Thermodynamic Study on the Dissociation and Complexation of Coumarinic Acid with Neodymium(III) and Dioxouranium(VI) in Aqueous Media

Abstract

In the frame of a systematic study on the sequestering ability of natural antioxidants towards metal cations, the complexation of coumarin-3-carboxilic acid (HCCA) with neodymium(III) and dioxouranium(VI) (uranyl, UO22+), and overall stability constants of the resulting complexes, were evaluated from the pH-potentiometric titration data at 37 °C and in an aqueous solution (i.e., 0.16 mol/L NaClO4). The graphic representation of the complex’s concentration curves is given by the distribution diagrams, which provide a depiction of all the species present in the solution in the selected pH ranges. The protonation constant of HCCA was also determined to evaluate the competition of the ligand for the metal cations and H+. The ligand-to-metal concentration ratio was varied between 1 and 10, and the hydrogen ion concentration was decreased stepwise until the incipient precipitation of a basic salt of the metal, which occurred at different values depending on the specific metal cation and the ligand to metal ratio. Speciation profiles obtained by potentiometric titrations and supported by UV-Vis data show that a complexation occurs at a ligand-to-Nd(III) and to –UO22+ ratio of 1:1 and 2:1, with different degrees of deprotonation: Nd(OH)(CCA)+, UO2(OH)(CCA), UO2(OH)2(CCA)−, and Nd(OH)(CCA)2, UO2(CCA)2 and (UO2)2(OH)2(CCA)2.

Insights on Stability Constants and Structures of Complexes between Coumarin Derivatives and Pb(II) in Aqueous Media

“…The complexation ability of coumarin-3-carboxylic acid towards selected metal cations in aqueous solutions at 37 • C and in 0.16 mol L −1 [i.e., inert salt NaCl or NaClO 4 depending on specific metal ions] has been reported [22][23][24][25]. In these previous investigations it has been verified that, with the single exception of Fe(III), HCCA exhibits only one mode of binding, in which the deprotonated carboxylic group acts as a bidentate chelating ligand.…”

“…All solutions were freshly prepared with bidistilled water, freed from any organic impurities by means of a Milli-Q system (Millipore, Burlington, MA, USA). Lead(II) perchlorate, perchloric acid, sodium perchlorate stock solutions, and the sodium hydroxide titrant solutions were prepared and standardized as previously described [23,24,27]. HCCA (Sigma, St. Louis, MO, USA, ≥99%) was kept in a desiccator over silica gel and was used without further purification.…”