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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. 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.

Its chemical formula is HOC₁₀H₆N=NC₁₀H₄(OH)(NO₂)SO₃Na, or C₂₀H₁₂O₇N₃SNa. Its CAS number is [1787-61-7] [1] and its SMILES structure is OC1=CC(S(=O) ([O-])=O)=C3C (C=CC([N+]([O-])=O)=C3) =C1N=NC2=CC= C(C=CC=C4) C4=C2O.[Na+]..

[edit] External link

• "Molecule of the Week!" article on Eriochrome Black T from the ACS

• Link page to external chemical sources.