Disclosed is a process for extracting or separating metal ions using a composition including: an ionic liquid of formula C+,−X, in which: C+ is an onium cation including at least one hydrocarbon chain R1 including from 6 to 20 carbon atoms; XP− is an anion of charge p, the ionic liquid having a solubility in water at 20° C. of at least 10 g/l; an acid; and water. The composition includes two liquid phases: a phase enriched in ionic liquid ϕIL; and a phase enriched in water ϕw, the pH of which is less than or equal to 4.7. The composition is useful for extracting a metal ion from an acidic aqueous medium including a metal ion, for separating metal ions from an aqueous medium including at least two metal ions or for purifying an acidic aqueous solution including a metal ion.