Spectrophotometric determination of the stability of La hydroxyl complexes at near neutral to alkaline pH from 25 to 75 °C

In this study, UV-Vis spectrophotometric experiments were conducted from 25 to 75 °C in near-neutral to alkaline NaOH-bearing aqueous solutions with varying lanthanum (La) concentrations (0 to ~0.23 mmol/kg). The color indicator m-cresol purple was used to determine in situ pH and derive the average OH- ligand number (n) and formation constants for the La hydroxyl complexes (LaOH2+, La(OH)2+, and La(OH)30). From 25 to 50 °C, n ranges between ~1 and 2 at pH from 7.0 to 9.3. At 75 °C, n ranges between ~1.5 and 3 at pH from 6.3 to 8.8. These results suggest the predominance of LaOH2+ and La(OH)2+ complexes from 25 to 50 °C, and an increased predominance of La(OH)30 at 75 °C. The cumulative formation constants (βn°, n = 1 to 3) are derived for the reaction La3+ + nOH- = La(OH)n3-n, and fitted between 25 and 250 °C by combining the UV-Vis and literature solubility data. The resulting logβn° are expressed as function of temperature (T in Kelvin): logβ1° = -1.786 +0.0133T +1.049·103/T; logβ2° = -5.797 +0.0267T +2.713·103/T; logβ3° = 6.435 +0.0223T +512.7/T. A comparison between these new fits and existing extrapolations using the Helgeson-Kirkham-Flowers equation of state indicates significant differences in the predicted hydrolysis of La. The latter extrapolations should therefore be updated for the hydrolysis of REE.