text: Diphenylamine was discovered by A. W. Hofmann in 1864 amongst the products of dry distillation of aniline dyes; it was first purposefully synthesized through deamination of a mix of aniline and its salts by a group of French chemists two years later.
  In 1872, diphenylamine was suggested as a means to detect nitrous acid in sulfuric acid due to its blue coloration in the presence of oxidizing agents. By 1875, it was also being used to detect nitrites and nitrates in drinking water.