Mistake in the Considerations of Hydrolysis Constants and Chloride-ion Complex Formation on the Solubility of Rare Earth Elements

Simplifications of chemical calculations are used frequently in teaching general and analytical chemistry to undergraduate students. However, oversimplifications can give inconsistent solutions to chemical calculation problems. In this paper we discuss this topic over a common example of solubility calculations of the Ln(III) hydroxide in 2M at 303 K. The procedures for the calculation of the solubility of rare earth elements (REE) considering both hydrolysis and chloride-ion complex formation are presented. Solubility products (Ksp), hydrolysis (βLn,H), and chloride-complex stability (βLn,Cl) constants for lanthanum, praseodymium, and lutetium in 2M-NaClO4 and 2M-NaCl at 303K were taken from the literature. It is important to distinguish between the minus logarithm of the hydronium concentration (pCH) and the experimental pH value measured in systems with high ionic strength. According to the results, when pCH(sat) ≤ pβLn,H – 1, Ln3+ is the only soluble species; for the interval pβLn,H – 1 < pCH(sat) < pβLn,H + 1, a two-solubility-equilibrium model is required due to the presence of the soluble first hydrolysis species, (Ln(OH)2+). Moreover, for a high chloride-ion concentration a soluble chloride complex may also be relevant. In fact, Ln(OH)2+ controls the behavior of the high-charge density REE, whereas LnCl2+ is the most import for the low-charge density REE.