Copper complexation with the mellitic acid series

The formation constants for copper complexes with the mellitic acid series, a series of aromatic carboxylic acids, were determined from potentiometric copper titrations. Formation constants were determined for copper complexation with hemimellitic acid, trimellitic acid, trimesic acid, pyromellitic acid, and mellitic acid at five pH values (3.00, 4.00, 5.00, 5.75, 6.75) for each organic acid. The ionic strength of the solutions was 0.1M and the temperature was 25°C for all titrations. Free copper-ion activity was monitored with an ion-specific electrode. Relevant complexation reactions and corresponding stability constants for description of the experimental data were determined by application of the nonlinear parameter optimization program FITEQL to the experimental data. Testing various possible complexes revealed that the simple 1:1 complexes of CuL and CuHL (where L represents the fully deprotonated organic ligand) could describe all of the data. The logarithms of the complex formation constants for the CuL species ranged from 2.67 for trimesic acid to 6.23 for mellitic acid, and for the CuHL species from 2.47 for trimellitic acid to 5.03 for mellitic acid. The logarithms of the extracted metal-organic ligand complex formation constants were found to correlate well with the logarithms of organic acid dissociation constants.