Limit test for Iron MCQs With Answer

Limit Test for Iron MCQs With Answer is a high-yield topic for B. Pharm students preparing for Pharmaceutical Analysis and Pharmacopoeial testing. The limit test for iron ensures trace iron impurities in APIs and excipients do not exceed specified limits by visual or spectrophotometric colorimetric comparison. In Indian Pharmacopoeia (IP), iron is detected as a purple ferrous thioglycolate complex in alkaline medium using thioglycolic acid, citric acid, and ammonia in matched Nessler cylinders. In USP/EP, a blood-red ferric thiocyanate complex is formed under acidic conditions. Understanding reagents, roles, pH control, masking, interferences, and comparison against a standard is crucial for accuracy and compliance. Now let’s test your knowledge with 50 MCQs on this topic.

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

• To ensure iron impurity does not exceed a specified pharmacopoeial limit
• To determine the valency of iron in the API
• To estimate the total heavy metals content
• To quantify iron content exactly by titration

Correct Answer: To ensure iron impurity does not exceed a specified pharmacopoeial limit

Q2. In the IP method, which colored complex indicates the presence of iron?

• Purple ferrous thioglycolate in alkaline medium
• Blue ferrous ferrocyanide in alkaline medium
• Green ferrous sulfate in acidic medium
• Yellow ferric chloride in neutral medium

Correct Answer: Purple ferrous thioglycolate in alkaline medium

Q3. Which reagent serves as the reducing and complexing agent in the IP limit test for iron?

• Thioglycolic acid
• EDTA
• Hydroxylamine hydrochloride
• Ammonium persulfate

Correct Answer: Thioglycolic acid

Q4. What is the role of ammonia in the IP limit test for iron?

• To provide an alkaline medium for color development
• To oxidize ferrous to ferric iron
• To act as a masking agent for copper
• To precipitate iron as hydroxide for measurement

Correct Answer: To provide an alkaline medium for color development

Q5. Why is citric acid added before alkalinization in the IP method?

• To keep iron in solution by complexation and prevent hydroxide precipitation
• To acidify strongly for the thiocyanate reaction
• To oxidize interfering reducing agents
• To remove chloride ions

Correct Answer: To keep iron in solution by complexation and prevent hydroxide precipitation

Q6. In a limit test, how is the test result typically judged?

• By comparing color intensity of the test with a standard prepared to the limit
• By calculating molarity from titration data
• By measuring pH at the end point
• By observing precipitate weight

Correct Answer: By comparing color intensity of the test with a standard prepared to the limit

Q7. Which apparatus is traditionally used for visual comparison in the iron limit test?

• Matched Nessler cylinders
• Pipette tips
• Burettes
• Separating funnels

Correct Answer: Matched Nessler cylinders

Q8. A standard iron solution for the test is commonly prepared from which salt?

• Ferrous ammonium sulfate (Mohr’s salt)
• Sodium chloride
• Zinc sulfate
• Silver nitrate

Correct Answer: Ferrous ammonium sulfate (Mohr’s salt)

Q9. Typical color development time in the IP iron limit test is:

• About 5 minutes before comparison
• Immediately upon mixing with no waiting time
• At least 60 minutes in the dark
• Overnight incubation at 2–8°C

Correct Answer: About 5 minutes before comparison

Q10. What is the purpose of preparing a reagent blank in the iron limit test?

• To compensate for any background color from reagents and water
• To serve as the oxidant
• To speed up complex formation
• To precipitate interfering ions

Correct Answer: To compensate for any background color from reagents and water

Q11. The USP/EP iron test commonly relies on formation of which complex?

• Blood-red ferric thiocyanate in acidic medium
• Purple ferrous thioglycolate in alkaline medium
• Blue Prussian blue in alkaline medium
• Yellow ferrous chromate in neutral medium

Correct Answer: Blood-red ferric thiocyanate in acidic medium

Q12. For the thiocyanate method, iron must be in which oxidation state for color formation?

• Ferric (Fe3+)
• Ferrous (Fe2+)
• Zero-valent iron
• Colloidal iron

Correct Answer: Ferric (Fe3+)

Q13. In thiocyanate-based iron testing, which reagent is often used as an oxidant?

• Ammonium persulfate
• Sodium sulfide
• Potassium iodide
• Sodium borohydride

Correct Answer: Ammonium persulfate

Q14. The limit test for iron is best described as:

• A semi-quantitative colorimetric comparison to a defined limit
• A precise quantitative assay by titrimetry
• A gravimetric determination of iron oxide
• A potentiometric redox titration

Correct Answer: A semi-quantitative colorimetric comparison to a defined limit

Q15. Why must all reagents used be “iron-free” in the limit test?

• To avoid false positives from extraneous iron
• To enhance the solubility of APIs
• To speed the color development
• To change the oxidation state of iron

Correct Answer: To avoid false positives from extraneous iron

Q16. What is the likely effect of strong oxidizing agents on the IP thioglycolic acid method?

• They can destroy thioglycolic acid and suppress color formation
• They intensify the purple color regardless of iron content
• They precipitate iron as ferric hydroxide
• They have no effect on the reaction

Correct Answer: They can destroy thioglycolic acid and suppress color formation

Q17. Which statement correctly distinguishes a limit test from an assay?

• A limit test checks compliance against a maximum allowable impurity; an assay quantifies the exact amount
• Both provide the exact concentration of analyte
• An assay only detects presence or absence
• A limit test requires calibration curves while an assay does not

Correct Answer: A limit test checks compliance against a maximum allowable impurity; an assay quantifies the exact amount

Q18. Which unit is most appropriate for expressing the iron limit in raw materials?

• ppm (µg/g or µg/mL)
• mol/L
• % w/w of iron oxide
• mEq/L

Correct Answer: ppm (µg/g or µg/mL)

Q19. What is the primary role of citric acid in the IP procedure besides maintaining solubility?

• Masking interfering metal ions
• Acting as a strong oxidant
• Providing an intense yellow background
• Precipitating chloride

Correct Answer: Masking interfering metal ions

Q20. If the test solution is darker than the standard in a limit test for iron, the material:

• Fails the test (iron exceeds the limit)
• Passes the test (iron below the limit)
• Must be retested with double volume
• Is inconclusive regardless of comparison

Correct Answer: Fails the test (iron exceeds the limit)

Q21. If the substance is not water-soluble, how is the test solution usually prepared?

• Using a suitable dilute acid as directed by the monograph
• Using a nonpolar solvent like hexane
• Suspending in vegetable oil
• Using solid-state IR analysis

Correct Answer: Using a suitable dilute acid as directed by the monograph

Q22. Which practice helps prevent adventitious iron contamination during the test?

• Acid-washing glassware and avoiding metal spatulas
• Using steel forceps for all transfers
• Rinsing with tap water only
• Mixing with iron rods for homogeneity

Correct Answer: Acid-washing glassware and avoiding metal spatulas

Q23. Matched Nessler cylinders are preferred because they:

• Provide the same optical path length for accurate visual comparison
• Allow heating the solution during the test
• Prevent evaporation due to tight sealing
• Increase the pH automatically

Correct Answer: Provide the same optical path length for accurate visual comparison

Q24. The standard solution in a limit test is prepared to contain:

• A known microgram amount of iron equivalent to the specified limit for the test quantity
• Ten times the allowed limit to exaggerate the color
• No iron at all to serve as a blank
• An unknown amount of iron for calibration

Correct Answer: A known microgram amount of iron equivalent to the specified limit for the test quantity

Q25. In the IP method, what happens if ammonia is added before citric acid?

• Iron may precipitate as hydroxide, reducing color development
• The color will intensify undesirably
• The test becomes specific for copper
• No change; order of addition is unimportant

Correct Answer: Iron may precipitate as hydroxide, reducing color development

Q26. The complex formed with thioglycolic acid in the IP method is best described as:

• Ferrous thioglycolate
• Ferric thioglycolate
• Ferrous sulfate
• Ferric hydroxide

Correct Answer: Ferrous thioglycolate

Q27. The colored species formed in the thiocyanate method is:

• Ferric thiocyanate
• Ferrous thioglycolate
• Ferrous carbonate
• Ferric citrate

Correct Answer: Ferric thiocyanate

Q28. For best visual comparison, cylinders should be viewed against:

• A white background under consistent lighting
• A black background in dim light
• A patterned background
• Direct sunlight only

Correct Answer: A white background under consistent lighting

Q29. Which factor most directly correlates with iron concentration in these tests?

• Color intensity of the formed complex
• Volume of ammonia added
• Temperature of the room
• Type of cylinder stopper

Correct Answer: Color intensity of the formed complex

Q30. Why must citric acid used in the test be iron-free?

• To avoid introducing background iron that increases color
• To reduce the pH to below 1
• To eliminate chloride interference
• To oxidize ferrous iron

Correct Answer: To avoid introducing background iron that increases color

Q31. The practical sensitivity of pharmacopoeial iron limit tests is generally in the:

• Microgram per liter to microgram per gram range
• Gram per liter range
• Kilogram per cubic meter range
• Nanogram per kilogram range only

Correct Answer: Microgram per liter to microgram per gram range

Q32. The measurement principle underlying both IP and USP/EP iron tests is:

• Colorimetry (visual or photometric)
• Conductometry
• Polarography
• Coulometry

Correct Answer: Colorimetry (visual or photometric)

Q33. In the IP method, the correct order of reagent addition is typically:

• Test solution → citric acid → thioglycolic acid → ammonia
• Test solution → ammonia → citric acid → thioglycolic acid
• Test solution → thioglycolic acid → oxidant → ammonia
• Test solution → thiocyanate → citric acid → ammonia

Correct Answer: Test solution → citric acid → thioglycolic acid → ammonia

Q34. How should thioglycolic acid reagent be stored to maintain performance?

• Protected from light and air in a tightly closed container
• Open to air to allow oxidation
• Under strong alkaline conditions
• Frozen at −80°C

Correct Answer: Protected from light and air in a tightly closed container

Q35. Why is it important to control iron impurities in pharmaceutical substances?

• Iron can catalyze oxidation and cause discoloration or degradation
• Iron improves product stability
• Iron has no effect on drug quality
• Iron lowers microbial load

Correct Answer: Iron can catalyze oxidation and cause discoloration or degradation

Q36. How does EDTA interfere with the iron limit test?

• It chelates iron and reduces color intensity
• It oxidizes Fe2+ to Fe3+
• It precipitates iron as EDTA salt
• It produces a red color with iron

Correct Answer: It chelates iron and reduces color intensity

Q37. Why is a reagent blank often used alongside the standard and test in visual methods?

• To ensure any inherent reagent coloration is accounted for
• To speed up color development
• To act as a standard iron source
• To provide oxidation potential

Correct Answer: To ensure any inherent reagent coloration is accounted for

Q38. Which handling practice is recommended to minimize contamination?

• Use plastic or glass spatulas instead of metal ones
• Use iron spoons for convenience
• Use unwashed glassware to avoid residue
• Use abrasive scrubbers that leave metal particles

Correct Answer: Use plastic or glass spatulas instead of metal ones

Q39. In the thiocyanate method, what is the effect of reducing agents present in the sample?

• They can reduce Fe3+ to Fe2+ and diminish the red color
• They intensify the red color by creating more Fe3+
• They have no effect on the reaction
• They cause precipitation of FeS

Correct Answer: They can reduce Fe3+ to Fe2+ and diminish the red color

Q40. When using a spectrophotometer for the thiocyanate method, which path length is standard?

• 1 cm matched cuvettes
• 10 cm gas cells
• 0.1 cm microcells only
• Open beakers without cuvettes

Correct Answer: 1 cm matched cuvettes

Q41. Which water quality is preferred for preparing test and standard solutions?

• Iron-free purified or deionized water
• Tap water
• Sea water
• Distilled water stored in rusty containers

Correct Answer: Iron-free purified or deionized water

Q42. If the sample naturally has color, what is the best approach in a limit test?

• Use a spectrophotometric method with appropriate blank correction
• Proceed with visual comparison ignoring background color
• Add activated charcoal without validation
• Dilute until the color disappears regardless of the monograph

Correct Answer: Use a spectrophotometric method with appropriate blank correction

Q43. The best description of a pharmacopoeial limit test procedure is:

• Prepare test and standard under identical conditions and compare colors
• Prepare only a test solution and estimate by eye
• Use any convenient solvent without controls
• Measure conductivity only

Correct Answer: Prepare test and standard under identical conditions and compare colors

Q44. What is the role of the standard iron solution in the limit test?

• To represent the maximum allowable iron for the specified quantity of sample
• To provide a zero baseline
• To neutralize acids
• To precipitate iron from solution

Correct Answer: To represent the maximum allowable iron for the specified quantity of sample

Q45. In alkaline medium without citric acid, iron may form:

• Ferric hydroxide precipitate
• Ferric thiocyanate complex
• Ferrous cyanide solution
• Ferric chloride gas

Correct Answer: Ferric hydroxide precipitate

Q46. Which color indicates a positive response for iron in the IP thioglycolic acid test?

• Purple/violet coloration
• Blue coloration
• Green coloration
• Orange-yellow coloration

Correct Answer: Purple/violet coloration

Q47. Why should results be read within the specified time window (e.g., about 5 minutes)?

• Because the color may change or fade with time affecting accuracy
• Because more time always decreases sensitivity
• Because iron precipitates only after an hour
• Because ammonia evaporates completely within seconds

Correct Answer: Because the color may change or fade with time affecting accuracy

Q48. Which statement about interferences is true for the iron limit test?

• Strong chelators (e.g., EDTA) can lower observed color by binding iron
• All anions enhance the color intensity
• Oxidants always increase color in every method
• Reducing agents have no effect on any iron test

Correct Answer: Strong chelators (e.g., EDTA) can lower observed color by binding iron

Q49. To minimize bias when comparing colors visually, one should:

• Use identical volumes and matched cylinders viewed side by side
• Use different volumes to amplify color
• View one cylinder today and one tomorrow
• Mix test and standard into one cylinder

Correct Answer: Use identical volumes and matched cylinders viewed side by side

Q50. Compared to visual comparison, spectrophotometric reading in the iron limit test generally offers:

• Higher sensitivity and less subjectivity
• Lower sensitivity and more subjectivity
• No difference at all
• Greater dependence on ambient light

Correct Answer: Higher sensitivity and less subjectivity