Eriochrome Black T is a complexometric indicator that is used in complexometric titrations, e.g. in the water hardness determination process. It is an azo dye. Eriochrome is a trademark of Huntsman Petrochemical, LLC.
In its deprotonated form, Eriochrome Black T is blue. It turns red when it forms a complex with calcium, magnesium, or other metal ions.