Acid orange 5 is a compound with formula Na(C<sub>6</sub>H<sub>5</sub>NHC<sub>6</sub>H<sub>4</sub>N=NC<sub>6</sub>H<sub>4</sub>SO<sub>3</sub>). 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.
Acid orange 5 may be prepared by diazotization of sulfanilic acid, followed by reaction with diphenylamine: