Separation of Lanthanides – Ion Exchange Method and Valency Change Method

The separation of lanthanides elements is difficult since their chemical properties are almost identical. The properties of metal ions are mainly determined by their size and charge. The lanthanides are all typically trivalent (charge) and are almost similar in size, and hence possess difficulty in separation. Only two methods are currently used for separating the lanthanides:

• Ion Exchange Method
• Valency Change Method

Separation of Lanthanides by Ion Exchange Method

For the separation and purification of Lanthanides, the ion-exchange method is the most significant, rapid, and reliable technique.

A lanthanide ion solution is passed through a column of -SO₃H containing synthetic ion-exchange resin (such as Dowex-50). The Ln³⁺ ions bind to the resin by replacing H⁺ ions in the SO₃H groups.

Thus, the Ln3+ ions get fixed to the resin, and the H⁺ ions produced are washed through the column.

Then, a buffered solution of citric acid/ammonium citrate (eluent) is passed through the resin. The Ln³⁺ ions are eluted/removed by this solution, which forms stable complexes with the citrate ion and moves down slowly. The smaller Ln³⁺ ions form citrate complexes first and possess more stability as compared to other citrate complexes. As a result, the lanthanide with smaller ions discharges out of the column first.

Since the ions leave the column at different speeds, separation is feasible. The solution coming out of the column is collected in various containers in various fractions; each container receives a solution component (elute) that is rich in a certain metal. The metal ion is precipitated as its oxalate when ammonium oxalate solution is added to each solution component, which when heated forms the oxide.

Separation of Lanthanides by Valency Change Method

The ability of some of the lanthanides to exist in variable valencies is employed in separating these elements.

Ce⁴⁺ is a stable ion that differs from Ce³⁺ and other Ln³⁺ ions in some properties. This property is used to distinguish Ce from other lanthanides. When NaOCl is added to a mixture of Ln³⁺ ions, Ce³⁺ is oxidized to Ce⁴⁺ resulting in the precipitation of The Ce⁴⁺ as Ce(OH)⁴ from the mixture, which can then be heated to generate CeO₂. Alternatively, tributyl phosphate can be used to extract Ce⁴⁺ ions.

When compared to other Ln3+ ions, Eu²⁺ has quite unique properties. The Eu³⁺ ions in the mixture are reduced electrostatically to Eu²⁺ and then precipitated as EuSO4 before being filtered out of the solution with the other (unaffected) Ln³⁺ ions. The separation of Sm and Yb via their divalent species follows a similar procedure.

The other two methods; the separation of lanthanides by fractional crystallization method and the liquid-liquid extraction method can be also applied for the separation of lanthanide elements.

• Fractional Crystallisation: This approach is based on the formation of isomorphous compounds with considerable variances in solubilities, as well as significant changes in solubilities as a function of temperature. The double nitrates of Mg and Ce, for example, can be distinguished from the double nitrates of Mg and other lanthanides.
• Liquid-liquid Extraction Method: In organic solvents, the solubilities of Ln³⁺ ions differ greatly. The metal in the combination may be complexed first, followed by the transfer of one or more of the products to the organic solvent layer. This approach can be used to extract Ce(IV) and Th(IV) from other Ln³⁺ ions. They produce H₂ when they react with dilute mineral acids.

Separation of Lanthanide elements Video

FAQ/MCQs

References

• J. D. Lee, Concise Inorganic Chemistry, 5th Edition, John Wiley and Sons. Inc. 2007.
• F. A. Cotton, G. Wilkinson & C. Gaus, Basic Inorganic Chemistry, 3 rd Edition, John Wiley & Sons (Asia), Pvt., Ltd., 2007.
• D. F. Shriver & P. W. Atkins, Inorganic Chemistry, 5th Edition, Oxford University Press, 2010.