pH-Metric Studies of Acid-Base Equilibria on the mixed Cu( II) Complexes with Pyrazine-2,3-Dicarboxylic Acid and Amino Acids

In this study, acid-base equilibria involved in the complex formation of Copper (II) ion with pyrazine-2,3-dicarboxylic acid (PDC) and amino acids (L) ligands have been determined potentiometrically in 20% (v/v) ethanol-water mixture, at 25 degrees C and an ionic strength (I) of 0.10 mol/L (NaClO4). The relative stabilities of the ternary complexes are compared with those of the corresponding binary complexes in terms of Delta log K values. Importantly, the possible formation of mixed-ligand complexes by simultaneous mechanisms has been confirmed by comparison with constructed theoretical curves. Additionally, species distributions in solution for all complexes were precisely evaluated. pH-studies of these systems clearly indicate that the concentration of the ternary complexes increases with increasing pH. This behaviour attests that complex formation is more favoured in physiological pH range (6-7), whereas protonated ternary complexes become highly favourable at lower pH's.