and Congo red (CR) and their 1:1:1 ternary complex and the protonation constants of the ligands were determined potentiometrically at a constant ionic strength of I = 0.10 mol dm−3 (NaClO4) in aqueous solutions at 15.0, 25.0 and 35 °C. The thermodynamic parameters ΔGo, ΔHo and ΔSo were reported for the complex formation reactions. The enthalpy changes of all complexation are found to be negative but entropy changes positive. The driving forces for the formation of Ag–CR, Ni–CR, Co–CR are the enthalpy decreases. But the driving force for Y–CR is the entropy decreases. The driving force of Ni–Glu, Co–Glu, Ni–Leu and Co–Leu systems are about equally enthalpy decrease and entropy increase while Ag–Glu and Ag–Leu complexation are enthalpy stabilized. But, the driving force of Y–Glu and Y–Leu systems are the entropy increase. For formed ternary complexes the reactions are all enthalpy-driven.