## 4 Discussion  
In this work, we have investigated the acid\(-\)base equilibrium of coumarinic acid and the chelating properties of its deprotonated form towards neodymium and uranyl at 37 \({}^{\circ}\)C and in 0.16 mol/L NaClO4. Coumarinic acid anion forms 1:1 and 2:1 complexes with both metal ions. The uranyl ion is also capable to form a polynuclear complex with stoichiometry 2.2. Furthermore, from the data reported in Table 1, it may be noted that the complex UO2(OH)(CCA) is stronger than the corresponding neodymium species, Nd(OH)(CCA)\({}^{+}\). The stability constants reported in Table 1 were used to represent the distribution diagrams (Figure 4a,b), in which the percentage of metal cations into the complexes as a function of pH is depicted. As can be seen in Figure 4a, the complexation between HCCA and Nd(III) starts at a pH higher than 3.5 with the neutral species Nd(OH)(CCA)\({}_{2}\), which is predominant in the whole pH range. The complex Nd(OH)(CCA)\({}^{+}\), instead, is a minor species with a percentage that does not reach values higher than 5%. The neodymium hydrolytical species Nd(OH)2\({}^{+}\) does not exist in the pH range investigated, supporting that the complexes between the ligand and metal ion could also involve the hydroxyl group. Regarding the system UO2\({}^{2+}\)-HCCA (Figure 4b), all of the complexes reach significant percentages. The complexation starts from pH 2 with the formation of the neutral species UO2(CCA)\({}_{2}\), which is predominant until pH 5. The anionic complex UO2(OH)2(CCA)\({}^{-}\) is the main from pH 5, while the other two minor species, (UO2)2(OH)2(CCA)\({}_{2}\) and UO2(OH)(CCA), start to have significant percentages at the same pH value (i.e., 4) and coexist in the whole investigated pH range. As can be seen in Figure 4b, the percentage of the predominant uranyl hydrolytical complexes, (UO2)2(OH)2\({}^{2+}\) and (UO2)3(OH)5\({}^{+}\), decreases drastically in the presence of coumarin-3-carboxylic acid, confirming that the complexes between UO2\({}^{2+}\) and HCCA could be mixed species involving hydroxyl ion too. To verify the effective formation of the complexes, a comparison of the UV-Vis spectra for free ligand and coordinated species is reported in Figure 5.  
Figure 4: Distribution diagrams in the presence of HCCA of (**a**) Nd3\({}^{+}\) (C\({}_{\text{M}}\) = 1.0 mM and \(C_{\text{L}}\) = 2.0 mM) and (**b**) UO2\({}^{2+}\) (C\({}_{\text{M}}\) = 2.5 mM and \(C_{\text{L}}\) = 2.5 mM). One (green line) and 2 (yellow line) in (**b**) are the hydrolytical species (UO2)2(OH)2\({}^{2+}\) and (UO2)3(OH)5\({}^{+}\), respectively. According to the literature [19], the UV-Vis spectrum of free coumarin-3-carboxylic acid shows two characteristic absorption bands, at 333 nm (low intensity) and 292.5 nm. The addition of the metal cations, Nd\({}^{3+}\) or UO\({}_{2}\)\({}^{2+}\), in the aqueous suspension containing HCCA, brought solid ligand into the aqueous phase as soluble complexes. Figure 5 shows the absorption spectra of the Nd\({}^{3+}\)-HCCA (brown line in Figure 5a) and UO\({}_{2}\)\({}^{2+}\)-HCCA (red line in Figure 5b) complexes. Due to the low electronic rearrangement, which is induced on the ligand upon complex formation, the free ligand absorption spectrum was not significantly affected by the complexation. Indeed, the band at 333 nm undergoes, in both spectra, a slight blue shift (i.e., at 320 nm). The most interesting spectral change can be detected for the band at 292.5 nm, which undergoes a significant hyposchromic shift, especially for the uranyl system (i.e., at 280 nm). As a further product of both syntheses, a solid was obtained, which was dissolved in a 1:1 ethanol:water mixture and analyzed, after the appropriate dilution with water, spectrophotometrically (red line in Figure 5a and blue line in Figure 5b).