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.
Highlights
Complexometry of metals with organic ligands find applications in numerous scientific accomplishments including complexometric titrations, solvent extraction, preconcentration and other analytical applications, investigation in protein and nucleic acids supramolecular assemblies, and environmental remediation
The color of the ternary complex Th:bromocresol orange (BCO):cetylpyridinium bromide (CPB) was used for the selective determination of Th4+ in the presence of many transition metals and a pKf of 12.0 was found with the indicator and a molar absorptivity, of 9.2 x 104 L mol-1 cm-1 at 560 nm (Khalifa and Hafez, 1998)
We studied the compounds A and F as chelating agents for metal complexometric titrations with spectrophotometric end point detection and determined their dissociation constants and formation constants with Fe3+, Cu2+, Zn2+, and Pb2+
Summary
Complexometry of metals with organic ligands find applications in numerous scientific accomplishments including complexometric titrations, solvent extraction, preconcentration and other analytical applications, investigation in protein and nucleic acids supramolecular assemblies, and environmental remediation. Most indicators are unstable, several of them bond irreversibly to metals so they should be back-titrated, (Skoog et al, 2006) and have unclear color changes For these reasons, other methods as electrochemical or spectrophotometric techniques are used for endpoint detection which allow a decrease in detection limits and titration of environmental samples where metal concentration is lower. The color of the ternary complex Th:BCO:CPB was used for the selective determination of Th4+ in the presence of many transition metals and a pKf of 12.0 was found with the indicator and a molar absorptivity, , of 9.2 x 104 L mol-1 cm-1 at 560 nm (Khalifa and Hafez, 1998). Titrations of metals by complexation reactions are performed using ethylenediaminotetraacetic acid (EDTA) when metal concentrations are 0.001 mol/L or higher. We studied the compounds A and F as chelating agents for metal complexometric titrations with spectrophotometric end point detection and determined their dissociation constants and formation constants with Fe3+, Cu2+, Zn2+, and Pb2+
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