Limit test for Arsenic MCQs With Answer

Limit Test for Arsenic MCQs with Answers is an essential topic for B. Pharm students studying pharmaceutical analysis and quality control. This pharmacopeial test (often the Gutzeit method) detects trace arsenic by generating arsine gas (AsH3) from the sample using arsenic-free zinc and acid, then forming a yellow to brown stain on mercuric bromide paper compared against a standard. Key aspects include the principle, reagents (HCl, KI, stannous chloride, lead acetate scrubber), apparatus, interferences (H2S, antimony), and safety. Modern alternatives include the silver diethyldithiocarbamate (SDDC) spectrophotometric method and ICP-MS aligned with ICH Q3D arsenic limits (e.g., oral PDE 15 µg/day). Typical pharmacopeial limits are around 1–2 ppm for drug substances. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the core principle of the classical limit test for arsenic (Gutzeit method)?

• Oxidation of arsenic to arsenate followed by titration
• Generation of arsine gas and its reaction with mercuric bromide paper to form a colored stain
• Precipitation of arsenic as arsenic trisulfide and gravimetric weighing
• Formation of a blue complex measured at 700 nm

Correct Answer: Generation of arsine gas and its reaction with mercuric bromide paper to form a colored stain

Q2. Which gas is produced from arsenic compounds during the Gutzeit limit test?

• Phosphine (PH3)
• Ammonia (NH3)
• Arsine (AsH3)
• Sulfur dioxide (SO2)

Correct Answer: Arsine (AsH3)

Q3. In the classical Gutzeit test, the detecting paper is impregnated with which reagent?

• Mercuric bromide
• Silver nitrate
• Potassium permanganate
• Cobalt thiocyanate

Correct Answer: Mercuric bromide

Q4. What is the typical color of the stain formed on the mercuric bromide paper by arsine?

• Blue to green
• Yellow to brown
• Purple
• Colorless

Correct Answer: Yellow to brown

Q5. Why is potassium iodide often added in the arsenic limit test procedure?

• To oxidize arsenic to arsenate
• To reduce pentavalent arsenic to trivalent arsenic
• To generate hydrogen gas
• To remove hydrogen sulfide

Correct Answer: To reduce pentavalent arsenic to trivalent arsenic

Q6. Which acid is commonly used to generate hydrogen from zinc in the Gutzeit test?

• Hydrochloric acid
• Nitric acid
• Acetic acid
• Perchloric acid

Correct Answer: Hydrochloric acid

Q7. Why must arsenic-free zinc be used in the test?

• To accelerate arsine oxidation
• To avoid background arsenic contamination
• To neutralize acids during the reaction
• To form a complex with mercuric bromide

Correct Answer: To avoid background arsenic contamination

Q8. The stain development time in the modified Gutzeit test is typically standardized to about:

• 10 minutes
• 20 minutes
• 40 minutes
• 90 minutes

Correct Answer: 40 minutes

Q9. What is the purpose of passing the gas through a plug of lead acetate cotton in the apparatus?

• To dry the gas
• To catalyze arsine formation
• To trap hydrogen sulfide that would otherwise cause interference
• To cool the gas stream

Correct Answer: To trap hydrogen sulfide that would otherwise cause interference

Q10. In the SDDC spectrophotometric method for arsenic, absorbance is typically measured near:

• 420 nm
• 480 nm
• 535 nm
• 620 nm

Correct Answer: 535 nm

Q11. The absorbing solution for the SDDC method commonly uses which solvent?

• Pyridine
• Methanol
• Water
• Acetone

Correct Answer: Pyridine

Q12. Which modern technique aligns with ICH Q3D for accurate quantification of arsenic in pharmaceuticals?

• Potentiometry
• ICP-MS
• Thin-layer chromatography
• Refractometry

Correct Answer: ICP-MS

Q13. According to ICH Q3D, the typical oral PDE (permitted daily exposure) for arsenic is approximately:

• 1.5 µg/day
• 5 µg/day
• 15 µg/day
• 50 µg/day

Correct Answer: 15 µg/day

Q14. In the limit test, the sample complies if the test stain is:

• Darker than the standard
• Equal to the standard
• Lighter than the standard
• Not visible at all times

Correct Answer: Lighter than the standard

Q15. Besides potassium iodide, which reagent is commonly used to assist pre-reduction of As(V) to As(III)?

• Stannous chloride
• Ferric chloride
• Potassium dichromate
• Sodium carbonate

Correct Answer: Stannous chloride

Q16. The oxidation state of arsenic in arsine (AsH3) is:

• +5
• +3
• 0
• -3

Correct Answer: -3

Q17. If hydrogen sulfide is not removed before contact with the detecting paper, the most likely outcome is:

• False low result due to bleaching
• False high result due to additional staining
• No effect on the result
• Complete prevention of arsine formation

Correct Answer: False high result due to additional staining

Q18. Which element can interfere in the Gutzeit test by forming a similar hydride gas?

• Antimony (forming stibine)
• Calcium (forming calcium hydride)
• Iron (forming ferrocene)
• Sodium (forming sodium hydride vapor)

Correct Answer: Antimony (forming stibine)

Q19. The immediate source of reducing power that converts arsenic to arsine in the classical test is:

• Zinc dust in alkaline medium
• Nascent hydrogen generated from zinc and acid
• Electrochemical reduction at a platinum electrode
• Ultraviolet irradiation

Correct Answer: Nascent hydrogen generated from zinc and acid

Q20. Which safety practice is essential when performing the Gutzeit test?

• Use of open flame near the generator
• Performing the test in a fume hood due to toxic arsine
• Handling acids without gloves to avoid contamination
• Inhaling briefly to check for gas leaks

Correct Answer: Performing the test in a fume hood due to toxic arsine

Q21. A primary starting material to prepare arsenic standard solution is:

• Arsenic trioxide (As2O3)
• Sodium arsenate (Na2HAsO4) only
• Arsenic pentafluoride (AsF5)
• Arsenic metal powder only

Correct Answer: Arsenic trioxide (As2O3)

Q22. Arsenic trioxide is typically dissolved first in which reagent to make a standard solution?

• Sodium hydroxide
• Hydrochloric acid
• Ammonium chloride
• Sodium chloride

Correct Answer: Sodium hydroxide

Q23. A common pharmacopeial limit for arsenic in many substances using the classical test is approximately:

• 0.1 ppm
• 1–2 ppm
• 10–20 ppm
• 100 ppm

Correct Answer: 1–2 ppm

Q24. In the Gutzeit apparatus, where is the mercuric bromide paper positioned?

• Immersed in the reaction mixture
• At the side arm before the gas stream
• At the top outlet so that arsine contacts it as it exits
• At the bottom below the zinc

Correct Answer: At the top outlet so that arsine contacts it as it exits

Q25. If the mercuric bromide paper is excessively moist or contaminated, the likely effect is:

• Sharper, well-defined stain
• Diffuse stain leading to overestimation
• No stain formation
• Blue coloration instead of yellow

Correct Answer: Diffuse stain leading to overestimation

Q26. The blank in the arsenic limit test should contain:

• Only the sample without reagents
• All reagents without any sample arsenic
• Only water
• Only zinc and acid

Correct Answer: All reagents without any sample arsenic

Q27. Which species can suppress arsine generation and cause a false low result?

• Chloride
• Nitrate
• Nitrite
• Sulfate

Correct Answer: Nitrite

Q28. The arsenic limit test is best described as:

• A fully quantitative method with calibration curve
• A semi-quantitative pass/fail comparison against a standard
• A qualitative identification test only
• A potentiometric titration

Correct Answer: A semi-quantitative pass/fail comparison against a standard

Q29. For highly colored or turbid samples, which variant is preferred to improve selectivity and sensitivity?

• Flame photometry
• Silver diethyldithiocarbamate spectrophotometry
• Thin-layer chromatography
• Polarimetry

Correct Answer: Silver diethyldithiocarbamate spectrophotometry

Q30. Which acid is least suitable for generating hydrogen from zinc in this test?

• Hydrochloric acid
• Sulfuric acid (dilute)
• Glacial acetic acid
• Phosphoric acid (dilute)

Correct Answer: Glacial acetic acid

Q31. Correct practice for comparing stains in the Gutzeit test is to compare:

• Only the length of stain regardless of time
• Intensity of stains under identical conditions and time
• Area of paper discolored after different times
• Stain color hue in dim light

Correct Answer: Intensity of stains under identical conditions and time

Q32. When should the timing for stain development begin?

• After adding zinc but before sealing the apparatus
• After placing and sealing the detecting paper at the outlet
• Before adding acid to the generator
• After removing the detecting paper

Correct Answer: After placing and sealing the detecting paper at the outlet

Q33. Under what lighting should stain intensity be compared for best consistency?

• Direct sunlight
• Standardized white light/daylight
• Blue LED light
• Red lamp light

Correct Answer: Standardized white light/daylight

Q34. Maintaining an acidic medium during the reaction primarily ensures:

• Enhanced oxidation of arsine
• Steady generation of hydrogen from zinc
• Formation of arsenic sulfide
• Complete evaporation of the solvent

Correct Answer: Steady generation of hydrogen from zinc

Q35. Which detecting paper is NOT used in the arsenic limit test?

• Mercuric bromide paper
• Mercuric chloride paper (in some variants)
• Cobalt thiocyanate paper
• Mixed mercuric halide paper

Correct Answer: Cobalt thiocyanate paper

Q36. In hydride-generation spectrometric methods for arsenic, the hydride-forming reductant commonly used is:

• Sodium borohydride
• Hydroxylamine
• Sodium nitrite
• Ascorbic acid

Correct Answer: Sodium borohydride

Q37. The SDDC reagent provides a characteristic response with:

• Arsine to form a red complex
• Oxygen to form a blue complex
• Ammonia to form a yellow complex
• Carbon dioxide to form a white precipitate

Correct Answer: Arsine to form a red complex

Q38. The WHO guideline value for arsenic in drinking water is approximately:

• 1 µg/L
• 5 µg/L
• 10 µg/L
• 50 µg/L

Correct Answer: 10 µg/L

Q39. If the standard stain appears unusually dark even at low standard amounts, a likely cause is:

• Acid too dilute
• Contamination of reagents or apparatus with arsenic
• Insufficient zinc
• Use of deionized water

Correct Answer: Contamination of reagents or apparatus with arsenic

Q40. Which hydride is NOT produced from arsenic during hydride generation?

• Arsine
• Stibine
• Germane
• Phosphine (as interference)

Correct Answer: Germane

Q41. After testing, mercuric bromide papers should be disposed of as:

• General laboratory waste
• Recyclable paper
• Hazardous mercury-containing waste
• Compostable waste

Correct Answer: Hazardous mercury-containing waste

Q42. The detection capability of the classical Gutzeit test is best described as being in the:

• Milligram range
• Microgram range
• Nanogram range
• Gram range

Correct Answer: Microgram range

Q43. Which species provides electrons for reducing arsenic to arsine in the classical test?

• Hydroxide ion
• Nascent hydrogen from zinc-acid reaction
• Dissolved oxygen
• Nitrite ion

Correct Answer: Nascent hydrogen from zinc-acid reaction

Q44. The Gutzeit limit test is designed to detect:

• Only arsenic metal
• Total inorganic arsenic after reduction
• Only arsenate ions
• Only organoarsenic compounds

Correct Answer: Total inorganic arsenic after reduction

Q45. The pre-reduction step for pentavalent arsenic is performed in which medium?

• Strongly alkaline medium
• Acidic iodide/stannous chloride medium
• Neutral buffered medium
• Non-aqueous solvent

Correct Answer: Acidic iodide/stannous chloride medium

Q46. In the SDDC method, the red color is best measured:

• Immediately and within about 15 minutes to avoid fading
• After 2 hours for complete stabilization
• Only the next day
• Only in the dark after 6 hours

Correct Answer: Immediately and within about 15 minutes to avoid fading

Q47. Which route of administration has the most stringent PDE for arsenic per ICH Q3D?

• Oral
• Parenteral
• Inhalation
• Transdermal

Correct Answer: Inhalation

Q48. Which glassware preparation minimizes background arsenic errors?

• Rinsing with tap water only
• Acid-washing with nitric acid followed by high-purity water rinse
• Heating empty glassware in an oven overnight
• Polishing with detergent

Correct Answer: Acid-washing with nitric acid followed by high-purity water rinse

Q49. Pharmacopeial arsenic limits are commonly expressed as:

• Percent w/w
• ppm (µg/g)
• mol/L
• mEq/L

Correct Answer: ppm (µg/g)

Q50. The fixed geometry of the official Gutzeit apparatus (distance to paper, gas path) primarily serves to:

• Increase temperature of the reaction
• Ensure reproducible contact time and gas dispersion for consistent staining
• Reduce the need for blanks
• Eliminate the use of reducing agents

Correct Answer: Ensure reproducible contact time and gas dispersion for consistent staining