5-Acetyl-8-Hydroxyquinoline and 5-Formyl-8- Hydroxyquinoline: Chelating Agents for Complexometric Titrations

Abstract

Metal–organic ligand complexes are essential to many technological developments from protein supramolecular assemblies to solvent extraction, complexometric titrations, and environmental remediation. We studied the chelating and acid-base properties in ethanol of two derivatives of 8-hydroxyquinoline (Q), 5-acetyl-8-hydroxyquinoline (A) and 5-formyl-8-hydroxyquinoline (F), of which applications as chelators are unknown. The ligands acidity followed the order F (pKa1 2.9; pKa2 9.5)> A (pKa1 4.8; pKa2 9.6)> Q (pKa1 5.3; pKa2 10.0) due to inductive effects of electronegative groups, formyl in F and acetyl in A. The higher acidity of F with respect to A was due to the higher electronegativity of the formyl group in F. This acidity order was reflected in larger formation constants of F-metal complexes with pKf values of 32 (Fe3+), 27.1 (Cu2+), 26.2 (Cu2+), and 24.9 (Pb2+), than with A, with pKf values of 31, 22.5, 21.4 and 21.2 with these metals, or the pKf values in the literature for Q. In summary, A and F resulted excellent chelators for metals in complexometric titrations with large formation constants; these large formation constants recommend the application of these compounds in masking, preconcentration, and titration of metals by complexation and oxidation-reduction reactions.