SEPRATION OF THE LANTHANIDES

The properties of metal ions are determined by their charge and size. Since all the lanthanide ions are of about the same size and also carry the same charge, being typically trivalent, their properties are almost identical. The separation of lanthanides from one another, therefore is as problematic as the separation of isotopes. Recently, methods based on repeated fractional crystallisation, solvent extraction, fractional precipitation, ion exchange chromatography etc. Which take advantage of slight differences of properties like solubility, complex ion formation and hydration, arising from very slight size differences of their trivalent ions, have been used. These methods are discussed below. 

1) Repeated Fractional Crystallisation :

   

                            Formation of simple salts like nitrates, sulphates, bromates, oxalates as well as double salts 2La ( NO₃)₃, 3Mɡ

(NO₃)₂.24H₂O, which from nice crystals, have been made use of in the separation of the lanthanide elements. Advantage is taken of slightly differences in their solubilities in water.

2) Solvent Extraction :

                                     The partition coefficients of the salts of these metals between water and organic solvents are slightly different. To take an example the partition coefficient of Gd(NO₃)₃ between water and normal butyl alcohol is 1.06 times greater than that of La (NO₃)₃ . This means that Gd (NO₃)₃ can be separated from La(NO₃)₃ by continuous extraction with water from a solution of these salts in butyl alcohol.

3) Fractional Precipitation :

                    If only a little amount of a precipitating agent is added to a mixture of lanthanide salts, the substance having the lowest solubility product will precipitate out first. Thus, if sodium hydroxide is added to a solution of lanthanide nitrates, Lu(OH), which is the weakest base and has the lowest solubility product, is precipitated first while La(OH)₃, which has the highest solubility product is, precipitated last. A partially separation takes place. However, by redissolving that the precipitate, the process can be again repeated a number of times until complete separation is affected.

4) Change of oxidation state :

            As already discussed, some of the lanthanide show +2 and +4 oxidation states in addition to the +3 oxidation state which is most characteristic of all the elements of the family. The properties of the M²⁺and M⁴⁺ ions are different from those of the usual M³⁺ ions. To take an example, cerium can be separated from a mixture of trivalent ions of all other lanthanide by oxidising it to Ce⁴⁺ state by reacting it with alkaline KMnO₄. Now Ce⁴⁺ ion has a greater charge than Ce³⁺ ion. Accordingly, it's smaller in size and, therefore, less basic and less soluble. The  precipitate as Ce(OH)₄ by the addition of a little amount of an alkali leaving all the trivalent lanthanide ions in solution. It has been possible to have 99 percent pure cerium from a mixture containing only 40 percent cerium by this method.

Europium can be separated from a mixture of trivalent lanthanide ions by reducing it first to the +2 oxidation state by means of zinc amalgam. Eu²⁺ ion can then be precipitated as EuSO₄ which is insoluble in water. The sulphates of all the trivalent lanthanides on the other hand, are soluble in water.

5) Ion Exchange Chromatography :

                  This is most successful and the most rapid method for the separation of lanthanides. We shall first explain a few terms which are used frequently in the discussion of this technique.