Azo violet
Azo violet (4-(4-Nitrophenylazo)resorcinol) is an organic compound with the chemical formula C12H9N3O4. It is used commercially as a violet dye and experimentally as a pH indicator, turning blue-purple in the presence of magnesium salt in a slightly alkaline, or basic, environment. Azo violet may also be used to test for the presence of ammonium ions. The color of ammonium chloride or ammonium hydroxide solution will vary depending upon the concentration of azo violet used.