Abstract: a) The present invention falls within the area of the synthesis of ionic liquids, namely it concerns a process of synthesis of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) with a high degree of purity and its use in the extraction and selective separation of metals, namely lanthanides. Thus, it is the object of the present invention, a process for the synthesis of a pure ionic liquid using in its constitution only the elements carbon, hydrogen, oxygen and nitrogen (CHON), assuming itself as a “green” alternative in the recovery of metals, thus reducing the environmental impact in the way they are recovered, as well as the guarantee of a more efficient extraction of these metals.