EDTA titrations are usually carried out in highly basic conditions, where the fully deprotonated form of EDTA, Y4−, actively complexes with the free metal ions in the solution. Several metal ions precipitate as hydrous oxide (hydroxides, oxides, or oxyhydroxides) under these conditions, lowering the concentration of free metal ions in the solution. For this reason, auxiliary complexing agents or ligands such as ammonia, tartrate, citrate, or triethanolamine are used in EDTA titrations to prevent unwanted precipitation. These ligands bind strongly to the metal ions to form metal−ligand complexes that are less stable than the metal−EDTA complexes. The addition of EDTA during titration displaces ligands from the metal−ligand complexes, forming more stable metal−EDTA complexes.
