Iodimetry and iodometry – principles and applications MCQs With Answer

Iodimetry and iodometry – principles and applications MCQs With Answer

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Iodimetry and iodometry – principles and applications MCQs With Answer

Iodimetry and iodometry are essential redox titrations in pharmaceutical analysis, used for assay, standardization and quality control of drugs and excipients. These methods rely on iodine/iodide chemistry, triiodide formation, starch endpoint detection and standard thiosulfate titrants. B.Pharm students should master stoichiometry, reaction mechanisms (iodate–iodide interactions), titrant preparation, common interferences, and pharmacopeial applications such as ascorbic acid and metal determinations. Practical considerations include solution stability, proper indicators, and precautionary storage. This focused set of MCQs reinforces conceptual understanding and calculation skills required for titrimetric methods in pharmaceutical laboratories. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which statement correctly defines iodometry?

  • Titration of reducing agents directly with standard iodine solution
  • Titration where an oxidizing analyte oxidizes iodide to iodine, and the liberated iodine is titrated with thiosulfate
  • Volumetric analysis using silver nitrate and iodide
  • Gravimetric determination of iodine content

Correct Answer: Titration where an oxidizing analyte oxidizes iodide to iodine, and the liberated iodine is titrated with thiosulfate

Q2. What is iodimetry?

  • Titration of oxidizing agents by generation of iodine from iodate
  • Titration of reducing agents by direct titration with standard iodine solution
  • Determination of iodide by precipitation with silver nitrate
  • Chromatographic separation of iodine species

Correct Answer: Titration of reducing agents by direct titration with standard iodine solution

Q3. Which reagent is most commonly used to standardize sodium thiosulfate solutions?

  • Potassium permanganate
  • Potassium iodate (KIO3)
  • Standard iodine in KI
  • Hydrochloric acid

Correct Answer: Potassium iodate (KIO3)

Q4. The blue color produced with starch indicator during iodometric titration is due to complexation of starch with which species?

  • Iodide ion (I-)
  • Triiodide ion (I3-)
  • Sodium thiosulfate (S2O3^2-)
  • Tetrathionate ion (S4O6^2-)

Correct Answer: Triiodide ion (I3-)

Q5. What is the molar stoichiometry between iodine (I2) and thiosulfate ion (S2O3^2-) in the titration reaction?

  • 1 mole I2 reacts with 1 mole S2O3^2-
  • 1 mole I2 reacts with 2 moles S2O3^2-
  • 1 mole I2 reacts with 4 moles S2O3^2-
  • 2 moles I2 react with 1 mole S2O3^2-

Correct Answer: 1 mole I2 reacts with 2 moles S2O3^2-

Q6. How is a standard iodine titrant usually prepared for iodimetric titrations?

  • Dissolve iodine in water without additives
  • Dissolve iodine in potassium iodide solution to form soluble triiodide
  • Dissolve iodide salts in organic solvent
  • Prepare by electrolytic generation of iodide

Correct Answer: Dissolve iodine in potassium iodide solution to form soluble triiodide

Q7. In redox terms, iodine (I2) typically acts as which of the following?

  • A reducing agent
  • An oxidizing agent
  • An inert spectator ion
  • A complexing ligand only

Correct Answer: An oxidizing agent

Q8. Why is strong acid avoided during the titration step with sodium thiosulfate?

  • Strong acid stabilizes thiosulfate, preventing reaction
  • Strong acid converts thiosulfate to sulfur and sulfide, decomposing it
  • Strong acid forms volatile iodine gas increasing endpoint error
  • Strong acid has no effect on these titrations

Correct Answer: Strong acid converts thiosulfate to sulfur and sulfide, decomposing it

Q9. In the standardization of Na2S2O3 using KIO3, how many moles of thiosulfate react per mole of KIO3?

  • 2 moles
  • 4 moles
  • 6 moles
  • 8 moles

Correct Answer: 6 moles

Q10. Before adding starch, what is the typical observable color of the solution containing free iodine?

  • Colorless
  • Pale yellow to brown
  • Deep blue
  • Bright red

Correct Answer: Pale yellow to brown

Q11. The starch indicator forms a deep blue complex most strongly with which iodine-containing species?

  • Elemental iodine dissolved in organic solvents
  • Triiodide ion (I3-)
  • Iodate ion (IO3-)
  • Iodide ion (I-)

Correct Answer: Triiodide ion (I3-)

Q12. Which common interfering substance reacts with iodine and can affect iodometric assays if present in samples?

  • Chloride ion (Cl-)
  • Sulfite ion (SO3^2-)
  • Potassium ion (K+)
  • Glucose at neutral pH

Correct Answer: Sulfite ion (SO3^2-)

Q13. Which pharmaceutical compound is commonly determined by iodometric titration in quality control?

  • Paracetamol
  • Ascorbic acid (vitamin C)
  • Lactose
  • Calcium carbonate

Correct Answer: Ascorbic acid (vitamin C)

Q14. What is the best practice for storing iodine/iodide titrant solutions?

  • In clear glass bottles exposed to light
  • In open beakers at room temperature
  • In amber stoppered bottles in the dark
  • Frozen in plastic bags

Correct Answer: In amber stoppered bottles in the dark

Q15. Starch is most useful as an indicator in which titration step?

  • During acid–base titrations only
  • To detect the presence of free iodine when titrating with thiosulfate
  • As a primary standard for thiosulfate
  • To precipitate iodide as silver iodide

Correct Answer: To detect the presence of free iodine when titrating with thiosulfate

Q16. In iodimetry, what serves as the titrant?

  • Standard sodium thiosulfate solution
  • Standard iodine solution (iodine in KI)
  • Standard potassium permanganate
  • Dilute sulfuric acid

Correct Answer: Standard iodine solution (iodine in KI)

Q17. Why is potassium iodide (KI) added when preparing iodine solutions?

  • KI reacts to form insoluble iodide precipitate
  • KI forms triiodide (I3-) and greatly increases iodine solubility in water
  • KI reduces iodine to iodide, eliminating oxidizing power
  • KI is a buffering agent for pH control

Correct Answer: KI forms triiodide (I3-) and greatly increases iodine solubility in water

Q18. The standard reduction potential for the I2/ I- half-reaction is approximately:

  • -0.76 V
  • +0.34 V
  • +0.54 V
  • -0.14 V

Correct Answer: +0.54 V

Q19. Why must sodium thiosulfate solutions be standardized frequently?

  • They increase in concentration on standing
  • They are unstable and slowly oxidize in air, changing concentration
  • They form complexes with glassware
  • They are light-sensitive and polymerize

Correct Answer: They are unstable and slowly oxidize in air, changing concentration

Q20. Adding excess iodide to a solution containing iodine shifts the equilibrium to form which species?

  • Elemental iodine vapor (I2 gas)
  • Triiodide ion (I3-)
  • Iodate (IO3-)
  • Tetrathionate (S4O6^2-)

Correct Answer: Triiodide ion (I3-)

Q21. In the reaction IO3- + 5 I- + 6 H+ → 3 I2 + 3 H2O, how many electrons are effectively transferred per IO3- ion?

  • 2 electrons
  • 4 electrons
  • 6 electrons
  • 8 electrons

Correct Answer: 6 electrons

Q22. In iodometric determination of copper(II), copper is typically reduced to which oxidation state or species?

  • Metallic copper (Cu0)
  • Cuprous ion (Cu+), often as CuI
  • Cupric hydroxide (Cu(OH)2)
  • Cuprous oxide (Cu2O)

Correct Answer: Cuprous ion (Cu+), often as CuI

Q23. Which concentration unit is often convenient for iodine–thiosulfate titrations because of electron equivalence considerations?

  • Mole fraction
  • Molality
  • Normality
  • Percentage by weight

Correct Answer: Normality

Q24. At the true endpoint of an iodometric titration using starch, what is observed after adding starch near the endpoint?

  • The blue color appears and remains permanently
  • The blue color appears briefly then disappears on completion of titration
  • The solution turns green
  • The solution becomes cloudy and precipitates

Correct Answer: The blue color appears briefly then disappears on completion of titration

Q25. For extraction of liberated iodine into an organic phase for visual detection, which solvent historically gives a violet/purple iodine layer?

  • Water
  • Carbon tetrachloride (CCl4)
  • Ethanol/water mixture
  • Acetone

Correct Answer: Carbon tetrachloride (CCl4)

Q26. How is iodometry classified within titrimetric methods?

  • Complexometric titration
  • Precipitation titration
  • Redox titration
  • Acid–base titration

Correct Answer: Redox titration

Q27. Which of the following pharmaceutical analytes can be determined by iodimetry (titration with iodine)?

  • Ferrous sulfate (Fe2+)
  • Sodium chloride
  • Magnesium oxide
  • Calcium carbonate

Correct Answer: Ferrous sulfate (Fe2+)

Q28. Which primary standard is preferred for preparing a known amount of free iodine via generation for thiosulfate standardization?

  • Potassium dichromate
  • Potassium iodate (KIO3)
  • Sodium carbonate
  • Oxalic acid

Correct Answer: Potassium iodate (KIO3)

Q29. How many equivalents are present in 1 mole of molecular iodine (I2) for redox calculations?

  • 1 equivalent
  • 2 equivalents
  • 3 equivalents
  • 4 equivalents

Correct Answer: 2 equivalents

Q30. Why is excess iodide often added when performing iodometric titration of oxidizing pharmaceutical analytes?

  • To precipitate the analyte as iodide salt
  • To ensure complete conversion of oxidant to soluble triiodide (I3-) and prevent loss of iodine
  • To neutralize the sample pH
  • To catalyze thiosulfate decomposition

Correct Answer: To ensure complete conversion of oxidant to soluble triiodide (I3-) and prevent loss of iodine

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