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 and it turns red when it forms 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.
 * Applications

Eriochrome Black T has other applications as well. It can be used to measure the hardness (i.e. the concentration of calcium and magnesium ions) in water. It is also used to detect the presence of rare earth metals.