Thermodynamics of complexation of lanthanides by some benzoic acid derivatives in aqueous solution

The thermodynamic parameters of formation of 1:1 complexes between lanthani de cations and some benzoic acid derivatives were determined by potentiomet ric and calorimetric titration methods in aqueous solution. The benzoic aci d derivatives studied were 4-aminobenzoate, 4-hydoxybenzoate, and 4-nitrobe nzoate. All complexation measurements were made at 25 degrees C in an aqueo us medium of ionic strength 0.1 M NaClO4. The thermodynamic parameters for the complexation are discussed in relation to the electronic effect of the substituent groups within the ligands. It has been found that the stabiliti es of the complexes are affected by the nature of the para-substituted grou p in the phenyl ring of the carboxylate ligands. Theoretical calculations o f the charge distributions on the carbon atoms of various substituted benzo ate ligands were also performed. It was found that the stability constants of the complexes increase with increasing negative charge shifts of carboxy late carbon atoms of substituted benzoate ligands in changing from the ioni zed form to the metal-complexed form. (C) 1999 Elsevier Science B.V. All ri ghts reserved.