Questions
2–3 questions in B.Pharm semester exams
Difficulty
Medium
Importance
Essential core topic for B.Pharm and GPAT exams
Overview
Phytochemical screening and extraction are fundamental processes in Pharmacognosy used to isolate and identify bioactive secondary metabolites from plant matrices. Mastering these techniques is essential for drug discovery and quality control, as they form the bridge between raw herbal material and standardized therapeutic agents.
Principles of Extraction
Extraction involves the separation of medicinally active portions of plant tissues from inactive/inert components using selective solvents. The efficiency of the process depends on the nature of the plant matrix, the solubility of the target constituent, and the extraction technique employed.
- Maceration: Solvent stays in contact with the drug for a specific period at room temperature
- Percolation: Continuous flow of solvent through the powdered drug column
- Soxhlet Extraction: Continuous hot extraction technique using a siphon principle
- Supercritical Fluid Extraction (SFE): Uses CO2 as a solvent at critical pressure and temperature
- Ultrasound-assisted extraction utilizes acoustic cavitation to disrupt cell walls
Preliminary Phytochemical Screening
These are rapid, qualitative chemical tests performed to determine the presence of major classes of phytoconstituents in an extract. A positive result is indicated by characteristic color changes or precipitation patterns.
- Alkaloids: Dragendorff's reagent, Mayer's reagent, Hager's reagent
- Flavonoids: Shinoda test (Magnesium ribbon + HCl)
- Tannins: Ferric chloride test (Blue-black or green color)
- Saponins: Foam test (persistence of foam for >10 mins)
- Glycosides: Borntrager's test (for anthraquinone glycosides)
Solvent Selection and Optimization
The 'like dissolves like' principle is the cornerstone of solvent selection, where polar solvents are used for polar glycosides and non-polar solvents for volatile oils. Factors like boiling point, toxicity, and cost must be balanced for industrial scaling.
- Petroleum ether: Used for lipids and volatile oils
- Ethanol/Methanol: Universal solvents for polar and non-polar compounds
- Water: Primarily for tannins and plant mucilage
- Dielectric constant dictates the solvent polarity gradient
- Solvent-to-drug ratio is typically maintained at 5:1 to 10:1
Exam Tip
Always memorize the specific color indications for standard reagents as these are the most common questions in viva and written exams.
Common Mistakes
- Confusing Dragendorff's reagent results (orange-red) with Mayer's reagent (cream-colored precipitate).
- Failing to mention the specific role of the siphon in a Soxhlet apparatus.
- Ignoring the necessity of drying the plant material before extraction, which leads to solvent dilution.
More Revision Notes
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