Complexometric titration involves the formation of a complex by reacting a metal ion with one or more ligands. A visual indicator often detects the end point of a complexometric titration. It is added to the metal solution before the titration, forming a stable metal–indicator complex and imparting color to the solution. As the titration approaches the equivalence point, the excess of the added ligand displaces the indicator from the metal–indicator complex, releasing the free indicator. The free indicator formation changes the color of the solution rapidly, signaling an endpoint. O,O′-dihydroxyazo compounds, such as eriochrome black T and calmagite, are frequently used as indicators in complexometric titrations.