Extraction techniques: solvent selection, successive/exhaustive methods MCQs With Answer

Introduction

Extraction techniques are foundational in phytochemistry for isolating bioactive compounds from plant matrices. This quiz focuses on solvent selection principles and the two common approaches: successive (sequential) and exhaustive extraction. You will review factors that guide solvent choice—polarity matching, miscibility, boiling point, toxicity, and selectivity—plus practical strategies such as pH manipulation, salting‑out, particle size control, and preventing emulsions. The questions emphasize method rationale (why a solvent or pH is chosen), process design (order of solvents in successive extraction), and operational considerations (Soxhlet vs maceration, supercritical CO2, co‑solvents, chelators). These MCQs will help M.Pharm students apply theory to real extraction scenarios.

Q1. Which statement correctly describes successive extraction?

• Using a single solvent repeatedly until no more analyte is extracted
• Applying solvents of increasing polarity sequentially to fractionate classes of compounds
• Extracting by increasing temperature gradually to improve yield
• Employing supercritical fluids in consecutive pressure stages

Correct Answer: Applying solvents of increasing polarity sequentially to fractionate classes of compounds

Q2. Which set of criteria is most important when selecting a solvent for phytochemical extraction?

• Color of solvent, supplier brand, and bottle size
• Polarity relative to target compound, boiling point, toxicity, and miscibility
• Market price only
• Odor and historical preference without analytical backing

Correct Answer: Polarity relative to target compound, boiling point, toxicity, and miscibility

Q3. The partition coefficient (K) in liquid–liquid extraction is defined as:

• The ratio of solvent volumes used
• The ratio of concentration of solute in the organic phase to that in the aqueous phase at equilibrium
• The difference between initial and final mass of solute
• The solubility of solute in water only

Correct Answer: The ratio of concentration of solute in the organic phase to that in the aqueous phase at equilibrium

Q4. To extract a weak acidic phytochemical into an organic solvent, you should:

• Adjust the aqueous pH above the compound’s pKa to form the anion
• Adjust the aqueous pH below the compound’s pKa to convert it to the non‑ionized form
• Add a strong oxidant to degrade interfering substances
• Use a nonpolar solvent without pH adjustment

Correct Answer: Adjust the aqueous pH below the compound’s pKa to convert it to the non‑ionized form

Q5. The salting‑out effect during liquid–liquid extraction helps by:

• Increasing solute solubility in water so it cannot be extracted
• Decreasing water’s ability to solvate nonpolar solutes, promoting transfer to the organic phase
• Causing chemical degradation of the target compound
• Lowering the boiling point of the aqueous phase

Correct Answer: Decreasing water’s ability to solvate nonpolar solutes, promoting transfer to the organic phase

Q6. Which extraction technique is most accurately described as exhaustive extraction?

• Maceration for a fixed short time without solvent replacement
• Soxhlet extraction where solvent is continuously cycled until the sample is depleted
• One‑time liquid–liquid partition with equal volumes
• Ultrasonic cleaning of glassware

Correct Answer: Soxhlet extraction where solvent is continuously cycled until the sample is depleted

Q7. For extracting polar glycosides from plant material with high efficiency, the most suitable solvent is:

• n‑Hexane
• Diethyl ether
• Methanol
• Carbon tetrachloride

Correct Answer: Methanol

Q8. Typical solvent order for a successive extraction from nonpolar to polar is:

• Methanol → Ethyl acetate → Chloroform → Hexane
• Hexane → Chloroform (or dichloromethane) → Ethyl acetate → Methanol (or water)
• Water → Methanol → Hexane → Benzene
• Acetonitrile → Hexane → Water → Ether

Correct Answer: Hexane → Chloroform (or dichloromethane) → Ethyl acetate → Methanol (or water)

Q9. Which statement about supercritical CO2 (scCO2) extraction is correct?

• scCO2 is highly polar and ideal for extracting sugars without modifiers
• scCO2 has tunable solvating power with pressure and temperature and often needs a polar co‑solvent for polar compounds
• scCO2 has a critical point at 100°C and 200 bar
• scCO2 is immiscible with nonpolar lipids

Correct Answer: scCO2 has tunable solvating power with pressure and temperature and often needs a polar co‑solvent for polar compounds

Q10. Which factor most strongly influences extraction rate (kinetics) from plant particles?

• Particle size of the plant material (smaller particles increase rate)
• Color of the solvent
• Brand of extraction apparatus
• Time of day the plant was harvested

Correct Answer: Particle size of the plant material (smaller particles increase rate)

Q11. A persistent emulsion forms during liquid–liquid extraction. Which practical measure is commonly used to break it?

• Add a neutral dye
• Add saturated sodium chloride (salt) or a suitable demulsifier and centrifuge
• Lower the temperature to freeze the emulsion
• Increase the stirring speed indefinitely

Correct Answer: Add saturated sodium chloride (salt) or a suitable demulsifier and centrifuge

Q12. Back‑extraction (or stripping) refers to:

• Removing solvent from a crude extract by rotary evaporation
• Transferring the analyte from the organic phase back into an aqueous phase by changing conditions such as pH
• Adding drying agent to the organic layer
• Using heat to desorb analyte from solid adsorbents

Correct Answer: Transferring the analyte from the organic phase back into an aqueous phase by changing conditions such as pH

Q13. To favor extraction of alkaloids (basic compounds) into an organic solvent, the aqueous phase should be:

• Acidified well below the pKa to keep alkaloids protonated
• Basified above the pKa so alkaloids are in the free base (non‑ionized) form
• Frozen to trap water and release alkaloids
• Made highly ionic using strong chaotropes

Correct Answer: Basified above the pKa so alkaloids are in the free base (non‑ionized) form

Q14. Which additive is commonly used to chelate metal ions and prevent metal‑catalyzed oxidation during extraction?

• EDTA (ethylenediaminetetraacetic acid)
• Sodium chloride solely for salting out
• Hydrochloric acid to oxidize metals
• Gaseous oxygen to stabilize phenolics

Correct Answer: EDTA (ethylenediaminetetraacetic acid)

Q15. A primary disadvantage of exhaustive extraction methods (e.g., prolonged Soxhlet) is:

• They always give incomplete recovery
• They require large solvent volumes and long times and may degrade thermolabile compounds
• They never extract nonpolar compounds
• They immediately purify the extract to final product

Correct Answer: They require large solvent volumes and long times and may degrade thermolabile compounds

Q16. Which solvent is fully miscible with water and often used as a polar organic extraction solvent?

• n‑Hexane
• Dichloromethane
• Ethanol
• n‑Pentane

Correct Answer: Ethanol

Q17. For selective extraction of nonpolar lipids from plant material, the preferred solvent is:

• Methanol
• Water
• n‑Hexane
• Ethanol–water (1:1)

Correct Answer: n‑Hexane

Q18. How does increasing extraction temperature generally affect yield and selectivity?

• It always improves selectivity and prevents any degradation
• It increases solubility and diffusion rates improving yield but can decrease selectivity and risk thermal degradation of sensitive compounds
• It has no effect on extraction
• It solidifies the solvent preventing extraction

Correct Answer: It increases solubility and diffusion rates improving yield but can decrease selectivity and risk thermal degradation of sensitive compounds

Q19. In scCO2 extraction of moderately polar flavonoids, adding a small percentage of ethanol functions as:

• A demulsifier to increase emulsion stability
• A co‑solvent (modifier) to increase solvating power for polar analytes
• An oxidizing agent to convert flavonoids
• A drying agent to remove water

Correct Answer: A co‑solvent (modifier) to increase solvating power for polar analytes

Q20. Which of the following is considered a green solvent choice for phytochemical extraction?

• Chloroform
• Ethanol (food‑grade)
• Benzene
• Carbon tetrachloride

Correct Answer: Ethanol (food‑grade)

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