Eriochrome Black T

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.


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