Eriochrome Black T is a complexometric indicator. During titrations of water hardness determination process. Eriochrome Black T is blue in colour but in the presence of calcium, magnesium, or other metal ions It turns red.

Eriochrome Black T, or EBT, has a prang of 6.3 to 11.5, and exhibits the following behaviour: with red as its acidic colour and orange its basic color.