Solubility of Nd(III) Trihydroxide Solid Phase at Neutral to Weakly Alkaline Conditions: A Colorimetric and Quantitative Study

The dissolution reaction of Nd(III) trihydroxide solid phase was quantitatively investigated in the pH_c range of 6.8 to 8.0 at 0.1 M NaCl medium under carbonate-free condition. Prior to the solubility experiment, the solid phase characterization was conducted on the starting material using various analytical methods such as XRD, TA-DTG, and SEM/TEM, to obtain stoichiometric information about the synthesized Nd(III) solid phase. For the solubility experiment, the Nd(III) solid phase was equilibrated by means of the undersaturation approach, and the concentrations of aqueous Nd(III) species were quantified using UV–Vis absorption spectroscopy coupled with the liquid waveguide capillary flow cell (LWCC) and ICP-MS/OES. After filtering the aqueous solution through both syringe and membrane filters, the measured aqueous Nd(III) concentration indicated that no or trivial contribution from the colloidal phase of Nd(III) during the solubility experiment. Based on chemical thermodynamic calculations using experimental results and literature data, the solubility product constant of Nd(III) trihydroxide solid phase was determined to be log^*K⁰_s,0 = 18.14 ± 0.53 at infinite dilution by applying the SIT approach (i.e., I = 0). This study demonstrated that UV–Vis absorption spectroscopy coupled with LWCC can be effectively employed for the quantitative analysis of aqueous lanthanide ions at low concentrations, even without the need for additional complexing agents.