Masking and demasking agents MCQs With Answer

Masking and demasking agents are essential concepts in analytical chemistry for B.Pharm students, especially in complexometric titrations and selective assays. Masking agents selectively bind interfering ions, forming stable, non-reactive complexes to protect the analyte, while demasking agents release those ions when analysis of the masked species is needed. Understanding mechanisms, selection criteria, pH dependence, stability constants, and common reagents (e.g., fluoride, citrate, tartrate) improves accuracy in drug analysis, quality control, and trace metal determination. Mastery of masking/demasking strategies also enhances method development and troubleshooting in pharmaceutical analysis. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary function of a masking agent in analytical chemistry?

• To precipitate the analyte
• To form a stable complex with interfering ions
• To oxidize interfering species
• To act as a titrant

Correct Answer: To form a stable complex with interfering ions

Q2. What does a demasking agent do?

• Removes the analyte from solution permanently
• Releases an interfering ion from its masked complex
• Acts as a color indicator
• Increases the pH without affecting complexes

Correct Answer: Releases an interfering ion from its masked complex

Q3. Which mechanism best describes masking in complexometric titrations?

• Permanent precipitation of interferent
• Covalent modification of the analyte
• Formation of a stable coordinate complex with the interferent
• Electrostatic shielding of the analyte

Correct Answer: Formation of a stable coordinate complex with the interferent

Q4. Which property is most important when selecting a masking agent?

• High volatility
• Strong, selective binding to the interfering ion without reacting with the analyte
• Ability to change the color of the solution
• Low molecular weight only

Correct Answer: Strong, selective binding to the interfering ion without reacting with the analyte

Q5. How does pH influence masking efficiency?

• pH has no effect on masking
• Masking efficiency often depends on pH because complex stability and ligand protonation change with pH
• Higher pH always improves masking for all systems
• Lower pH always degrades all masking agents

Correct Answer: Masking efficiency often depends on pH because complex stability and ligand protonation change with pH

Q6. Which of the following is a commonly used masking ion for aluminum?

• Sulfide
• Fluoride
• Permanganate
• Bromide

Correct Answer: Fluoride

Q7. Which statement about demasking is correct?

• Demasking always involves dilution only
• Demasking can be achieved by acidification, oxidation, reduction, or addition of a stronger complexing agent
• Demasking never changes pH
• Demasking is only used to destroy the analyte

Correct Answer: Demasking can be achieved by acidification, oxidation, reduction, or addition of a stronger complexing agent

Q8. In complexometric titrations, why might a masking agent be preferred over selective precipitation?

• Masking always requires less reagent
• Masking preserves all species in solution, allowing direct titration of the analyte without co-precipitation errors
• Selective precipitation is always faster
• Masking agents are cheaper than precipitating agents

Correct Answer: Masking preserves all species in solution, allowing direct titration of the analyte without co-precipitation errors

Q9. Which factor describes why a masking agent should be reversible in many assays?

• To allow later demasking when the masked ion must be measured separately
• To permanently inactivate the interferent
• So it can precipitate with the analyte
• To form an irreversible covalent bond

Correct Answer: To allow later demasking when the masked ion must be measured separately

Q10. How does a high formation (stability) constant (Kf) for the interferent–mask complex affect masking?

• High Kf decreases masking efficiency
• High Kf leads to more effective masking of the interfering ion
• Kf is irrelevant to masking
• High Kf causes the analyte to precipitate

Correct Answer: High Kf leads to more effective masking of the interfering ion

Q11. Which of these is NOT a desirable characteristic of a masking agent?

• Selective complexation with the interfering ion
• Non-interference with the titrant or indicator
• Formation of colored complexes that alter detection
• Stability under assay conditions

Correct Answer: Formation of colored complexes that alter detection

Q12. Which analytical technique commonly uses masking/demasking strategies?

• Complexometric titration
• Simple gravimetric weighing only
• Refractive index measurement of polymers
• Thin-layer chromatography without complex formation

Correct Answer: Complexometric titration

Q13. Why must the masking agent not react with the titrant?

• Reaction with the titrant would consume titrant and cause inaccurate results
• Reaction would speed up the titration appropriately
• Titrant reactions are always desirable
• It will make the solution colorless

Correct Answer: Reaction with the titrant would consume titrant and cause inaccurate results

Q14. Which demasking approach uses a stronger ligand to displace the original masking ligand?

• Acidification
• Competitive complexation with a stronger chelator
• Heating without reagents
• Adding an inert salt

Correct Answer: Competitive complexation with a stronger chelator

Q15. How can masking improve spectrophotometric assays?

• By increasing background absorption from interferents
• By preventing interferents from forming colored species that overlap with the analyte signal
• By precipitating the analyte to remove it
• By changing the solvent polarity only

Correct Answer: By preventing interferents from forming colored species that overlap with the analyte signal

Q16. Which of the following is a safety consideration when selecting masking agents?

• Only biodegradability matters
• Toxicity and disposal issues (e.g., cyanide) must be considered
• Masking agents are always safe
• Only pH matters for safety

Correct Answer: Toxicity and disposal issues (e.g., cyanide) must be considered

Q17. What is the likely result if an insufficient amount of masking agent is used?

• No change—masking is independent of concentration
• Incomplete masking leads to interference and inaccurate analysis
• The analyte concentration will always be overestimated
• The titration will proceed faster without errors

Correct Answer: Incomplete masking leads to interference and inaccurate analysis

Q18. Which is an example of demasking by chemical destruction of the masking ligand?

• Adding a solvent that dissolves both complex and analyte
• Oxidizing the masking ligand to break its complex with the interferent
• Cooling the solution to precipitate the ligand
• Filtering the solution to remove the ligand

Correct Answer: Oxidizing the masking ligand to break its complex with the interferent

Q19. In a multi-metal analysis by EDTA titration, why might masking be necessary?

• To increase ionic strength only
• To prevent certain metals from reacting with EDTA so that a specific metal can be determined selectively
• EDTA works only with masked metals
• To change the solvent dielectric constant

Correct Answer: To prevent certain metals from reacting with EDTA so that a specific metal can be determined selectively

Q20. Which factor would you adjust to demask a metal complex that is sensitive to protonation?

• Lower the temperature drastically
• Acidify the solution to protonate and release the masking ligand
• Add more masking agent
• Add a nonpolar solvent

Correct Answer: Acidify the solution to protonate and release the masking ligand

Q21. What role does kinetics play in choosing a masking agent?

• Kinetics are irrelevant if thermodynamics favor complexation
• The masking reaction must be sufficiently fast to form the complex under assay conditions
• Slower kinetics are always preferred
• Only solubility matters, not kinetics

Correct Answer: The masking reaction must be sufficiently fast to form the complex under assay conditions

Q22. Which of the following is a correct statement about irreversible masking?

• Irreversible masking is preferred when the interferent must be recovered intact
• Irreversible masking permanently removes the interferent, making later quantification difficult
• Irreversible masking is the same as reversible masking
• Irreversible masking always improves precision

Correct Answer: Irreversible masking permanently removes the interferent, making later quantification difficult

Q23. How can masking agents affect the choice of indicator in titrations?

• They never affect the indicator
• If the masking agent forms colored complexes, a different indicator may be required to avoid spectral overlap
• Indicators are always incompatible with masked systems
• One should choose an indicator that reacts with the masking agent

Correct Answer: If the masking agent forms colored complexes, a different indicator may be required to avoid spectral overlap

Q24. Which experimental practice helps ensure effective masking in an assay?

• Adding masking agent in stoichiometric or slight excess relative to the interfering ion
• Using the masking agent only after titration is complete
• Avoiding pH control
• Diluting the masking agent to negligible concentration

Correct Answer: Adding masking agent in stoichiometric or slight excess relative to the interfering ion

Q25. For quality control in pharmaceuticals, masking agents aid primarily by:

• Causing drugs to precipitate
• Improving selectivity and accuracy when trace metals or impurities interfere with assays
• Replacing active ingredients
• Enhancing drug potency

Correct Answer: Improving selectivity and accuracy when trace metals or impurities interfere with assays

Q26. Which is an appropriate demasking method when the masking ligand is oxidizable?

• Add a reducing agent
• Add an oxidizing agent to break down the masking ligand
• Lower the ionic strength only
• Add EDTA as a masking agent

Correct Answer: Add an oxidizing agent to break down the masking ligand

Q27. What is the effect of a masking agent on the equilibrium between analyte, interferent and titrant?

• It shifts equilibrium by binding the interferent, reducing its free concentration and minimizing side reactions
• It increases the free interferent concentration
• It always shifts equilibrium toward precipitation
• It has no effect on equilibria

Correct Answer: It shifts equilibrium by binding the interferent, reducing its free concentration and minimizing side reactions

Q28. When analyzing a sample containing Ca2+ and Mg2+, which masking strategy is conceptually correct to determine Ca2+ alone?

• Use a masking agent that selectively complexes Mg2+ so Ca2+ can be titrated
• Mask Ca2+ and titrate Mg2+ instead without demasking
• Precipitate both ions before titration
• Ignore the presence of Mg2+

Correct Answer: Use a masking agent that selectively complexes Mg2+ so Ca2+ can be titrated

Q29. Which outcome indicates successful masking during an assay?

• Increased interference and blurred end point
• Sharper end point and expected stoichiometric titration of the analyte
• Complete loss of analyte signal
• Formation of multiple precipitates

Correct Answer: Sharper end point and expected stoichiometric titration of the analyte

Q30. Why is documentation of masking/demasking procedures important in pharmaceutical analysis?

• Procedures are trivial and need not be recorded
• Detailed documentation ensures reproducibility, traceability and regulatory compliance for assay methods
• Documentation only increases paperwork without benefit
• It prevents any chemical reactions from occurring

Correct Answer: Detailed documentation ensures reproducibility, traceability and regulatory compliance for assay methods

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