STABILITY OF COPPER-HUMIC ACID COMPLEXES IN ACIDIFIED SOLUTION

The stability of copper-humic acid complexes in acidified solutions was investigated by the pH and conductometric titration methods. Fluka humic acid and humic acids extracted from peat (Shinshinotsu, Sarobetsu, Bibai in Hokkaido) were used in this work. The evaluation of stability of the copper complexes were performed by comparing with the titration curves of model ligands (EDTA, citric acid, phenylalanine, hydroxyhydroquinone). It was found that the copper complexes with the humic acids of Fluka and Shinshinotsu were stable in acidified solutions, while the humic acids of Sarobetsu and Bibai dissociable. The FTIR spectra of the humic acids and the pH titration curves of the model ligands suggested that, in the cases of the stable complexes, the carboxyl groups in the humic acids which have low pKa values would contribute to the complex formation. In the cases of the dissociable complexes, the phenolic hydroxyl groups would contribute to the formation.