## 2 Materials and Methods
### Potentiometry and Spectrophotometry  
The potentiometric apparatus used to carry out the titrations was described previously [21]. The glass electrodes were manufactured by Metrohm (Swiss) and acquired, after the addition of the reagents, a constant potential within 45-60 min that remained unchanged within \(\pm\)0.01 mV. To avoid carbonate interference, a slow stream of nitrogen  
Figure 1: Coordination sites of HCCA.  
gas was passed into the test solutions, kept under magnetic stirring during titrations. The cell assembly was placed in a thermostat kept at (37.0 \(\pm\) 0.1) \({}^{\circ}\)C.  
The complexation equilibria were studied, at 37 \({}^{\circ}\)C and in 0.16 mol/L NaClO\({}_{4}\), by measuring, with a glass electrode (GE), the competition of HCCA for H\({}^{+}\) and metal cations. Measurements were performed as potentiometric titrations with cell (G)  
\[\text{RE}\mid\text{Test Solution}\mid\text{GE}\qquad\qquad\text{(G)}\]  
in which RE is the reference electrode (Ag \(\mid\)AgCl\(\mid\) 0.01 mol/L NaCl\(\mid\) 0.15 mol/L NaClO\({}_{4}\)\(\mid\) 0.16 mol/L NaClO\({}_{4}\)) and the Test Solution contained \(C_{\text{M}}\) mol/L M(ClO\({}_{4}\))\({}_{n}\) (i.e., Nd(ClO\({}_{4}\))\({}_{3}\) and UO\({}_{2}\)(ClO\({}_{4}\))\({}_{2}\)), \(C_{\text{L}}\) mol/L HCCA, \(C_{\text{A}}\) mol/L HClO\({}_{4}\), \(C_{\text{B}}\) mol/L NaOH, and (0.16 \(-\)\(n\)\(C_{\text{M}}\) - \(C_{\text{A}}\) - \(C_{\text{B}}\)) mol/L NaClO\({}_{4}\). The EMF of cell (G) can be written, in mV, at the temperature of 37 \({}^{\circ}\)C, as Equation (1):  
\[E=E^{\circ}+61.54\log[\text{H}^{+}]+E_{j} \tag{1}\]  
where \(E^{\circ}\) was constant for each series of measurements and \(E_{j}\) is the liquid junction potential, which is a function of [H\({}^{+}\)] only [23]. \(E_{j}\) value under our experimental conditions (i.e., 37 \({}^{\circ}\)C and 0.16 mol/L NaClO\({}_{4}\)) was determined by acid-base titration with cell (G) when metal ions and HCCA were absent. The data for [H\({}^{+}\)] \(\leq\) 0.100 mol/L could be well approximated by the linear slope \(E_{j}\) ([H\({}^{+}\)]\({}_{l}\) = -\(j_{l}\) [H\({}^{+}\)]. The value of \(j\) (mV/M) is 337 \(\pm\) 1.  
Each titration was divided into two parts. In the first part, \(E^{\circ}\) was determined in the absence of HCCA and metal cations; in the [H\({}^{+}\)] range 10\({}^{-4}\)-10\({}^{-2}\) mol/L values constant to within 0.1 mV were calculated according to the [PERSON]'s method [24; 25]. In the second part, after the addition of the reagents, the acidity was stepwise decreased by adding known volumes of NaOH standard solution. The concentration of HCCA (\(C_{\text{L}}\)) was varied within 1 and 5\(\cdot\)10\({}^{-3}\) mol/L, while the concentration of metal ions (\(C_{\text{M}}\)) ranged within 0.5 and 5\(\cdot\)10\({}^{-3}\) mol/L, and the ligand to metal concentration ratio was varied within 1 and 10. The hydrogen ion concentration was varied from 1\(\cdot\)10\({}^{-2}\) mol/L (pH 2.0) to incipient precipitation of a neutral salt, which takes place in the range [H\({}^{+}\)] = (3.2-0.032)\(\cdot\)10\({}^{-5}\) mol/L (pH 4.5-6.5) depending on the specific metal ion and on the ligand-to-metal ratio. The primary \(C_{\text{M}}\), \(C_{\text{L}}\), \(C_{\text{A}}\), \(C_{\text{B}}\) and [H\({}^{+}\)] data form the basis of the treatment to obtain the stability constants.  
The spectrophotometric measurements were carried out with a Varian Cary 50 Scan UV Visible Spectrophotometer. Absorbance values between 210 and 450 nm were measured each 1 nm. The temperature of the cell holder was kept at (37.0 \(\pm\) 0.3) \({}^{\circ}\)C by a Grant circulating water bath. Matched quartz cells of thickness 1 cm were employed. The absorbance, \(A_{\lambda}\), was recorded to 0.001 units. The acquisition of data was controlled with the aid of a computer connected to the instrument.