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3 Limitations of Chemical Color Tests

3 Limitations of Chemical Color Tests

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7



Chemical Screening



Fig. 7.1 A spot plate typically used in color-screening tests.



7.5



Documentation



Comprehensive documentation of chemical color tests would include (at minimum): a complete description of reagents (expiration dates, color, physical properties, photographs, etc.), a complete description of test substance (color, physical properties,

irregularities, notable markings, identifying characteristics, photographs, etc.), observations during test performance (testing

conditions, testing equipment, glassware, spot plate, transition colors from initial mixing to end of test, photographs, etc.),

complete description of results (final color, positive or negative, comparison to published results, deviations, supporting evidence for observed results, photographs, etc.). Simply recording a positive (+) or negative (−) result should be avoided

because it does not provide adequate information for subsequent peer or technical review.

Note: Photographing a chemical color test may or may not provide adequate documentation. A photograph cannot illustrate

short-lived color transitions that may have been observed during the examination; however, they can accurately prove selected

results.



7.6



Chemical Color Tests



We have selected some of the most common and most reliable color-screening tests for discussion. It should be noted that

this list is not intended to represent a comprehensive collection of color-screening tests.



7.6.1



Chen’s Test



Reagent 1:

• 1% (m/v) copper(II) sulfate (CuSO4) in water

Reagent 2:

• 8 g sodium hydroxide in 100-ml water (2 M NaOH)

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add two drops of reagent 1, then two drops of reagent 2 and

note the color.

Results:

• Produces purple color with ephedrine/pseudoephedrine, phenylpropanolamine, and lidocaine.

Note: This test requires a blank because the reagents are light blue.

The formation of a copper complex produces the color. Copper(II) acts as a chelating agent connecting two of the target

molecules. Ephedrine is shown as an example.



7.6



Chemical Color Tests



83



Diagram 7.5



7.6.2



Dille–Koppanyi’s Test



Reagent 1:

• 0.1 g cobalt(II) acetate or, 0.1 g cobalt(II) acetate tetrahydrate

• 0.2 ml glacial acetic acid

• 100 ml methanol (absolute)

Reagent 2:

• 5 ml isopropylamine

• 95 ml methanol (absolute)

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add three drops of reagent 1, then three drops of reagent 2 and

note the color.

Results:

• Purple: glutethimide, theophylline, chlorzoxazone, all barbiturates (except thiobarbs)

• Blue/purple: dilantin

The colored complex contains cobalt(II) and two target molecules stabilized by two molecules of isopropylamine.

Barbiturates are shown as an example.



Diagram 7.6



7.6.3



Mecke’s Test



Reagent:

• 1% selenous acid (H2SeO3) in concentrated sulfuric acid

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of the reagent and note the color.

Results:

• Purple: codeine, diazepam, methcathinone, flunitrazepam, phenylacetone, and oxycodone

• Green: opiate alkaloids, i.e., morphine and heroin

The reagent oxidizes opiates to a green-colored ortho-quinone of apomorphine. Heroin is shown as an example.



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7



Chemical Screening



Diagram 7.7



7.6.4



Marquis’ Test



Reagent 1:

• Concentrated sulfuric acid

Reagent 2:

• Eight to ten drops of 37% formaldehyde in 10 ml of glacial acetic acid

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of reagent 1, then one drop of reagent 2 and note

the color.

Results:

• Orange-to-brown: N,N-dimethylamphetamine, amphetamine, methamphetamine, mescaline, and the pethidine group

• Purple: opiate alkaloids, morphine, heroin, and codeine

• Brown-red-purple: opium

• Pink-to-violet: methadone

• Green-to-black: MDA and MDMA (when reagent 2 is added first)

• Purple-to-black: MDA and MDMA (when reagent 1 is added first)

Color formation from opiate alkaloids is most likely from a complex containing two molecules of the opiate and two

molecules of formaldehyde. Its formation is promoted by the presence of a strong acid (e.g., sulfuric acid). The colored

complex of heroin is shown.



Diagram 7.8



A bimolecular orange-to-brown-colored carbenium ion complex is formed between two amphetamine or methamphetamine molecules in the presence of the Marquis’ reagent. Amphetamine is shown as an example.



Diagram 7.9



7.6



Chemical Color Tests



85



A bimolecular-molecular green-to-black-colored complex containing two carbenium ions is formed when either MDMA

or MDA reacts with formaldehyde in the presence of sulfuric acid. This will occur when reagent 2 is added first.



Diagram 7.10



A bimolecular-molecular purple-to-black-colored complex containing two carbenium ions is formed when either MDMA

or MDA reacts with sulfuric acid in the presence of formaldehyde. This will occur when reagent 1 is added first.



Diagram 7.11



7.6.5



Nitric Acid Test



Reagent:

• Concentrated nitric acid

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of the reagent and note the color.

Results:

• Orange-to-red: morphine

• Orange: codeine

• Yellow: heroin

The aromatic portion of heroin, codeine, and morphine (benzene ring) is nitrated at the ortho position. The highly polar

nitro group (NO2) generates the colored complex through intramolecular ring closure via hydrogen bonding. Heroin is shown

as an example.



Diagram 7.12



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7.6.6



7



Chemical Screening



Primary Amine Test



Reagent 1:

• 1 g sodium nitroprusside (nitroferricyanide)

• 10 ml acetone

• 90 ml water

Reagent 2:

• 2% sodium carbonate in water

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of reagent 1, then one drop of reagent 2 and note

the color.

Results:

• Blue is positive for the presence of primary amines.



7.6.7



Secondary Amine Test



Reagent 1:

• 1 g sodium nitroprusside (nitroferricyanide)

• 10 ml acetaldehyde

• 90 ml of water

Reagent 2:

• 2% sodium carbonate in water

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of reagent 1 then one drop of reagent 2 and note

the color.

Results:

• Blue color is positive with some secondary amines, such as MDMA and methamphetamine. This test cannot be used to

screen the secondary amines pseudoephedrine, ephedrine, and ketamine.



7.6.8



Tertiary Amine Test



Neutral reagent:

• 2% cobalt(II) thiocyanate in water solution

Acidified reagent:

• 2% cobalt(II) thiocyanate in water solution

• Add a few drops of concentrated HCl.

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of the reagent and note color.

Results on neutral test:

• Blue: cocaine HCl, ketamine, pethidine, methadone, methylphenidate, and methaqualone

Results on acidified test:

• Blue: cocaine base, phencyclidine (PCP), pethidine, methadone, methylphenidate, and methaqualone



7.6.9



Van-Urk’s Test



Reagent:

• 1 g para-dimethylaminobenzaldehyde (p-DMBA)

• 10 ml concentrated HCl

• 90 ml ethanol

• Preparation: Dissolve p-DMBA in ethanol, then add HCl.

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of the reagent and note the color.



7.6



Chemical Color Tests



87



Results:

• Purple: LSD (lysergic acid diethylamide)

• Blue: indoles, pyrroles, and tryptophan

• Yellow: procaine and benzocaine

The purple-colored trimolecular complex formed with LSD involves two molecules of LSD and one modified reagent

molecule.



7.6.10 Duquenois–Levine Test

Reagent 1:

• Petroleum ether

Reagent 2:

• 97.5 ml of 2% vanillin solution in methanol (absolute)

• 2.5 ml of acetaldehyde

Reagent 3:

• Concentrated hydrochloric acid

Reagent 4:

• Chloroform

Procedure:

• Wash plant material with petroleum ether.

• Place the petroleum-ether extract in a spot-plate well and allow the ether to evaporate.

• Add a few drops of reagent 2.

• Add a few drops of reagent 3 and note the color.

• Add a few drops of reagent 4 and note the color in the chloroform layer.

Results:

• A positive test for cannabis resin requires two observations: (1) a purple color must form after the addition of reagent 3,

and (2) the color must transfer into the chloroform layer after the addition of reagent 4.



7.6.11 Froehde’s Test

Reagent:

• 0.5% sodium molybdate (Na2MoO4) solution in concentrated sulfuric acid

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of the reagent and note the color.

Results:

• Purple: opiate alkaloids



7.6.12 Janovsky Test

Reagent 1:

• 0.2% (m/v) meta-dinitrobenzene in 2-propanol

Reagent 2:

• 10% (m/v) potassium hydroxide in methanol (absolute)

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of reagent 1, then one drop of reagent 2 and note

the color.

Results:

• Purple: diazepam, methcathinone, flunitrazepam, phenylacetone, oxycodone



88



7



Chemical Screening



7.6.13 Weber Test

Reagent 1:

• Freshly prepared 0.1% (m/v) Fast Blue B or Diazo Blue B (O-dianisidine, tetrazotized) solution in water

Reagent 2:

• Concentrated hydrochloric acid

Place either 1–2 mg or 1–2 drops of sample in a spot plate, add one drop of reagent 1, then one drop of reagent 2 and note

the color.

Results:

• Red color after the addition of reagent 1, followed by a blue-green color after the addition of reagent 2 indicates the presence of psilocin or psilocybin.



7.7



Summary of Chemical Color Tests



The flowchart below is a summary of the color-screening methods discussed in this chapter (Fig. 7.2). The chart illustrates

the progressive nature of chemical screening and the correlation that exists among and between different tests. Although the

chart could be used to isolate atypical controlled substances (using different combinations), the results shown are representative of those most frequently encountered. Remember, color-screening methods are presumptive tests and should never be

used as definitive proof in the identification of a substance.



7.7



Summary of Chemical Color Tests



Fig. 7.2 Results of color-screening tests on frequently encountered controlled substances.



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