Ehrlich's reagent
Ehrlich's reagent or Ehrlich reagent is a reagent containing "p"-dimethylaminobenzaldehyde (DMAB) and thus can act as an indicator to presumptively identify indoles and urobilinogen. Several Ehrlich tests use the reagent in a medical test; some are drug tests and others contribute to diagnosis of various diseases or adverse drug reactions. It is named after Nobel Prize winner Paul Ehrlich who used it to distinguish typhoid from simple diarrhoea. The Ehrlich reagent works by binding to the C2 position of two indole moieties to form a resonance stabilised carbenium ion compound. Medical testing. Ehrlich reagent can be  used to detect urobilinogen, which can indicate jaundice or other liver-related issues. A very common Ehrlich test is a simple spot test to identify possible psychoactive compounds such as tryptamines (e.g. DMT) and ergoloids (e.g. LSD). It gives a negative test-result for 25I-NBOMe and many other non-indole-related psychoactives. The reagent will also give a positive result for opium, because of the presence of tryptophan in natural opium. Pyridoxine, present in vitamin supplements, can give false-positive results to the Ehrlich test. Preparation. The reagent is prepared by dissolving 0.5–2.0 g of "p"–dimethylaminobenzaldehyde (DMAB) in 50  mL of 95% ethanol and 50 mL of concentrated hydrochloric acid and is best used when fresh. Other alcohols, such as 1-propanol, can also be used as well. The Ehrlich reagent is similar to a number of other indole tests: