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

References

Azo dyes
PH indicators
Benzenesulfonates
Organic sodium salts
Acid dyes