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:
 * HO3SC6H4N2+ + (C6H5)2NH  ->  HO3SC6H4N\dNC6H4N(H)C6H5  +  H+