Introduction: Redox titration with potassium iodate is a vital analytical technique in pharmaceutical analysis, combining principles of oxidation–reduction, iodometry and titrimetric calculations. Potassium iodate (KIO3) acts as a reliable primary standard to generate iodine in acidic medium, which is then titrated with standardized sodium thiosulfate using starch as an indicator. B. Pharm students must master reaction stoichiometry, standardization procedures, endpoint detection, interferences, and safety when applying this method to assay vitamins, reducing agents and oxidants. Key terms: potassium iodate, redox titration, iodometry, iodimetric, starch indicator, standardization, equivalence point, pharmaceutical analysis. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the balanced redox equation for the reaction of iodate with iodide in acidic medium to produce iodine?
- IO3- + 5 I- + 6 H+ → 3 I2 + 3 H2O
- IO3- + 3 I- + 3 H+ → 2 I2 + 3 H2O
- IO3- + I- + 2 H+ → I2 + H2O
- IO3- + 6 I- + 6 H+ → 4 I2 + 3 H2O
Correct Answer: IO3- + 5 I- + 6 H+ → 3 I2 + 3 H2O
Q2. Why is potassium iodate (KIO3) commonly used as a primary standard in iodometric titrations?
- Because it is a strong reducing agent
- Because it is stable, non-hygroscopic and has known high purity
- Because it reacts directly with thiosulfate without additives
- Because it forms a colored complex with starch
Correct Answer: Because it is stable, non-hygroscopic and has known high purity
Q3. In a typical KIO3-based iodometric titration, which reagent is used as the titrant to determine liberated iodine?
- Silver nitrate
- Potassium permanganate
- Sodium thiosulfate solution
- Hydrogen peroxide
Correct Answer: Sodium thiosulfate solution
Q4. Which indicator is conventionally used to detect the endpoint in iodine–thiosulfate titrations?
- Phenolphthalein
- Methyl orange
- Starch solution
- Potassium permanganate
Correct Answer: Starch solution
Q5. During an iodometric titration with starch, the endpoint is signalled by which colour change?
- Colorless to blue-black
- Blue-black to colorless
- Yellow to red
- Pink to colorless
Correct Answer: Blue-black to colorless
Q6. The titration which involves liberation of iodine followed by titration with thiosulfate is classified as:
- Iodimetric titration
- Iodometric titration
- Complexometric titration
- Acid-base titration
Correct Answer: Iodometric titration
Q7. What is the stoichiometric mole ratio between KIO3 and Na2S2O3 in the complete sequence (KIO3 → I2 → titration with S2O3^2-)?
- 1 KIO3 : 3 Na2S2O3
- 1 KIO3 : 2 Na2S2O3
- 1 KIO3 : 6 Na2S2O3
- 1 KIO3 : 1 Na2S2O3
Correct Answer: 1 KIO3 : 6 Na2S2O3
Q8. For standardization of sodium thiosulfate, why is a KIO3 solution treated with excess KI and acid before titration?
- To reduce KIO3 to iodide directly
- To generate a known amount of free iodine (I2) quantitatively
- To precipitate interfering anions
- To form a colored complex for spectrophotometry
Correct Answer: To generate a known amount of free iodine (I2) quantitatively
Q9. What is the primary role of adding acid (HCl or H2SO4) in the reaction between IO3- and I-?
- To neutralize the sample
- To provide H+ required for conversion of IO3- to I2
- To oxidize iodide to iodate
- To precipitate iodine
Correct Answer: To provide H+ required for conversion of IO3- to I2
Q10. Which impurity in a pharmaceutical sample most likely interferes with KIO3–iodometric titration by consuming iodine?
- Sucrose
- Nitrite (NO2-) ions
- Sodium chloride
- Calcium carbonate
Correct Answer: Nitrite (NO2-) ions
Q11. Why should starch indicator be added near the endpoint rather than at the start of an iodometric titration?
- Starch reacts with KIO3 and degrades the titrant
- The starch–iodine complex is reversible and starch is unstable at high iodine concentrations, causing slower reactions or false endpoints if added too early
- Starch is insoluble in titration media
- Early addition causes the solution to become alkaline
Correct Answer: The starch–iodine complex is reversible and starch is unstable at high iodine concentrations, causing slower reactions or false endpoints if added too early
Q12. What is the standard electrode potential (E°) for the I2/I- redox couple at standard conditions (approx.)?
- +1.23 V
- +0.54 V
- 0.00 V
- -0.76 V
Correct Answer: +0.54 V
Q13. How should a prepared standard KIO3 solution be stored to maintain stability for use in standardization?
- In a clear plastic bottle at room light
- In an amber glass bottle, away from light, and standardized periodically
- In direct sunlight to prevent contamination
- Frozen at -20°C without any standardization
Correct Answer: In an amber glass bottle, away from light, and standardized periodically
Q14. What safety precaution is most important when handling solid potassium iodate in the laboratory?
- Handle it near open flames to sterilize reagents
- Store and use away from combustible materials because it is an oxidizer
- Always mix it with organic solvents
- No special precautions required
Correct Answer: Store and use away from combustible materials because it is an oxidizer
Q15. If 25.00 mL of 0.1000 M Na2S2O3 is used to titrate iodine produced from a sample, how many moles of KIO3 were present? (Use KIO3 : Na2S2O3 = 1 : 6)
- 4.167 × 10^-4 mol
- 2.500 × 10^-3 mol
- 1.500 × 10^-3 mol
- 6.000 × 10^-5 mol
Correct Answer: 4.167 × 10^-4 mol
Q16. Which statement correctly distinguishes iodometric from iodimetric titration?
- Iodometric titration involves titration of iodine liberated by an oxidizing analyte; iodimetric involves generating iodine externally
- Iodometric titration measures iodine produced and titrated with thiosulfate; iodimetric titration directly titrates an oxidizing analyte with iodide
- They are identical and interchangeable terms with no difference
- Iodometric uses ammonia, iodimetric uses chloride
Correct Answer: Iodometric titration measures iodine produced and titrated with thiosulfate; iodimetric titration directly titrates an oxidizing analyte with iodide
Q17. Which property is NOT desirable in a primary standard like solid KIO3?
- High chemical purity
- Hygroscopic nature
- Stable on storage
- Known composition and high equivalent weight
Correct Answer: Hygroscopic nature
Q18. How does increasing temperature typically affect iodometric titrations using KIO3?
- Always increases accuracy due to faster kinetics
- May increase volatility of iodine and cause endpoint errors, so moderate temperature is preferred
- Has no effect on titration
- Prevents the starch–iodine complex from forming
Correct Answer: May increase volatility of iodine and cause endpoint errors, so moderate temperature is preferred
Q19. What is the principal role of potassium iodide (KI) when added to a KIO3 titration mixture?
- To act as the titrant
- To provide iodide ions which are oxidized to iodine by iodate
- To act as a pH buffer
- To form a precipitate with iodate
Correct Answer: To provide iodide ions which are oxidized to iodine by iodate
Q20. If excess sodium thiosulfate is accidentally added beyond the endpoint, what immediate observation will be made?
- The solution will turn deep blue
- The solution will become colorless because iodine is reduced to iodide
- The starch–iodine complex will intensify permanently
- The solution will precipitate iodine
Correct Answer: The solution will become colorless because iodine is reduced to iodide
Q21. Which pharmaceutical analyte is commonly assayed by iodine liberated from KIO3 and titrated with thiosulfate?
- Ascorbic acid (vitamin C)
- Paracetamol
- Ibuprofen
- Chlorpheniramine
Correct Answer: Ascorbic acid (vitamin C)
Q22. What mole ratio relates IO3- to I2 in the conversion producing iodine?
- 1 IO3- : 1 I2
- 1 IO3- : 2 I2
- 1 IO3- : 3 I2
- 1 IO3- : 6 I2
Correct Answer: 1 IO3- : 3 I2
Q23. Why is KI often added in excess when preparing the iodine for titration from KIO3?
- To ensure complete and rapid conversion of all IO3- to I2 and prevent side reactions
- To neutralize the acid
- To precipitate iodine immediately
- To act as an indicator
Correct Answer: To ensure complete and rapid conversion of all IO3- to I2 and prevent side reactions
Q24. Which of the following is a common source of systematic error in KIO3–iodometric assays?
- Use of freshly standardized titrant
- Iodine volatilization and light-induced decomposition leading to loss of titratable iodine
- Using too little starch at the endpoint only
- Performing titration at ambient temperature
Correct Answer: Iodine volatilization and light-induced decomposition leading to loss of titratable iodine
Q25. The starch–iodine complex forms a blue-black color. This complex is most sensitive at what stage of titration?
- At the beginning when iodine concentration is highest
- Near the endpoint when iodine concentration becomes trace
- At pH above 10 only
- Only in non-aqueous media
Correct Answer: Near the endpoint when iodine concentration becomes trace
Q26. For potentiometric detection of the iodometric endpoint, which electrode combination is appropriate?
- Glass pH electrode and calomel reference
- Platinum indicator electrode and Ag/AgCl reference electrode
- Ion-selective electrode for potassium
- Calomel as indicator and pH electrode as reference
Correct Answer: Platinum indicator electrode and Ag/AgCl reference electrode
Q27. Why cannot KIO3 be titrated directly with sodium thiosulfate without adding KI and acid?
- KIO3 is insoluble in water
- Thiosulfate reacts only with molecular iodine (I2), so I2 must be generated from IO3- using iodide and acid first
- Thiosulfate oxidizes KI instead of KIO3
- KIO3 precipitates in presence of thiosulfate
Correct Answer: Thiosulfate reacts only with molecular iodine (I2), so I2 must be generated from IO3- using iodide and acid first
Q28. How many grams of KIO3 (M ≈ 214 g·mol^-1) are required to prepare 500.0 mL of 0.0100 M solution?
- 2.14 g
- 1.07 g
- 0.214 g
- 10.7 g
Correct Answer: 1.07 g
Q29. Why must sodium thiosulfate solutions be standardized frequently before use?
- They are highly acidic and evaporate quickly
- Thiosulfate slowly oxidizes to tetrathionate and loses strength on standing
- They form stable complexes with starch over time
- They precipitate when stored in glass
Correct Answer: Thiosulfate slowly oxidizes to tetrathionate and loses strength on standing
Q30. If nitrite impurity is present in a sample undergoing KIO3–iodometric titration, what is the expected effect on the measured result for a reducing drug?
- No effect, nitrite is inert in this titration
- An erroneously high consumption of thiosulfate, giving an overestimated drug concentration
- An erroneously low consumption of thiosulfate, giving an underestimated drug concentration
- Complete precipitation preventing titration
Correct Answer: An erroneously high consumption of thiosulfate, giving an overestimated drug concentration
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