Forensic Chemistry: Post-Blast Colour Tests (Griess, Janowski, Diphenylamine)

This medium-difficulty drill covers the field and laboratory colour tests used to detect explosive residues in post-blast debris, targeting UGC-NET Forensic Science Paper II Unit VI. The set opens with the Modified Griess test: the reaction of sulfanilic acid with nitrite ions under acidic conditions to produce a diazonium salt, which couples with alpha-naphthylamine to yield an orange-red azo dye. The questions address which explosive class generates nitrite fragments on detonation, the pH and temperature conditions for reliable colour development, and the critical interference from soil nitrate that demands a blank control. The Janowski test for nitroaromatic explosives such as TNT and picric acid follows: methyl ethyl ketone and potassium hydroxide in an organic solvent produce a violet to magenta Meisenheimer complex with the aromatic ring, and the questions probe which structural feature of the nitroaromatic is essential, why aliphatic nitro compounds fail the test, and how picric acid's additional phenolic group alters the colour endpoint. Diphenylamine in concentrated sulphuric acid, which oxidises to a deeply coloured quinoidal product in the presence of nitrate or nitrite ions, is examined across five questions: the blue-to-violet colour progression, sensitivity limits reported by Yinon, and the strong interference from oxidising pharmaceuticals and vegetable fertilisers that forces the examiner to confirm with a second test. The aluminium and sodium hydroxide test for azides, used on primary explosive residues from detonators, is addressed through three questions on the mechanism of hydrogen azide gas evolution and flame test confirmation. Thin-layer chromatography on silica gel with chloroform-acetone mobile phases and UV visualisation covers explosive class separation and Rf-value interpretation. The final set addresses Indian post-blast laboratory workflow: NSG and NIA field kit protocols, CFSL submission requirements, and the role of GC-MS as the confirmatory technique after positive colour-test screening.

Intended for UGC-NET Forensic Science Paper II aspirants working through Unit VI, NFSU MSc Forensic Chemistry students, FACT aptitude candidates, and scientific officers rotating through CFSL and state FSL explosive-analysis sections.

Topics covered:

• Modified Griess test: sulfanilic acid, alpha-naphthylamine, azo dye mechanism
• Janowski test: nitroaromatics, Meisenheimer complex, violet/magenta endpoint
• Diphenylamine test in sulphuric acid: nitrates, nitrites, blue colour
• Aluminium and NaOH test: azides, hydrogen azide gas, primary explosives
• TLC on silica gel: chloroform-acetone, UV visualisation, Rf values
• False positives and interferences: fertilisers, pharmaceuticals, urine
• Indian post-blast workflow: NSG/NIA kits, CFSL protocols, GC-MS confirmation

Work through each question before reading the explanation, then revisit every wrong answer against the Yinon, Beveridge, Saferstein, Sharma, and Houck and Siegel references cited. Allow 30 minutes.