Naphthionic acid

Naphthionic acid is an aromatic sulfonic acid. It appears as a light grey powder and is used in the synthesis of azo dyes such as Rocceline (a. k. a. Solid Red A), during which the amino group of the acid (in the form of a salt) is diazotated and then coupled with, in the case mentioned, β-naphthol. ...
Found on http://en.wikipedia.org/wiki/Naphthionic_acid
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