Comparative Study of Binding Behaviour of Cu(II) with Humic Acid and Simple Organic Compounds by Ultrasound Spectrometry
Several simple organic (hydroxy)acids as models of principal functional moieties of humic acids and the copper ion as a model ion were used to investigate the metal-humic interactions. The quantum chemical calculations have been performed showing differences in the interaction strength and stability of the metal-acid complexes formed by different aromatic mono- and polycarboxylic acids and their hydroxyl-containing analogues. In this study, new results from the high resolution ultrasound spectroscopy will be presented. The ultrasound propagation velocity, which is sensitive to the intermolecular interactions, has been measured during titration of the models by solution of the copper(II) salt. The dependence of ultrasound velocity on added amount of the copper(II) ions enabled to follow the metal interactions with the individual functional groups and to elucidate the effect of structure of complexing molecule on its binding with the metal. Changes in the slope of the dependence were used to find the saturation of binding and to follow changes of hydration of interacting species. The differences observed for the individual models show that there are active centers not only with various strength and stability of the formed complexes but also with their various rigidity and ability of conformational changes.