The Ehrlich reagent works by binding to the C2 position of two indole moieties to form a resonance stabilised carbenium ion compound.[1]
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 lysergamides (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.[3]
Pyridoxine, present in vitamin supplements, can give positive results to the Ehrlich test, showing a pink colour change.[4]
Preparation
The reagent is prepared by dissolving 0.5[5]–2.0 g of p–dimethylaminobenzaldehyde (DMAB) in 50 mL of 95% ethanol and 50 mL of concentrated hydrochloric acid[6][7] and is best used when fresh. Other alcohols, such as 1-propanol, can also be used as well.[8]
The Ehrlich reagent is similar to a number of other indole tests:
The van Urk reagent, which uses 0.125 g of p-DMAB, 0.2 mL of ferric chloride solution (25 mg/mL) in 100 mL of 65% sulfuric acid.[9][10][11] This is sometimes referred to as the Hofmann reagent or p-DMAB-TS (Test Solution) and gives slightly different colours with different indoles.
^Kovar, Karl-Artur; Laudszun, Martina (February 1989). "Chemistry and Reaction Mechanisms of Rapid Tests for Drugs of Abuse and Precursors Chemicals" (PDF). UNODC. p. 15. Retrieved 3 January 2016.
^ a bde Faubert Maunder, MJ (1975). "Field and laboratory test for raw and prepared opium". Bulletin on Narcotics. 27 (1): 71–6. PMID 1039285.
^"Pyridoxine Monograph for Professionals". Drugs.com. Retrieved 2021-08-21.
^Spratley, Trinette (2004). "Analytical Profiles for Five "Designer" Tryptamines" (PDF). Microgram Journal. 3 (1–2): 55. Retrieved 2013-10-09.
^O’Neal, Carol L; Crouch, Dennis J; Fatah, Alim A (April 2000). "Validation of twelve chemical spot tests for the detection of drugs of abuse". Forensic Science International. 109 (3): 189–201. doi:10.1016/S0379-0738(99)00235-2. PMID 10725655.
^"Color Test Reagents/Kits for Preliminary Identification of Drugs of Abuse" (PDF). Law Enforcement and Corrections Standards and Testing Program. July 2000. Retrieved 2011-07-24.
^"Ehrlich's Reagent Safety Data Sheet" (PDF). Labchem. 2 July 2014. Retrieved 11 January 2015.
^Ehmann, A. (1977). "The van URK-Salkowski reagent — a sensitive and specific chromogenic reagent for silica gel thin-layer chromatographic detection and identification of indole derivatives" (PDF). Journal of Chromatography A. 132 (2): 267–276. doi:10.1016/S0021-9673(00)89300-0. PMID 188858. 2.03 g FeCI3. 6 H2O were dissolved in 500 ml water and 300 ml conc. H2SO4
^Sunshine, Irving (1969). Handbook of analytical toxicology. Chemical Rubber Co. p. 408. p-DMAB-TS: To a cool soln of 65 ml H2S04 in 35 ml H2O, add 125 mg para-dimethylaminobenzaldehyde, dissolve, add 1-2 drops of FeCI3-TS.
^"Basic Tests for Pharmaceutical Substances: 5. Reagents". WHO: Essential Medicines and Health Products Information Portal. Archived from the original on October 25, 2015. Retrieved 2019-12-20.
^Maunder, M. J. de Faubert (August 1974). "A field test for hallucinogens: further improvements". Journal of Pharmacy and Pharmacology. 26 (8): 637–638. doi:10.1111/j.2042-7158.1974.tb10677.x. PMID 4155730. S2CID 97915487.