Solvent extraction techniques MCQs With Answer

Solvent extraction techniques MCQs With Answer offers a focused set of practice questions tailored for M.Pharm students studying Bioprocess Engineering and Technology. This collection emphasizes both fundamental principles and practical aspects of liquid–liquid extraction as applied to pharmaceutical processes: partition and distribution coefficients, pH control, choice of extractants and solvents, equipment (mixer-settler, centrifugal contactor), process variables, salting-out, complexation, stripping, and solvent recovery. Questions combine conceptual clarity with process-oriented scenarios to help learners prepare for exams and laboratory decision-making. Answers are provided to reinforce learning and enable self-assessment for students aiming to design, optimize, or troubleshoot extraction steps in drug development and production.

Q1. What is the partition coefficient (K) in liquid–liquid extraction?

• The ratio of solute concentration in the organic phase to that in the aqueous phase at equilibrium
• The ratio of solvent densities in organic and aqueous phases
• The distribution of solvent between two immiscible liquids
• The mass transfer coefficient times phase contact time

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

Q2. How does the distribution ratio (D) differ from the partition coefficient (K)?

• D is measured at non-equilibrium, while K is equilibrium-only
• D accounts for all chemical forms of the solute in both phases; K refers to a single molecular species
• K includes complexation effects but D does not
• D is independent of pH while K strongly depends on pH

Correct Answer: D accounts for all chemical forms of the solute in both phases; K refers to a single molecular species

Q3. For extraction of a weak acid drug from water into an organic solvent, which pH adjustment will generally increase extraction into the organic phase?

• Increase pH to make the acid fully ionized
• Lower the pH to convert the acid to its unionized form
• Adjust pH to the drug’s isoelectric point
• Maintain pH at neutral (pH 7) regardless of drug pKa

Correct Answer: Lower the pH to convert the acid to its unionized form

Q4. What is the primary purpose of adding salt (salting-out) to the aqueous phase during extraction?

• To increase the solubility of the solute in water
• To decrease the density difference between phases
• To reduce aqueous solubility of the solute and drive it into the organic phase
• To form complexes that retain solute in the aqueous phase

Correct Answer: To reduce aqueous solubility of the solute and drive it into the organic phase

Q5. Which solvent property is most critical when selecting an extractant for an organic solute?

• High partition coefficient for the target solute
• Bright color for visual monitoring
• Low vapor pressure only for safety reasons
• High miscibility with water

Correct Answer: High partition coefficient for the target solute

Q6. Which liquid–liquid extraction unit is most commonly used for large-scale continuous pharmaceutical separations?

• Batch separatory funnel
• Rotary evaporator
• Mixer-settler
• Thin-film evaporator

Correct Answer: Mixer-settler

Q7. How does increasing the organic-to-aqueous phase volume ratio (O/A) affect extraction of a neutral solute (assuming constant K)?

• Decreases the fraction extracted into organic phase
• Has no effect on extraction efficiency
• Increases the fraction of solute transferred to the organic phase
• Makes equilibrium unattainable

Correct Answer: Increases the fraction of solute transferred to the organic phase

Q8. Which measure is commonly used to break emulsions that form during liquid–liquid extraction?

• Add a demulsifier or coalescing agent and allow settling
• Increase agitation intensity indefinitely
• Reduce interfacial tension by adding surfactant intentionally
• Cool rapidly to freeze the emulsion

Correct Answer: Add a demulsifier or coalescing agent and allow settling

Q9. Which strategy is most appropriate to extract an ionizable basic drug into an organic solvent?

• Adjust aqueous pH to make the drug ionized and hydrophilic
• Adjust aqueous pH to convert the drug to its unionized base form
• Add strong acid to convert drug to quaternary salt
• Increase aqueous ionic strength to hold drug in water

Correct Answer: Adjust aqueous pH to convert the drug to its unionized base form

Q10. Is the distribution ratio (D) affected by temperature?

• No, D is a constant independent of temperature
• Yes, D typically changes with temperature because equilibrium constants are temperature dependent
• Only if the solvent is polar
• Only for non-ionizable solutes

Correct Answer: Yes, D typically changes with temperature because equilibrium constants are temperature dependent

Q11. What is the main purpose of the stripping (back-extraction) step in a solvent extraction process?

• To salt-out impurities into the organic phase
• To recover the solute from the organic phase into a new aqueous phase for further processing
• To evaporate the solvent and concentrate the organic phase
• To neutralize the organic solvent

Correct Answer: To recover the solute from the organic phase into a new aqueous phase for further processing

Q12. How is selectivity of an extractant for a target compound commonly defined?

• As the solvent’s boiling point divided by its density
• As the ratio of partition coefficients (or distribution ratios) of target over impurity
• As the absolute concentration of extractant in the organic phase
• As the total phase volume used in extraction

Correct Answer: As the ratio of partition coefficients (or distribution ratios) of target over impurity

Q13. Why are chelating or complexing agents used in metal or drug extractions?

• They reduce extraction temperature requirements
• They form hydrophobic complexes that transfer more readily into the organic phase
• They neutralize the organic solvent
• They always increase aqueous solubility of the target

Correct Answer: They form hydrophobic complexes that transfer more readily into the organic phase

Q14. What is the “third phase” phenomenon encountered in some solvent extraction systems?

• Generation of a gaseous phase due to solvent boiling
• Formation of a separate viscous or solid-like middle phase when overloaded with complexed solute
• A transient microemulsion that enhances extraction efficiency permanently
• Complete miscibility of the two phases into one homogeneous phase

Correct Answer: Formation of a separate viscous or solid-like middle phase when overloaded with complexed solute

Q15. Which property is least relevant when selecting an extractant for efficient separation (assuming safety and environmental constraints are handled separately)?

• Partition coefficient for the solute
• Density difference between phases
• Viscosity affecting mass transfer
• The color of the solvent

Correct Answer: The color of the solvent

Q16. Compared to co-current extraction, what is the main advantage of counter-current liquid–liquid extraction?

• It requires less solvent but achieves similar separation efficiency
• It is always easier to operate at lab scale
• It eliminates the need for a settling stage
• It prevents any back-mixing of phases

Correct Answer: It requires less solvent but achieves similar separation efficiency

Q17. The basic distribution law describing equilibrium partitioning of a solute between two immiscible liquids is named after which scientist?

• Henry
• Nernst
• Raoult
• Fick

Correct Answer: Nernst

Q18. Which extraction technique is preferred for gentle separation of proteins and large biomolecules where organic solvents would denature the product?

• Classical organic solvent LLE with dichloromethane
• Aqueous two-phase extraction (ATPS) using polymers or salts
• Supercritical CO2 extraction at high temperature
• Soxhlet extraction with hot organic solvent

Correct Answer: Aqueous two-phase extraction (ATPS) using polymers or salts

Q19. Why might a co-solvent (modifier) be added to the organic phase in a pharmaceutical extraction?

• To permanently polymerize the organic phase
• To increase miscibility with water and always prevent phase separation
• To tune solvent polarity and improve solute solubility in the organic phase while balancing phase separation
• To neutralize the aqueous phase pH

Correct Answer: To tune solvent polarity and improve solute solubility in the organic phase while balancing phase separation

Q20. Which method is most commonly used for recovery and reuse of organic solvents after extraction in pharmaceutical processes?

• Membrane filtration of the organic phase
• Distillation to separate solute and recover solvent
• Direct landfill disposal
• Lyophilization of the solvent

Correct Answer: Distillation to separate solute and recover solvent

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