Effect of Chloride Ions on the Hydrolysis of Trivalent Lanthanum,Praseodymium and Lutetium in Aqueous Solutions of 2M Ionic Strength

The solubility product of the solid hydroxides and the first hydrolysis constants of trivalent ions of lanthanum, praseodymium and lutetium, were determined in 2 M NaClO₄(aq) and 2 M NaCl(aq) at 303 K, where M denotes the concentration in mol-L⁻¹. Solubility diagrams (pLn_(aq)−pC_H) were measured by means of a radiochemical method. The pC_H borderlines of precipitation and the solubility products were determined from these diagrams. The fitting of the solubility equation with the experimental values from the pLn_(aq)−pC_H diagrams also allowed the calculation of the first hydrolysis constants and the solubility products. In separate experiments, the stability constants for the first monohydroxide species were determined by means of potentiometric pH titrations, where the data were treated with both the program SUPERQUAD and by fitting of the results to the mean ligand number equation. Values of the log₁₀ < eqid20 > _1,Cl constants for the LnCl²⁺ species were also calculated at 2 M ionic strength and 303 K, using the hydrolysis constants obtained in both perchlorate and chloride media. The quantitative effects of chloride ions on the hydrolysis reactions and solubilities were determined for these three rare-earths spanning the lanthanide series.