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

Here, we develop a robust experimental UV-Vis spectrophotometric method using m-cresol purple to determine in situ pH from 25 to 75 °C. This method is used to derive the average ligand number and hydrolysis constants of erbium (Er) at 25 °C in aqueous solutions with low ionic strength (≤ 0.001 mol/L) at pH from ~7 to 9.5 and in the presence of Er concentrations from 0 to 0.057 mM. The average ligand number ranges between 1 and 3 indicating that Er(OH)2+, Er(OH)2+ and Er(OH)30 control speciation in the experiments. The logarithm of the Er hydrolysis constants (log*βn°, n= 1 to 3) derived at infinite dilution for the reaction Er3+ + nH2O = Er(OH)n3-n + nH+ are: *β1°= −7.22 ± 0.10, *β2°= −14.52 ± 0.08, *β3°= −23.24 ± 0.04.