Introduction: Limit tests for impurities in drugs are essential qualitative and semi‑quantitative checks used in pharmaceutical quality control to ensure safety and compliance with pharmacopeial standards. These tests screen for specific impurities—such as heavy metals, chlorides, sulfates, arsenic and sulfides—using classical analytical principles (precipitation, colorimetry, gravimetry and titrimetry). B.Pharm students must understand test principles, common reagents (e.g., silver nitrate, barium chloride, sodium rhodizonate), sample preparation, possible interferences and interpretation of results. Knowledge of limit tests supports regulatory compliance, batch release and patient safety. Key terms: limit tests, impurities, heavy metals, chloride, sulfate, arsenic, pharmacopeia, analytical methods, quality control. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary objective of a pharmacopeial limit test?
- To determine the exact concentration of an active pharmaceutical ingredient
- To provide a qualitative or semi‑quantitative check that a specified impurity does not exceed a stated limit
- To replace full quantitative assays for release testing
- To measure dissolution rate of formulations
Correct Answer: To provide a qualitative or semi‑quantitative check that a specified impurity does not exceed a stated limit
Q2. Which reagent is commonly used in the limit test for chlorides based on precipitation?
- Potassium permanganate
- Silver nitrate with potassium chromate indicator (Mohr’s method)
- Barium chloride
- Sodium nitroprusside
Correct Answer: Silver nitrate with potassium chromate indicator (Mohr’s method)
Q3. The limit test for sulfates typically forms which insoluble precipitate?
- Silver chloride
- Barium sulfate
- Lead sulfide
- Calcium carbonate
Correct Answer: Barium sulfate
Q4. Which principle is commonly used in the limit test for heavy metals?
- Formation of a colored complex measured by UV absorbance
- Precipitation of metal sulfides and comparison against a reference standard (lead acetate/ sulfide test)
- Gas evolution and volumetric measurement
- High performance liquid chromatography
Correct Answer: Precipitation of metal sulfides and comparison against a reference standard (lead acetate/ sulfide test)
Q5. Which test reagent is classically used to detect arsenic in a limit test?
- Gutzeit reagent producing a yellow‑brown stain with arsine (arsenic) on paper
- Nessler’s reagent producing a brown color
- Flame photometry
- Potassium permanganate titration
Correct Answer: Gutzeit reagent producing a yellow‑brown stain with arsine (arsenic) on paper
Q6. Why are limit tests considered semi‑quantitative rather than fully quantitative?
- They use advanced instrumentation that gives exact values
- They indicate whether impurity is above or below a defined threshold, not a precise concentration
- They always overestimate impurity levels
- They cannot detect any impurities
Correct Answer: They indicate whether impurity is above or below a defined threshold, not a precise concentration
Q7. In the chloride limit test, what is the role of potassium chromate indicator?
- To precipitate chloride as a colored complex
- To indicate the endpoint by forming red silver chromate when excess silver is present
- To dissolve the silver chloride precipitate
- To reduce silver ions
Correct Answer: To indicate the endpoint by forming red silver chromate when excess silver is present
Q8. Which of the following is a common interference in the sulfate limit test and how is it controlled?
- Carbonates cause turbidity; control by acidifying the solution to remove carbonate as CO2
- Chlorides form the same precipitate; control by adding silver nitrate
- Organic solvents increase turbidity; control by heating
- Phosphate forms barium phosphate; control by adding hydrochloric acid
Correct Answer: Carbonates cause turbidity; control by acidifying the solution to remove carbonate as CO2
Q9. How is the sensitivity of a limit test typically established?
- By using the most sensitive analytical instrument available
- By testing a reference solution containing the specified limit concentration and confirming visibility/effect
- By diluting the sample until no color remains
- By comparing with unrelated standards
Correct Answer: By testing a reference solution containing the specified limit concentration and confirming visibility/effect
Q10. Which analytical technique can be used to confirm a positive result from a classic limit test?
- Thin layer chromatography (TLC) only
- Atomic absorption spectroscopy (AAS) or ICP‑MS for elemental confirmation
- Visual inspection alone
- pH paper
Correct Answer: Atomic absorption spectroscopy (AAS) or ICP‑MS for elemental confirmation
Q11. In heavy metals limit tests using sulfide precipitation, which gas or reagent is commonly used to generate sulfide?
- Hydrogen sulfide (H2S) gas
- Nitrogen gas
- Oxygen gas
- Carbon dioxide
Correct Answer: Hydrogen sulfide (H2S) gas
Q12. What is a major safety consideration when performing classical limit tests for arsenic and heavy metals?
- No safety precautions are necessary
- Handling and disposal of toxic reagents and gases (arsine, H2S) require appropriate controls and PPE
- Only use of gloves is sufficient
- Safety is not needed for limit tests because quantities are small
Correct Answer: Handling and disposal of toxic reagents and gases (arsine, H2S) require appropriate controls and PPE
Q13. Which factor can cause a false positive in the chloride limit test?
- Presence of bromide or iodide that also precipitate with silver
- High temperature during titration only
- Low sample volume only
- Using distilled water exclusively
Correct Answer: Presence of bromide or iodide that also precipitate with silver
Q14. For a limit test, why is sample preparation and dissolution important?
- It has no impact on the test outcome
- Proper dissolution ensures the impurity is in a measurable form and removes insoluble excipients
- It only affects the color of the solution
- It replaces the need for a reference standard
Correct Answer: Proper dissolution ensures the impurity is in a measurable form and removes insoluble excipients
Q15. How are limit values for impurities typically specified in pharmacopeias?
- As exact concentrations only in mg per tablet
- As maximum allowable concentrations (e.g., ppm, % w/w, or specified test limits) for safety and quality
- As a range with no upper limit
- Pharmacopeias do not specify limits
Correct Answer: As maximum allowable concentrations (e.g., ppm, % w/w, or specified test limits) for safety and quality
Q16. Which of the following best describes a positive outcome in a limit test for heavy metals?
- No visible difference between test and blank
- Test develops color or turbidity greater than that produced by the standard reference solution indicating limit exceeded
- The solution evaporates completely
- pH becomes neutral
Correct Answer: Test develops color or turbidity greater than that produced by the standard reference solution indicating limit exceeded
Q17. Which reagent is used in some arsenic limit tests to reduce arsenic species to arsine gas?
- Sodium hypochlorite
- Zinc and acid (to generate arsine from arsenic)
- Sodium hydroxide alone
- Hydrogen peroxide only
Correct Answer: Zinc and acid (to generate arsine from arsenic)
Q18. What role does acidification play in many anion limit tests (e.g., sulfate)?
- To oxidize the anion
- To remove interfering carbonates and prevent co‑precipitation, ensuring proper formation of the target precipitate
- To evaporate the solvent
- To neutralize the sample completely
Correct Answer: To remove interfering carbonates and prevent co‑precipitation, ensuring proper formation of the target precipitate
Q19. When validating a limit test, which parameter is least relevant?
- Sensitivity (limit of detection relative to specified limit)
- Specificity (ability to detect target impurity without interference)
- Linearity over a wide concentration range beyond purpose
- Precision at the specified limit level
Correct Answer: Linearity over a wide concentration range beyond purpose
Q20. A limit test uses a reference standard that produces slight turbidity when it contains the maximum allowable impurity. If the sample produces identical turbidity, how is the sample interpreted?
- Sample is acceptable because it equals the specified limit
- Sample is unacceptable because any turbidity is prohibited
- Test must be repeated with no reference
- Sample should be discarded without further testing
Correct Answer: Sample is acceptable because it equals the specified limit
Q21. Which modern instrumental method is often used as an alternative to classical limit tests for elemental impurities?
- UV‑Vis spectrophotometry only
- Inductively coupled plasma mass spectrometry (ICP‑MS)
- Polarimetry
- Gas chromatography with flame ionization detector
Correct Answer: Inductively coupled plasma mass spectrometry (ICP‑MS)
Q22. In a chloride limit test by titration, what indicates the endpoint?
- Disappearance of any precipitate
- Appearance of red silver chromate color indicating excess silver
- Sample turning blue
- Evolution of gas
Correct Answer: Appearance of red silver chromate color indicating excess silver
Q23. Which statement accurately contrasts a limit test and a full quantitative assay?
- Limit tests quantify the active ingredient; assays only detect impurities
- Limit tests determine pass/fail relative to a threshold; assays provide exact concentration of an analyte
- Limit tests are more precise than assays
- Assays are always faster and cheaper than limit tests
Correct Answer: Limit tests determine pass/fail relative to a threshold; assays provide exact concentration of an analyte
Q24. In practice, why might a laboratory follow a positive classical limit test with an instrumental analysis?
- To increase the observed turbidity
- To confirm identity and quantify the impurity with higher specificity and sensitivity
- To avoid reporting results
- Instrumental analysis is always required regardless of initial result
Correct Answer: To confirm identity and quantify the impurity with higher specificity and sensitivity
Q25. Which of the following is a common way to express limits for elemental impurities?
- In molar fractions only
- Parts per million (ppm) or micrograms per gram (µg/g)
- As a pH value
- In degrees Celsius
Correct Answer: Parts per million (ppm) or micrograms per gram (µg/g)
Q26. What is the effect of organic solvents or color in the sample matrix on classical limit tests that rely on visual comparison?
- No effect because visual tests are always accurate
- They can mask or mimic turbidity or color, causing false positives or negatives
- They enhance the sensitivity of the test
- They convert impurities to harmless forms
Correct Answer: They can mask or mimic turbidity or color, causing false positives or negatives
Q27. During a sulfate limit test, why is a blank run important?
- To standardize volumetric glassware exclusively
- To detect background turbidity or contamination from reagents that could affect interpretation
- Blanks are unnecessary in limit tests
- To change the chemical composition of the reagents
Correct Answer: To detect background turbidity or contamination from reagents that could affect interpretation
Q28. Which of the following best describes interference control in limit tests?
- Ignoring interferences as they are rare
- Using sample pretreatment, masking agents or adjusting pH to remove or prevent interfering species
- Substituting reagents randomly
- Relying solely on visual judgment without controls
Correct Answer: Using sample pretreatment, masking agents or adjusting pH to remove or prevent interfering species
Q29. For regulatory release, why are pharmacopeial limit tests still used despite availability of advanced instruments?
- They are legally recognized, simple, cost‑effective screening tools and useful where instrumentation is unavailable
- They are always more accurate than instruments
- Instruments are never accepted by regulators
- Limit tests are faster but never reliable
Correct Answer: They are legally recognized, simple, cost‑effective screening tools and useful where instrumentation is unavailable
Q30. When documenting a limit test result in quality control, what must be recorded?
- Only the date and analyst name
- Test procedure, reagent batch, observations, reference standard used, and conclusion (pass/fail)
- Only the conclusion without supporting data
- Only the instrument serial number
Correct Answer: Test procedure, reagent batch, observations, reference standard used, and conclusion (pass/fail)
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
Mail- Sachin@pharmacyfreak.com