Limit test for Heavy metals MCQs With Answer

Limit Test for Heavy Metals MCQs With Answer is a concise, exam-focused resource for B. Pharm students to master pharmacopeial controls of elemental impurities. This topic covers the principle of the heavy metals limit test (expressed as lead), sulfide precipitation chemistry, use of acetate buffer and thioacetamide, visual comparison in Nessler cylinders, and common interferences. It also links classical tests in IP/USP/BP to modern ICH Q3D risk-based strategies and instrumental methods like ICP-MS, ICP-OES, and AAS. Keywords: limit test for heavy metals, B. Pharm, pharmacopoeia, lead standard solution, sulfide method, thioacetamide reagent, Nessler cylinder, ppm, ICH Q3D, elemental impurities. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary principle of the classical pharmacopoeial limit test for heavy metals?

• Formation of colored metal sulfides whose intensity is compared to a lead standard
• Complexation with EDTA and potentiometric endpoint
• Precipitation as metal hydroxides at high pH and gravimetry
• Fluorescence quenching proportional to mercury content

Correct Answer: Formation of colored metal sulfides whose intensity is compared to a lead standard

Q2. In the sulfide precipitation method, heavy metals are conventionally expressed as which element?

• Cadmium
• Lead
• Mercury
• Arsenic

Correct Answer: Lead

Q3. Which reagent is commonly used to generate sulfide ions in situ, replacing direct H2S gas in many compendial procedures?

• Thioacetamide
• Thioglycolic acid
• Dithizone
• Hydroxylamine hydrochloride

Correct Answer: Thioacetamide

Q4. What is the role of the acetate buffer in the heavy metals limit test?

• To maintain pH ~3–4 and control which metal sulfides precipitate
• To complex Pb2+ and prevent precipitation
• To oxidize organic impurities
• To dissolve formed sulfides

Correct Answer: To maintain pH ~3–4 and control which metal sulfides precipitate

Q5. Visual comparison of color intensity in the heavy metals test is traditionally performed in which apparatus?

• Volumetric flasks
• Quartz cuvettes
• Nessler cylinders
• Burettes

Correct Answer: Nessler cylinders

Q6. The color formed in the classical heavy metals limit test mainly corresponds to which compound for lead?

• Lead sulfate (PbSO4)
• Lead sulfide (PbS)
• Lead oxide (PbO)
• Lead chromate (PbCrO4)

Correct Answer: Lead sulfide (PbS)

Q7. Why is the heavy metals limit test described as nonspecific?

• It only detects mercury
• It responds to several metals that form sulfides, not just lead
• It cannot detect any metal below 1000 ppm
• It measures pH instead of metal content

Correct Answer: It responds to several metals that form sulfides, not just lead

Q8. Which compendial chapter largely replaced USP heavy metals <231> with risk-based elemental impurity limits?

• USP <905>
• USP <467>
• USP <232> and <233>
• USP <711>

Correct Answer: USP <232> and <233>

Q9. According to ICH Q3D, which elements are Class 1 elemental impurities of highest toxic concern?

• Pb, Cd, As, Hg
• Cu, Zn, Fe, Mn
• Pt, Pd, Rh, Ru
• Ni, V, Co, Mo

Correct Answer: Pb, Cd, As, Hg

Q10. The unit “ppm” in the context of heavy metals limits typically refers to:

• mg/kg or μg/g
• g/mL
• % w/w
• mol/L

Correct Answer: mg/kg or μg/g

Q11. What is the purpose of preparing a standard lead solution for the test?

• To titrate the sample directly
• To provide a color reference corresponding to the specified limit
• To act as a pH indicator
• To precipitate interfering ions

Correct Answer: To provide a color reference corresponding to the specified limit

Q12. Which heavy metals are poorly detected by the sulfide method at acetate-buffered pH and thus require separate tests?

• Arsenic and antimony
• Copper and bismuth
• Lead and mercury
• Cadmium and silver

Correct Answer: Arsenic and antimony

Q13. Why is thioacetamide preferred over bubbling H2S gas in many labs?

• It is more selective for lead only
• It avoids handling toxic H2S gas and generates sulfide in situ
• It removes the need for a buffer
• It prevents any precipitation

Correct Answer: It avoids handling toxic H2S gas and generates sulfide in situ

Q14. In a valid test, the color intensity of the test solution must be:

• Darker than the standard
• Exactly double the standard
• Not darker than the standard
• Independent of the standard

Correct Answer: Not darker than the standard

Q15. What is the main limitation of visual, comparator-based heavy metals tests?

• They require ICP-MS instrumentation
• They are subjective and have poor sensitivity and selectivity
• They cannot be performed in glassware
• They only work for gases

Correct Answer: They are subjective and have poor sensitivity and selectivity

Q16. Which instrumental technique provides multi-element, sub-ppb detection for elemental impurities?

• ICP-MS
• TLC
• UV-Visible spectrophotometry
• Polarimetry

Correct Answer: ICP-MS

Q17. A typical pretreatment for organic-rich samples before instrumental analysis is:

• Microwave digestion with nitric acid and hydrogen peroxide
• Dissolution in hexane
• Evaporation to dryness at 200 °C
• Addition of sodium hydroxide pellets

Correct Answer: Microwave digestion with nitric acid and hydrogen peroxide

Q18. In the sulfide method, maintaining pH around 3–4 helps to:

• Maximize precipitation of all metal sulfides
• Favor precipitation of easily precipitated sulfides while suppressing others like ZnS
• Convert PbS to soluble complexes
• Eliminate the need for a standard

Correct Answer: Favor precipitation of easily precipitated sulfides while suppressing others like ZnS

Q19. A blank solution in the heavy metals test is used to:

• Adjust the pH of the standard
• Correct for color from reagents and solvents
• Calibrate the balance
• Precipitate excess lead

Correct Answer: Correct for color from reagents and solvents

Q20. Dithizone is associated with which type of heavy metals determination?

• Extraction colorimetry of metals as dithizonates
• Gravimetry as metal hydroxides
• Flame emission spectroscopy
• Potentiometric titration with iodine

Correct Answer: Extraction colorimetry of metals as dithizonates

Q21. Which visual comparator practice improves precision in the limit test?

• Using different cylinder diameters for test and standard
• Using identical Nessler cylinders with the same path length and viewing conditions
• Viewing under changing light sources
• Filling cylinders to different volumes

Correct Answer: Using identical Nessler cylinders with the same path length and viewing conditions

Q22. The term “as Pb” in the test specification means:

• Only lead is measured
• Response is calibrated to an equivalent amount of lead
• Lead must be added to the sample
• Lead is used to oxidize other metals

Correct Answer: Response is calibrated to an equivalent amount of lead

Q23. Which sample property most commonly invalidates the visual sulfide test?

• High melting point
• Strong inherent color or turbidity
• Low density
• High boiling point

Correct Answer: Strong inherent color or turbidity

Q24. For samples that are intrinsically colored, a better approach is to:

• Skip testing
• Use an extraction colorimetric or instrumental method
• Dilute until color disappears without adjusting standard
• Increase buffer concentration arbitrarily

Correct Answer: Use an extraction colorimetric or instrumental method

Q25. The heavy metals limit test is best described as:

• An assay for total Pb content
• A semi-quantitative screening test calibrated against Pb
• A microbiological limit test
• A dissolution test

Correct Answer: A semi-quantitative screening test calibrated against Pb

Q26. Which of the following can cause falsely low readings in the sulfide test?

• Presence of complexing agents like EDTA
• Use of freshly prepared reagents
• Acetate buffer at pH 3–4
• Performing a reagent blank

Correct Answer: Presence of complexing agents like EDTA

Q27. Which statement about ICH Q3D is correct?

• It prescribes only visual tests
• It sets permitted daily exposures based on toxicity and route
• It applies only to excipients
• It bans the use of catalysts

Correct Answer: It sets permitted daily exposures based on toxicity and route

Q28. For accurate elemental impurity quantification at low levels, the preferred specimen introduction for ICP is:

• Electrothermal vaporization without digestion
• Nebulization of properly digested clear solutions
• Solid sampling by direct insertion
• Manual pipetting into a flame

Correct Answer: Nebulization of properly digested clear solutions

Q29. Which metal sulfide typically produces a black or brown coloration aiding visual detection?

• PbS
• ZnS
• CaS
• Na2S

Correct Answer: PbS

Q30. The main safety hazard associated with the classical H2S-based test is:

• Explosive peroxide formation
• Toxicity and flammability of hydrogen sulfide gas
• Radiation exposure
• Osmotic shock

Correct Answer: Toxicity and flammability of hydrogen sulfide gas

Q31. A key reason compendia moved away from USP <231> is that it:

• Was too sensitive for all elements
• Lacked specificity and did not align with toxicological risk
• Required ICP-MS
• Measured only volatile metals

Correct Answer: Lacked specificity and did not align with toxicological risk

Q32. In visual comparison, turbidity affects results by:

• Increasing path length
• Scattering light and artificially darkening the appearance
• Improving color discrimination
• Eliminating reagent blanks

Correct Answer: Scattering light and artificially darkening the appearance

Q33. When preparing lead standard solutions, a common stock is made from:

• Lead nitrate dissolved in acidified water
• Lead carbonate in acetone
• Lead sulfide in buffer
• Lead oxide in hexane

Correct Answer: Lead nitrate dissolved in acidified water

Q34. The sulfide method primarily detects metals that:

• Form insoluble sulfides under mildly acidic conditions
• Form soluble nitrates
• Form volatile hydrides
• Form chlorides at high pH

Correct Answer: Form insoluble sulfides under mildly acidic conditions

Q35. In the context of ICH Q3D, the most stringent PDEs often apply to which route?

• Oral
• Topical
• Inhalation
• Buccal

Correct Answer: Inhalation

Q36. If a sample contains sulfide or sulfide-reducing species, a recommended pretreatment before the sulfide test is to:

• Add more sodium sulfide
• Oxidize with nitric acid or hydrogen peroxide to remove endogenous sulfide
• Increase buffer strength to 2 M
• Neutralize with sodium bicarbonate

Correct Answer: Oxidize with nitric acid or hydrogen peroxide to remove endogenous sulfide

Q37. The color threshold in visual limit tests depends most on:

• Observer, lighting, and path length
• Boiling point of the solvent
• Magnetic stirring speed
• Altitude of the laboratory

Correct Answer: Observer, lighting, and path length

Q38. Which of the following is an advantage of ICP-MS over the classical heavy metals test?

• Lower cost and no calibration needed
• Element-specific quantification at sub-ppb levels
• No sample preparation required
• Better color discrimination

Correct Answer: Element-specific quantification at sub-ppb levels

Q39. The function of a masking agent in heavy metals testing is to:

• Enhance precipitation of all metals
• Prevent interference by selectively complexing certain ions
• Increase turbidity for better visibility
• Generate sulfide ions

Correct Answer: Prevent interference by selectively complexing certain ions

Q40. Which approach aligns testing with patient safety and process knowledge?

• Risk assessment following ICH Q3D and targeted instrumental control
• Universal application of a single visual test
• Skipping testing for low-dose products
• Using random limits without justification

Correct Answer: Risk assessment following ICH Q3D and targeted instrumental control

Q41. If the test solution is slightly yellow due to reagents, the analyst should:

• Ignore it
• Match the test and standard against a reagent blank
• Add dye to mask the color
• Increase sulfide concentration tenfold

Correct Answer: Match the test and standard against a reagent blank

Q42. Which of the following best describes the acceptance decision in the classical test?

• Quantitative calculation of ppm
• Pass if test is not darker than standard corresponding to the limit
• Pass if any precipitate forms
• Fail if pH is below 7

Correct Answer: Pass if test is not darker than standard corresponding to the limit

Q43. For heavy metals determination by extraction colorimetry using dithizone, the colored complex is typically extracted into:

• Chloroform
• Water
• Ethanol
• Glycerin

Correct Answer: Chloroform

Q44. Which of the following metals is least likely to interfere in the sulfide test at pH ~3–4?

• Iron(III)
• Lead(II)
• Mercury(II)
• Bismuth(III)

Correct Answer: Iron(III)

Q45. In preparing the test solution, digestion with nitric acid primarily serves to:

• Reduce metals to zero valence
• Oxidize organics and release metal ions from matrices
• Generate sulfide ions
• Neutralize acidic components

Correct Answer: Oxidize organics and release metal ions from matrices

Q46. Which statement is true regarding the scope of the classical heavy metals limit test?

• It identifies each metal separately
• It is a group limit test indicating combined heavy metals response
• It is only for finished products
• It is only for water testing

Correct Answer: It is a group limit test indicating combined heavy metals response

Q47. A key quality control practice when performing visual comparator tests is to:

• Document lighting conditions and use consistent illumination
• Allow different analysts to alternate mid-comparison
• Use opaque cylinders
• Read results after 48 hours

Correct Answer: Document lighting conditions and use consistent illumination

Q48. Under ICH Q3D, palladium, platinum, and rhodium are typically categorized as:

• Class 1 elements
• Class 2B catalysts
• Class 3 elements
• Volatile hydrides

Correct Answer: Class 2B catalysts

Q49. One practical reason to acidify lead stock solutions slightly is to:

• Prevent adsorption or precipitation of lead species
• Increase volatility of lead
• Create a stronger color by itself
• Neutralize the buffer

Correct Answer: Prevent adsorption or precipitation of lead species

Q50. The most educationally sound way for B. Pharm students to bridge classic tests and modern practice is to:

• Memorize only visual endpoints
• Understand reaction principles and map them to ICH Q3D-driven instrumental controls
• Avoid learning instrumentation
• Use arbitrary safety factors

Correct Answer: Understand reaction principles and map them to ICH Q3D-driven instrumental controls