Eriochrome Black T
is a complexometric indicator
that is part of the complexometric titrations
, eg. in the water hardness
determination process. It is an azo dye
In its protonated form, Eriochrome Black T is blue. It turns red when it forms a complex with calcium, magnesium, or other metal ions. Its chemical formula can be written as HOC10H6N=NC10H4(OH)(NO2)SO3Na.
When used as an indicator in an EDTA
titration, the characteristic blue end-point is reached when sufficient EDTA is added and metal ions form complexes with the EDTA instead of the Eriochrome.
Eriochrome Black T has also been used to detect the presence of rare earth metals.