Pharmacognosy and Phytochemistry: Crude Drugs and Their Markers
What the domain tests
Pharmacognosy covers the biological source, cultivation, collection, morphology, chemical constituents, identification tests and evaluation of natural drugs, plus phytochemical extraction and standardisation. Questions typically pair a crude drug with its marker compound or a colour test, or ask about adulteration and WHO herbal quality requirements.
Core rules to hold
- Chemical classification: alkaloids (tropane in belladonna and datura, indole in vinca and ergot, isoquinoline in opium and ipecac), glycosides (cardiac in digitalis and strophanthus, saponin in liquorice and ginseng, anthraquinone in senna and aloe), tannins, volatile oils, resins, gums and mucilages.
- Colour tests: Dragendorff gives an orange-red precipitate with alkaloids; Mayer gives cream; Wagner gives brown; Keller–Kiliani detects deoxy sugars of cardiac glycosides; Borntrager detects anthraquinones (pink-red with ammonia); Vitali–Morin detects tropane alkaloids; Van Urk detects ergot alkaloids.
- Evaluation of crude drugs: organoleptic, microscopic (stomata, trichomes, calcium oxalate), physical (ash value, extractive value, swelling index, foaming index), chemical and biological assays.
- Adulteration is detected through ash values, foreign matter limits and chromatographic fingerprints; WHO guidelines require marker-based standardisation.
Worked example
A powdered leaf sample gives an intense pink-red colour in the ammonia layer after extraction with dilute alcohol and benzene. The Borntrager reaction identifies anthraquinone glycosides, consistent with senna or aloe rather than digitalis, whose glycosides would require the Keller–Kiliani reaction for deoxy sugars.
Traps that cost marks
Mixing up Borntrager (anthraquinones) with Keller–Kiliani (cardiac glycosides); assuming every alkaloid gives the same colour with Dragendorff; confusing ash value (inorganic residue) with extractive value; forgetting that swelling index applies to mucilage-containing drugs such as isabgol.