Preparation and standardization of sodium thiosulphate solution is a fundamental practical topic for B. Pharm students, covering accurate solution preparation, titrimetric standardization, and quality control. This topic highlights essential keywords such as sodium thiosulphate, iodometry, titration, primary standard, molarity, normality, starch indicator, stoichiometry, end-point detection, and reagent stability. Students learn stepwise preparation, choice of hydrate form, concentration calculations, standardization against iodine or iodate/dichromate, common sources of error, and proper storage to prevent decomposition. Clear understanding is crucial for assay validation and pharmacopoeial compliance. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary purpose of standardizing a sodium thiosulphate solution?
- To determine the exact concentration of the thiosulphate solution
- To remove impurities from the sodium thiosulphate
- To increase the shelf-life of the solution
- To convert it into sodium sulfate
Correct Answer: To determine the exact concentration of the thiosulphate solution
Q2. Which reaction represents the redox stoichiometry between iodine and thiosulphate?
- I2 + 2 S2O3^2- → 2 I^- + S4O6^2-
- I2 + S2O3^2- → 2 I^- + S2O6^2-
- I2 + 4 S2O3^2- → 4 I^- + S8O6^2-
- I2 + S2O3^2- → I^- + S4O6^2-
Correct Answer: I2 + 2 S2O3^2- → 2 I^- + S4O6^2-
Q3. Which indicator is commonly used to detect the end point in thiosulphate titrations?
- Methyl orange
- Starch
- Phenolphthalein
- Bromothymol blue
Correct Answer: Starch
Q4. Why is starch indicator added near the end of an iodometric titration rather than at the start?
- Starch decomposes in acidic solution
- Starch forms a complex with excess iodine and causes slow end point if added too early
- Starch reacts with thiosulphate to give colored products
- Starch will oxidize iodine if present at the beginning
Correct Answer: Starch forms a complex with excess iodine and causes slow end point if added too early
Q5. Which of the following is a suitable primary standard for preparing a standard iodine solution used to standardize thiosulphate?
- Sodium chloride
- Potassium dichromate
- Potassium hydrogen phthalate (KHP)
- Glucose
Correct Answer: Potassium dichromate
Q6. Which hydrate form of sodium thiosulphate is commonly used in laboratories for preparing solutions?
- Anhydrous Na2S2O3
- Na2S2O3·2H2O
- Na2S2O3·5H2O (pentahydrate)
- Na2S2O3·7H2O
Correct Answer: Na2S2O3·5H2O (pentahydrate)
Q7. Calculate grams of Na2S2O3·5H2O (MW ≈ 248.18 g/mol) required to prepare 1.0 L of 0.1 M solution.
- 15.81 g
- 24.82 g
- 2.48 g
- 248.18 g
Correct Answer: 24.82 g
Q8. When standardizing sodium thiosulphate with an iodine solution, the titration endpoint is reached when:
- The solution turns permanently brown
- All iodine has been reduced and the blue starch-iodine color disappears
- The solution becomes strongly acidic
- Gas evolution ceases
Correct Answer: All iodine has been reduced and the blue starch-iodine color disappears
Q9. During preparation, which practice helps minimize decomposition of sodium thiosulphate solution?
- Expose the solution to sunlight to sterilize
- Store in dark, airtight glass bottles at cool temperature
- Always keep solution in plastic containers at room temperature
- Add trace amounts of chlorine to preserve
Correct Answer: Store in dark, airtight glass bottles at cool temperature
Q10. What effect does dissolved oxygen have on sodium thiosulphate solutions?
- Oxidizes thiosulphate slowly, decreasing concentration
- Reduces thiosulphate to sulfide, increasing concentration
- No effect on stability
- Converts it into an inert salt that improves stability
Correct Answer: Oxidizes thiosulphate slowly, decreasing concentration
Q11. What is the role of potassium iodide (KI) in iodometric standardization when using potassium dichromate?
- Acts as a solvent
- Provides iodide which is oxidized by dichromate to liberated iodine
- Serves as the starch indicator
- Precipitates dichromate
Correct Answer: Provides iodide which is oxidized by dichromate to liberated iodine
Q12. Which glassware practice ensures accurate titration volumes for standardization?
- Use uncalibrated beakers for titration
- Use a clean, calibrated burette and rinse with titrant before filling
- Use graduated cylinders for end-point detection
- Use a pipette without calibration for transferring reagents
Correct Answer: Use a clean, calibrated burette and rinse with titrant before filling
Q13. If 25.00 mL of iodine solution required 18.50 mL of Na2S2O3 titrant and the thiosulphate concentration is 0.100 M, what is the molarity of iodine assuming 1:2 stoichiometry (I2:S2O3^2-)?
- 0.0370 M
- 0.0740 M
- 0.148 M
- 0.0123 M
Correct Answer: 0.0370 M
Q14. Which error will cause a higher measured concentration of sodium thiosulphate than true value during standardization?
- Not accounting for thiosulphate decomposition prior to titration
- Using an overfilled burette so titrant drops out during setup
- Adding starch indicator too early causing slow end point
- Loss of titrant by evaporation during titration
Correct Answer: Not accounting for thiosulphate decomposition prior to titration
Q15. Which statement about the normality of sodium thiosulphate in iodometric titration is correct?
- Normality equals molarity because thiosulphate transfers one electron
- Normality depends on the equivalence with respect to iodine: N = 2 × M for I2 reactions
- Normality is always half the molarity in redox titrations
- Normality is irrelevant in titrations
Correct Answer: Normality depends on the equivalence with respect to iodine: N = 2 × M for I2 reactions
Q16. Which precaution is important when preparing sodium thiosulphate solution from pentahydrate crystals?
- Weigh crystals in a humid environment without a desiccator
- Dry crystals completely at 200 °C before weighing
- Weigh quickly and avoid prolonged exposure to air to prevent moisture changes
- Add concentrated acid during dissolution to stabilize
Correct Answer: Weigh quickly and avoid prolonged exposure to air to prevent moisture changes
Q17. In an iodometric standardization, what is the function of dilute sulfuric acid when using potassium dichromate and potassium iodide?
- To provide oxidizing environment for thiosulphate
- To acidify medium so dichromate oxidizes iodide to iodine
- To act as an indicator
- To precipitate excess iodide
Correct Answer: To acidify medium so dichromate oxidizes iodide to iodine
Q18. Which storage condition would most likely accelerate decomposition of sodium thiosulphate solution?
- Storing refrigerated in amber glass bottle
- Storage in direct sunlight at elevated temperature
- Storage under nitrogen atmosphere
- Storage with minimal headspace and airtight cap
Correct Answer: Storage in direct sunlight at elevated temperature
Q19. What is the chemical consequence if excess acid is used during iodometric generation of iodine from iodate?
- No iodine will be produced
- Side reactions may occur, altering available iodine and affecting titration accuracy
- Acid neutralizes thiosulphate making titration impossible
- Excess acid stabilizes iodine leading to sharper end point
Correct Answer: Side reactions may occur, altering available iodine and affecting titration accuracy
Q20. Which analytical documentation is essential after standardizing a sodium thiosulphate solution in a B.Pharm lab?
- Only the date of preparation
- Concentration, date, standardization method, indicators used, and analyst signature
- Just the batch number of sodium thiosulphate crystals
- Storage location and container color only
Correct Answer: Concentration, date, standardization method, indicators used, and analyst signature
Q21. Which species is the oxidizing agent when iodine is reduced in the titration with thiosulphate?
- S2O3^2-
- I^-
- I2
- S4O6^2-
Correct Answer: I2
Q22. Which titration technique improves accuracy when the end point color change is faint?
- Perform titration near boiling temperature
- Use back titration or potentiometric detection as alternative
- Never repeat the titration
- Add more indicator at the start
Correct Answer: Use back titration or potentiometric detection as alternative
Q23. In quality control, why is it advisable to standardize sodium thiosulphate freshly before performing critical assays?
- Because standardization is legally required monthly
- Because thiosulphate concentration can change over time due to oxidation
- Because fresh standardization increases reagent toxicity
- Because standard solutions never change and it is unnecessary
Correct Answer: Because thiosulphate concentration can change over time due to oxidation
Q24. Which of the following is NOT a common source of error in thiosulphate standardization?
- Incomplete liberation of iodine from oxidant
- Using contaminated glassware
- Inaccurate weighing of primary standard
- Using freshly boiled distilled water
Correct Answer: Using freshly boiled distilled water
Q25. What is the molar relationship between iodate (IO3^-) and iodine (I2) when converting for iodometric titration?
- 1 IO3^- yields 1 I2
- 1 IO3^- yields 0.5 I2
- 1 IO3^- yields 3 I2
- 1 IO3^- yields 0.2 I2
Correct Answer: 1 IO3^- yields 0.5 I2
Q26. For a standardization titration, which blank or correction is important to run?
- A reagent blank to account for iodine demand of reagents
- A blank with double the sample volume
- No blank is necessary in titrations
- A blank that contains starch only
Correct Answer: A reagent blank to account for iodine demand of reagents
Q27. Which laboratory safety precaution is particularly relevant when handling potassium dichromate for iodine generation?
- Potassium dichromate is non-toxic and needs no special precautions
- Use gloves and work in a fume hood because it is toxic and a carcinogen
- Store open on bench to avoid pressure buildup
- Mix with organic solvents to neutralize its toxicity
Correct Answer: Use gloves and work in a fume hood because it is toxic and a carcinogen
Q28. If 20.00 mL of 0.0200 M potassium dichromate solution is used to generate iodine and that iodine is titrated with 25.00 mL of Na2S2O3, what is the molarity of Na2S2O3? (Dichromate to iodine stoichiometry: Cr2O7^2- + 6 I^- → 2 Cr^3+ + 3 I2)
- 0.0288 M
- 0.0173 M
- 0.0346 M
- 0.0100 M
Correct Answer: 0.0288 M
Q29. Why is it recommended to standardize thiosulphate against an accurately known oxidant rather than using theoretical calculation alone?
- Theoretical calculations are always correct and standardization is redundant
- Impurities, hydrate water content, and solution losses make actual concentration differ from theoretical
- Because oxidants are cheaper than thiosulphate
- Standardization makes the solution sterile
Correct Answer: Impurities, hydrate water content, and solution losses make actual concentration differ from theoretical
Q30. After standardization, how should the labeled sodium thiosulphate solution typically state its information?
- Only the container color and analyst name
- Concentration, date of standardization, standardization method, expiry or restandardization date, and analyst initials
- Just the word “Standard”
- Manufacturer address only
Correct Answer: Concentration, date of standardization, standardization method, expiry or restandardization date, and analyst initials
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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