Limit test for Lead MCQs With Answer

Limit Test for Lead MCQs with Answer is designed to help B. Pharm students master the classical pharmacopoeial method for detecting trace lead impurity in pharmaceuticals. This topic covers the principle of the limit test for lead (dithizone complexation), sample preparation, ammonia–citrate buffer, masking agents (e.g., potassium cyanide, ammonium citrate), chloroform extraction, color comparison with a standard lead solution, and interpretation as per IP/BP/USP. You will also learn common interferences, pH control, reagent blank, safety, and alternatives like AAS/ICP-MSpharmaceutics, pharmaceutical analysis, and quality control to ensure patient safety from heavy metals. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary objective of the limit test for lead in pharmaceuticals?

• To ensure lead impurity is below the prescribed pharmacopoeial limit
• To determine the potency of the active ingredient
• To quantify total organic carbon
• To measure chloride content

Correct Answer: To ensure lead impurity is below the prescribed pharmacopoeial limit

Q2. What is the fundamental principle of the classical limit test for lead?

• Precipitation of lead as lead sulfide
• Complexation of lead with dithizone and extraction into chloroform
• Oxidation of lead with potassium permanganate
• Titration of lead with EDTA using eriochrome black T

Correct Answer: Complexation of lead with dithizone and extraction into chloroform

Q3. Dithizone, used in the lead limit test, is also known as:

• Diphenylthiocarbazone
• Dimethylglyoxime
• Eriochrome black T
• Phenolphthalein

Correct Answer: Diphenylthiocarbazone

Q4. The lead–dithizone complex in chloroform typically appears:

• Colorless
• Blue
• Red-violet (magenta)
• Yellow

Correct Answer: Red-violet (magenta)

Q5. The color of free dithizone in chloroform (in the absence of lead) is:

• Green
• Red
• Purple
• Orange

Correct Answer: Green

Q6. Which organic solvent is classically used to extract the lead–dithizone complex?

• Chloroform
• Hexane
• Ethyl acetate
• Toluene

Correct Answer: Chloroform

Q7. What is the primary role of ammonium citrate in the lead limit test?

• To precipitate lead as lead citrate
• To complex interfering metals and help maintain alkaline conditions
• To oxidize lead(II) to lead(IV)
• To act as a color indicator

Correct Answer: To complex interfering metals and help maintain alkaline conditions

Q8. Potassium cyanide is added in the classical method mainly to:

• Mask copper, cadmium, and zinc by forming stable cyanide complexes
• Increase the solubility of lead in water
• Act as a reducing agent for iron
• Adjust the pH to acidic conditions

Correct Answer: Mask copper, cadmium, and zinc by forming stable cyanide complexes

Q9. Hydroxylamine hydrochloride is included in the procedure to:

• Reduce Fe3+ to Fe2+ and remove oxidizing interferences
• Precipitate lead as lead hydroxide
• Enhance the volatility of lead
• Increase the partition coefficient into chloroform

Correct Answer: Reduce Fe3+ to Fe2+ and remove oxidizing interferences

Q10. The extraction of the lead–dithizone complex is most effective at which pH range?

• 1–2
• 4–5
• 8–9
• 12–13

Correct Answer: 8–9

Q11. The standard lead solution used for visual comparison in the limit test commonly represents:

• 10 ppm lead
• 1 ppm lead
• 50 ppm lead
• 200 ppm lead

Correct Answer: 10 ppm lead

Q12. Which salt is typically used to prepare the stock standard solution of lead?

• Lead nitrate
• Lead sulfate
• Lead acetate trihydrate
• Lead chloride

Correct Answer: Lead nitrate

Q13. The control (standard) solution in the lead limit test is prepared by:

• Treating a standard lead solution with the same reagents as the test
• Using only chloroform without reagents
• Adding sulfuric acid to lead nitrate crystals
• Heating lead powder with nitric acid

Correct Answer: Treating a standard lead solution with the same reagents as the test

Q14. In a visual comparison, if the test extract is more intensely colored than the standard extract, the sample:

• Passes the test
• Fails the test
• Requires dilution and retesting
• Is unsuitable for chloroform extraction

Correct Answer: Fails the test

Q15. Why should all glassware be acid-washed before performing the lead limit test?

• To increase the extraction rate
• To remove trace lead contamination that could cause a false failure
• To improve color contrast
• To sterilize the apparatus

Correct Answer: To remove trace lead contamination that could cause a false failure

Q16. Which pharmacopoeias describe the limit test for lead?

• Indian Pharmacopoeia (IP)
• British Pharmacopoeia (BP)
• United States Pharmacopeia (USP)
• All of the above

Correct Answer: All of the above

Q17. Sensitivity of the limit test can be improved by:

• Using only one large extraction with dithizone
• Performing multiple small, successive extractions with dithizone
• Using water instead of chloroform
• Eliminating masking agents

Correct Answer: Performing multiple small, successive extractions with dithizone

Q18. Safe handling of potassium cyanide in the laboratory requires:

• Open bench work and acid rinsing
• Work in a fume hood with PPE and segregated waste
• Heating to boiling to ensure dissolution
• Neutralization with strong acid after use

Correct Answer: Work in a fume hood with PPE and segregated waste

Q19. Which of the following is a major positive interference if not adequately masked?

• Sodium ions
• Copper(II) ions
• Chloride ions
• Nitrate ions

Correct Answer: Copper(II) ions

Q20. For spectrophotometric reading of the lead–dithizone complex, the absorbance maximum is typically near:

• 280 nm
• 340 nm
• 520 nm
• 700 nm

Correct Answer: 520 nm

Q21. The classical lead limit test is best described as:

• A precise quantitative assay
• A semi-quantitative, pass–fail color comparison
• A gravimetric determination
• A chromatographic separation

Correct Answer: A semi-quantitative, pass–fail color comparison

Q22. The reagent blank in this test is used to:

• Calibrate the pH meter
• Correct for color contributed by reagents and solvent
• Increase the amount of lead extracted
• Precipitate interfering ions

Correct Answer: Correct for color contributed by reagents and solvent

Q23. Ammonia is added in the procedure mainly to:

• Provide an alkaline medium for complex formation
• Precipitate lead hydroxide
• Oxidize lead(II) to lead(IV)
• Decrease the solubility of dithizone

Correct Answer: Provide an alkaline medium for complex formation

Q24. Classical masking agents used in the lead limit test include:

• Ammonium citrate and potassium cyanide
• Hydrochloric acid and sulfuric acid
• Ethanol and acetone
• Phenolphthalein and methyl orange

Correct Answer: Ammonium citrate and potassium cyanide

Q25. For dissolving solid samples prior to testing, a commonly used acid is:

• Acetic acid (glacial)
• Nitric acid (dilute)
• Hydrofluoric acid
• Carbonic acid

Correct Answer: Nitric acid (dilute)

Q26. In the limit test for lead, the color comparison is made using which layer?

• The aqueous layer
• The chloroform layer containing the lead–dithizone complex
• The precipitated solid layer
• The headspace vapor

Correct Answer: The chloroform layer containing the lead–dithizone complex

Q27. For reliable visual comparison, the test tubes should be viewed against:

• A black background
• A patterned background
• A white background under consistent lighting
• Direct sunlight

Correct Answer: A white background under consistent lighting

Q28. Dithizone solution for this test is typically prepared in:

• Water
• Methanol
• Chloroform
• Ethanol

Correct Answer: Chloroform

Q29. After exhaustive extraction of lead, the color of the chloroform dithizone layer should remain:

• Green
• Red
• Blue
• Colorless

Correct Answer: Green

Q30. Which reagent in the procedure mitigates oxidizing interferences that may degrade dithizone?

• Potassium cyanide
• Hydroxylamine hydrochloride
• Sodium chloride
• Ammonium sulfate

Correct Answer: Hydroxylamine hydrochloride

Q31. Compared with the general “Heavy metals” test (as Pb), the lead limit test with dithizone is:

• Less specific and more general
• More specific for lead under controlled pH and masking
• Only applicable to gases
• Based on gravimetry

Correct Answer: More specific for lead under controlled pH and masking

Q32. The lead–dithizone complex partitions into chloroform because it is:

• Ionic and highly polar
• Non-polar and soluble in organic solvent
• Gaseous at room temperature
• Hydrogen-bonded to water

Correct Answer: Non-polar and soluble in organic solvent

Q33. Formation of persistent emulsions during extraction can be minimized by:

• Violently shaking for extended periods
• Allowing the mixture to stand for clear phase separation
• Adding strong acid directly to the emulsion
• Heating the mixture to boiling

Correct Answer: Allowing the mixture to stand for clear phase separation

Q34. A false low result (apparent pass) may occur if:

• The extraction is done at too low a pH
• Ammonia is added to make the solution alkaline
• The reagent blank is included
• Multiple extractions are performed

Correct Answer: The extraction is done at too low a pH

Q35. Pre-cleaning (purifying) the dithizone solution before use helps to:

• Increase its acidity
• Remove traces of metal–dithizonate contaminants
• Increase the density of chloroform
• Oxidize lead to a higher valency

Correct Answer: Remove traces of metal–dithizonate contaminants

Q36. Dithizone solutions should be stored in amber containers primarily because:

• They are base-labile
• They are highly volatile
• They are light-sensitive and can degrade
• They freeze at room temperature

Correct Answer: They are light-sensitive and can degrade

Q37. Visual comparisons should be performed promptly after extraction because:

• Lead evaporates quickly
• The color may fade due to photodegradation or oxidation
• Chloroform solidifies over time
• Ammonia becomes acidic

Correct Answer: The color may fade due to photodegradation or oxidation

Q38. Pharmacopoeial lead limits are commonly expressed as:

• mg of Pb per tablet
• % w/w of Pb
• ppm (µg/g or µg/mL) of Pb
• moles of Pb per liter

Correct Answer: ppm (µg/g or µg/mL) of Pb

Q39. If the sample contains sulfide or strong reducing agents, pretreatment is needed because they can:

• Increase chloroform density
• Interfere by reducing or reacting with dithizone
• Improve masking of copper
• Stabilize the complex color

Correct Answer: Interfere by reducing or reacting with dithizone

Q40. An appropriate analytical blank for this test contains:

• Only distilled water
• All reagents except the sample
• Only chloroform
• Standard lead solution without reagents

Correct Answer: All reagents except the sample

Q41. Modern instrumental alternatives that can quantify trace lead with high sensitivity include:

• Melting point apparatus
• Atomic absorption spectroscopy (AAS) or ICP-MS
• Thin-layer chromatography
• Polarimetry

Correct Answer: Atomic absorption spectroscopy (AAS) or ICP-MS

Q42. EDTA, if present in the sample solution, may cause:

• Enhanced color development
• Suppressed extraction by chelating Pb2+
• Formation of a red precipitate
• Oxidation of dithizone

Correct Answer: Suppressed extraction by chelating Pb2+

Q43. To minimize emulsions during shaking with chloroform, it is good practice to:

• Shake moderately and consistently for a fixed time
• Bubble air vigorously through the mixture
• Add strong acid to break the emulsion
• Use boiling water bath during extraction

Correct Answer: Shake moderately and consistently for a fixed time

Q44. A standard addition check near the limit concentration helps to:

• Lower the detection limit
• Identify matrix effects that suppress or enhance color
• Increase the partition coefficient of chloroform
• Eliminate the need for a standard solution

Correct Answer: Identify matrix effects that suppress or enhance color

Q45. Digestion or dissolution of solid samples prior to testing ensures:

• Lead remains in the solid state
• Complete conversion of lead into a measurable solution
• Formation of volatile lead species
• Elimination of the need for masking

Correct Answer: Complete conversion of lead into a measurable solution

Q46. Why is a single “limit” standard used rather than a full calibration curve in the classical test?

• The test is a pass–fail comparison at the specified limit concentration
• Calibration curves are prohibited by pharmacopoeias
• The color is independent of concentration
• Instruments cannot read multiple standards

Correct Answer: The test is a pass–fail comparison at the specified limit concentration

Q47. In the classical method, lead is measured in which oxidation state?

• Pb0
• Pb2+
• Pb4+
• Mixed valency only

Correct Answer: Pb2+

Q48. For a pharmacopoeial limit test, a critical validation attribute is:

• Retention factor (Rf)
• Specificity at the limit concentration
• Boiling point of chloroform
• Particle size of glassware

Correct Answer: Specificity at the limit concentration

Q49. If a repeat extraction with fresh dithizone does not change the color of the chloroform layer, this suggests:

• Lead is still present in the aqueous phase
• Lead has been completely extracted under the conditions used
• Dithizone is too concentrated
• The pH is excessively acidic

Correct Answer: Lead has been completely extracted under the conditions used

Q50. Appropriate disposal for cyanide-containing waste generated during the test is to:

• Neutralize with acid and pour into the sink
• Mix with all other halogenated solvents
• Collect separately and dispose as hazardous waste per institutional protocol; never acidify
• Evaporate in a fume hood

Correct Answer: Collect separately and dispose as hazardous waste per institutional protocol; never acidify