UV-Vis spectrophotometric determination of rare earth elements (REE) speciation at near-neutral to alkaline pH. Part II: hydrolysis of Er from 25 to 75 °C

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Here, UV-Vis spectrophotometry experiments were conducted as a function of temperature between 35 and 75 ℃ to determine the hydrolysis of Er in near-neutral to alkaline solutions using mCP as an in situ pH color indicator. The experiments were conducted with Er concentrations from 0 to ~0.253 mmol/kg in low ionic strength solutions (≤ 0.001 mol/kg). The average OH− ligand number coordinated to Er3+ increases from 1.0 to 2.7 at 35 ℃ and from 1.5 to 3.2 at 75 ℃ over a pH range from 6.3 to 9.0. The measured speciation shows an increased predominance of Er(OH)30 over the Er(OH)2+ and Er(OH)2+ species with increased temperature. The logarithmic values of Er hydrolysis constants (log*βn◦, n =1 to 3) were derived for the reaction Er3+ + nH2O = Er(OH)n3-n + nH+ at each temperature of 35, 50, and 75 ℃: log*β1◦ of −7.02 ± 0.09, −6.19 ± 0.06, −5.43 ± 0.16; log*β2◦ of −14.39 ± 0.15, −13.98 ± 0.01, and −13.45 ± 0.24; log*β3◦ of −22.85 ± 0.15, −21.79 ± 0.14, and −20.32 ± 0.09. These equilibrium constants were fitted to a function to predict the hydrolysis constants of Er between 25 and 75 ℃, and show a systematic increase in the stability of REE hydroxyl complexes with higher ligand numbers with increased temperature.

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