Metal ions can be separated by complexation with organic ligands to form uncharged chelates. The chelating agent must contain hydrophobic groups and behave as a weak acid, losing a proton to bind with the metal. The metal–ligand complex is formed in the aqueous phase and extracted into the organic phase. The distribution coefficient of the metal complex depends only on the pH of the aqueous phase when a large excess of complexing reagent is used. Consider the plot of extraction efficiency versus pH for copper(II) and lead(II) ions. When the aqueous phase pH is less than 5, copper can be quantitatively extracted into the organic phase. Then, the pH can be buffered to about 9.5 for the selective extraction of lead.