Eriochrome Black T

Eriochrome Black T is a complexometric indicator that is used in complexometric titrations, e.g. in the water hardness determination process. It is an azo dye. Eriochrome is a trademark of Huntsman Petrochemical, LLC.^[1]

Eriochrome Black T

Wireframe model of an eriochrome black T minor tautomer
Sample of Eriochrome Black T
Names
Preferred IUPAC name Sodium 1-[1-Hydroxynaphthylazo]-6-nitro-2-naphthol-4-sulfonate
Systematic IUPAC name Sodium 4-[2-(1-hydroxynaphthalen-2-yl)hydrazin-1-ylidene]-7-nitro-3-oxo-3,4-dihydronaphthalene-1-sulfonate
Other names Sodium 4-[2-(1-hydroxynaphthalen-2-yl)hydrazin-1-ylidene]-7-nitro-3-oxonaphthalene-1-sulfonate; Solochrome Black T; ET-00; Erio T
Identifiers
CAS Number	1787-61-7 Y
3D model (JSmol)	Interactive image
Abbreviations	EBT
Beilstein Reference	4121162
ChemSpider	10483790 N
ECHA InfoCard	100.015.683
EC Number	217-250-3
MeSH	Eriochrome+black+T
PubChem CID	6808871 5359641 (4E) 5351620
RTECS number	QK2197000
UNII	MPC0KHJ23C Y
UN number	2923
CompTox Dashboard (EPA)	DTXSID4061969
InChI InChI=1S/C20H13N3O7S.Na/c24-17-10-18(31(28,29)30)15-9-12(23(26)27)6-7-14(15)19(17)22-21-16-8-5-11-3-1-2-4-13(11)20(16)25;/h1-10,24-25H,(H,28,29,30);/q;+1/p-1/b22-21+; Y Key: AMMWFYKTZVIRFN-QUABFQRHSA-M Y
SMILES C1=CC=C2C(=C1)C=CC(=C2O)/N=N/C3=C4C=CC(=CC4=C(C=C3O)S(=O)(=O)[O-])[N+](=O)[O-].[Na+]
Properties
Chemical formula	C20H12N3O7SNa
Molar mass	461.381 g/mol
Appearance	dark red/brown powder
Acidity (pKa)	6.2, 11.55
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

In its deprotonated form, Eriochrome Black T is blue. It turns red when it forms a complex with calcium, magnesium, or other metal ions.

EBT is blue in a buffered solution at pH 10. It turns red when Ca²⁺ ions are added.

Applications

When used as an indicator in an EDTA titration, the characteristic blue end-point is reached when sufficient EDTA is added and the metal ions bound to the indicator are chelated by EDTA, leaving the free indicator molecule.

Eriochrome Black T has also been used to detect the presence of rare earth metals.^[2]