Various chemical reagents have been developed during the past century. These were formerly used to determine a molecular structure, when more performant methods were not available. Nowadays, the use of these reagents has been replaced by NMR or chromatographic methods which are the standard ways for determining and characterizing new molecules.
However, some of these reagents find an application for the detection and identification of drugs.
The most famous reagents are Marquis, Mecke, Mandelin, Simon's and its variant Robadope. This page is dedicated to less common reagents.
Sulfuric acid
Sulfuric acid is not a reagent in the same sense as the others, because it is not selective towards a particular chemical group. It acts mainly as an acid catalyst and a dehydrating agent. It is especially reactive towards alcohols, phenols, ketones and aldehydes.
Common colorations vary from yellow to orange.[1]
When assessing reagent test results for a new identified compound, it is recommended to primarily test it with sulfuric acid. Since many reagents use sulfuric acid as a solvent, testing with sulfuric acid prevents any misinterpretation of the coloration given by another testing reagent, since the later could result from reaction of the compound with sulfuric acid and not with the chemical reagent.Preparation
Concentrated sulfuric acid must be used.
Apply sulfuric acid directly to the sample on a white tile or in a test tube.[1]
Froehde
When testing with Froehde's reagent, it is recommended to also test the sample with sulfuric acid, and to compare the obtained colorations.Preparation
Dissolve 0.5 g of molybdic acid or sodium molybdate in 100 mL of hot concentrated sulfuric acid.[2]
Liebermann
This reagent is composed of sodium nitrite dissolved in sulfuric acid. When testing with Liebermann's reagent, it is recommended to also test the sample with sulfuric acid, and to compare the obtained colorations.
The obtained coloration can be orange to orange-brown. Other colorations are also possible.Preparation and use
Add 5 g of sodium nitrite to 50 mL of (concentrated) sulfuric acid with cooling and swirling to absorb the brown fumes.[clarke]
Add 2 or 3 drops of the reagent to the sample on a white tilt. Occasionally it is necessary to carry out the test in a tube and heat in a water bath at 100°C.[1]
Ehrlich
The Ehrlich reagent is used to detect indoles including ergot alkaloids (violet coloration), phenols or phenolic amines (red or orange coloration turing into violet on dilution), and some cannabinoids (red to violet upon dilution)Preparation and use
Add 0.5 - 2 g of para-dimethylaminobenzaldehyde (p-DMAB) in a mixture of 50 mL ethanol and 50 mL of concentrated hydrochloric acid. [1][2]
Add the reagent to the sample in a test tube, warming if necessary. Observe any color produced and then carefully dilute with water.[1]Safety
Concentrated hydrochloric acid: Produces noxious fumes. Corrosive. Avoid skin contact.The Ehrlich's reaction
p-DMAB plays the role on an electrophile.
Cobalt thiocyanate
Preparation
Dissolve 2.0 g of cobalt (II) thiocyanate in 100 mL of distilled water. [2]
references
- ^ a b c d eClarke's isolation and identification of drugs. Second Edition. Edited by A. C. Moffat. Pharmaceutical press: London. 1986.
- ^ a b cCarol L. O’Neal, Dennis J. Crouch, Alim A. Fatah (2000). Validation of twelve chemical spot tests for the detection of drugs of abuse. Forensic Science International 109: 189–201