Complex formation of zinc(II) ion with glycine, N-acetyl- and N-benzoyl-glycine in aqueous and ethanolic solution by polarographic method.

The complex formation reactions of glycine, N-acetyl- and N-benzoyl-glycine with the Zn(II) ion were investigated using the polarographic method in aqueous and ethanolic solution to identify the number and type of complex species present in solution and to calculate their stability constants. In aqueous basic solution (pH 6–9), only the glycinate anion reacts with the metal ion forming three complexes of the type ZnL+, ZnL2 and ZnL−3. On the contrary in ethanolic solution all three amino acids react; the N-protected amino acid anions form four complexes of the type ZnL+, ZnL2, ZnL−3 and ZnL2−4, while in the presence of the glycine anion two complexes only of the type ZnL+ and ZnL2 were identified. The results obtained for these systems in aqueous and ethanolic solution, discussed in comparison with those of the corresponding cadmium(II) systems, also suggest an amino acid coordination to the metal ion through different coordination sites in the solvents examined.