Metal ions can be separated from one another by complexation with organic ligands–the chelating agent– to form uncharged chelates. Here, the chelating agent must contain hydrophobic groups and behave as a weak acid, losing a proton to bind with the metal. Since most organic ligands used in this process are insoluble or undergo oxidation in the aqueous phase, the chelating agent is initially added to the organic phase and extracted into the aqueous phase. The metal-ligand complex is formed in the aqueous phase and is extracted back into the organic phase. During this process, the distribution of the metal-chelate complex between the organic and aqueous phases is independent of the initial metal concentration. It depends only on the pH of the aqueous phase and ligand concentration in the organic phase. For instance, consider the separation of divalent copper and lead. Aqueous copper(II) and lead(II) ions can be separated from each other by extracting with the mixture of dithizone in carbon tetrachloride. The plot of extraction efficiency versus pH reveals that copper can be quantitatively extracted into the organic phase if the pH of the aqueous phase is less than 5.5. After copper extraction, the pH of the aqueous phase can be increased to approximately 9.5, and lead ions can be extracted.
