Eriochrome Black T

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Eriochrome Black T

IUPAC name	sodium (4Z)-4-[(1-hydroxynaphthalen-2-yl- hydrazinylidene]-7-nitro-3-oxo Y-naphthalene-1-sulfonate
Other names	Eriochrome Black T, Solochrome Black T, EBT
Identifiers
CAS number	[1787-61-7]
PubChem	5351620
SMILES	C1=CC=C2C(=C1)C=CC(=C2O)N/N= C\3/C4=C(C=C(C=C4)[N+](=O)[O-]) C(=CC3=O)S(=O)(=O)[O-].[Na+]
Properties
Molecular formula	C₂₀H₁₂N₃O₇SNa
Molar mass	461.381 g/mol
Appearance	Dark red/brown powder
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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.

Eriochrome Black T is blue, but turns red in the presence of metals.

Applications

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 other applications as well. It can be used to measure the hardness (i.e. the concentration of calcium and magnesium ions) in water.^[1] It is also used to detect the presence of rare earth metals.^[2]