Editing Lanthanide (section)

===Separation of lanthanides===
The similarity in ionic radius between adjacent lanthanide elements makes it difficult to separate them from each other in naturally occurring ores and other mixtures. Historically, the very laborious processes of [[Cascade (chemical engineering)|cascading]] and [[fractional crystallization (chemistry)|fractional crystallization]] were used. Because the lanthanide ions have slightly different radii, the [[lattice energy]] of their salts and hydration energies of the ions will be slightly different, leading to a small difference in [[solubility]]. Salts of the formula Ln(NO<sub>3</sub>)<sub>3</sub>·2NH<sub>4</sub>NO<sub>3</sub>·4H<sub>2</sub>O can be used. Industrially, the elements are separated from each other by [[solvent extraction]]. Typically an aqueous solution of nitrates is extracted into kerosene containing [[Tributyl phosphate|tri-''n''-butylphosphate]]. The [[stability constants of complexes|strength of the complexes]] formed increases as the ionic radius decreases, so solubility in the organic phase increases. Complete separation can be achieved continuously by use of [[countercurrent exchange]] methods. The elements can also be separated by [[ion-exchange chromatography]], making use of the fact that the [[stability constants of complexes|stability constant]] for formation of [[EDTA]] complexes increases for log K ≈ 15.5 for [La(EDTA)]<sup>−</sup> to log K ≈ 19.8 for [Lu(EDTA)]<sup>−</sup>.<ref name = "Greenwood&Earnshaw"/><ref>Pettit, L. and Powell, K. [http://www.acadsoft.co.uk/scdbase/scdbase.htm SC-database] {{Webarchive|url=https://web.archive.org/web/20170619235720/http://www.acadsoft.co.uk/scdbase/scdbase.htm |date=19 June 2017 }}. Acadsoft.co.uk. Retrieved on 15 January 2012.</ref>