Acid orange 5 is a compound with formula Na(C₆H₅NHC₆H₄N=NC₆H₄SO₃). It is an azo dye. It is also used as a pH indicator; it is red in pH under 1.4, orange-yellow in pH over 3.2.^[1]

Quick Facts Names, Identifiers ...

Acid orange 5

Names
Preferred IUPAC name Sodium 4-[(E)-(4-anilinophenyl)diazenyl]benzene-1-sulfonate
Other names Fast yellow; Tropaeolin OO
Identifiers
CAS Number	554-73-4 Y
3D model (JSmol)	Interactive image
ChemSpider	10660
ECHA InfoCard	100.008.248
PubChem CID	11133
UNII	VR8Q3R288W Y
CompTox Dashboard (EPA)	DTXSID8060294
InChI InChI=1S/C18H15N3O3S.Na/c22-25(23,24)18-12-10-17(11-13-18)21-20-16-8-6-15(7-9-16)19-14-4-2-1-3-5-14;/h1-13,19H,(H,22,23,24);/q;+1/p-1/b21-20+; Key: MLVYOYVMOZFHIU-ANVLNOONSA-M InChI=1/C18H15N3O3S.Na/c22-25(23,24)18-12-10-17(11-13-18)21-20-16-8-6-15(7-9-16)19-14-4-2-1-3-5-14;/h1-13,19H,(H,22,23,24);/q;+1/p-1/b21-20+; Key: MLVYOYVMOZFHIU-LGLICYIRBD
SMILES c1ccc(cc1)Nc2ccc(cc2)/N=N/c3ccc(cc3)S(=O)(=O)[O-].[Na+]
Properties
Chemical formula	C18H14N3NaO3S
Molar mass	375.38 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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Acid orange 5 may be prepared by diazotization of sulfanilic acid, followed by reaction with diphenylamine:

HO₃SC₆H₄N+2 + (C₆H₅)₂NH → HO₃SC₆H₄N=NC₆H₄N(H)C₆H₅ + H⁺