- Put a tiny bit of a sample powder into one of the wells of the spot plate.
- Add a few drops of the test reagent onto the sample powder.
- A few minutes later the mixture becomes a characteristic color.
Drug powder turns a specific color after a test reagent is added
Scientific name: Some reagents obtain a characteristic color after the addition of various drugs
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Such reagents as Marquis, Mandelin, Mecke and Simon's reagents (available in the test reagent kit) become characteristic colors in the reactions with some alkaloids and drugs. These reagents are used to identify illicit substances, but the demonstration may be carried out with common drugs, such as aspirin.
Marquis reagent is a mixture of formaldehyde and concentrated sulfuric acid. The mechanism of Marquis test reaction is not completely understood. Generally, formaldehyde molecules exist in protonated form in presence of concentrated sulfuric acid. The reaction of electrophilic substitution proceeds between drug molecules and protonated formaldehyde and form a carbocation. Another drug molecule reacts with this carbocation with the formation of a dimer. This dimer is oxidized by concentrated sulfuric acid and gives a colored stabilized carbocation. Marquis reagent forms deep red products with aspirin.
Mandelin reagent is a mixture of ammonium metavanadate and concentrated sulfuric acid. It changes color to grayish olive green with aspirin.
Mecke reagent is composed of selenious acid and concentrated sulfuric acid. Mecke reagent forms grayish purple products with aspirin.
Simon's test is used for detecting secondary amines. A blue complex ( (RR')NH2+ + [OCHCH2NO Fe(CN)5]3- is formed in the reaction of secondary amines with sodium nutroprusside, acetaldyhyde and sodium carbonate mixture. There is no characteristic reaction between Simon's reagent and aspirin.