Q. Distribution coefficients of the salts?

The distribution coefficients of the salts of lanthanide elements between organic and water solvents are slightly different. Thus, the individual elements are selectively extracted from aqueous solutions of their salts into an organic solvent. This technique of separation is define as solvent extraction. Tributyl phosphate is a very good solvent for this process. The solubility of lanthanides in +3 oxidation state in tributyl phosphate increases with atomic number. Separation is performed by using a continuous counter-current process in which the aqueous solution of lanthanide nitrates and the solvent are passed through a column continueusly in opposite directions. This process is much less tedious than performing several crystallisations.