Estimation of sodium benzoate by non-aqueous titration MCQs With Answer

Estimation of sodium benzoate by non-aqueous titration MCQs With Answer

Non-aqueous titration is a key titrimetric technique for the accurate estimation of sodium benzoate in pharmaceutical samples when aqueous media are unsuitable. This topic covers sample preparation (conversion to benzoic acid), choice of non-aqueous solvents (e.g., glacial acetic acid, methanol), appropriate titrants (perchloric acid for bases, sodium methoxide for acids), indicators, end-point detection (visual and potentiometric), calculations, and common sources of error. Understanding stoichiometry, equivalent weight (MW ≈ 144.11 g/mol), titrant standardization, and pharmacopoeial assay requirements is essential for B. Pharm students performing validated, precise and accurate assays. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary analytical principle behind non-aqueous titration used to estimate sodium benzoate?

• Using aqueous buffers to stabilize the end-point
• Measuring mass loss after drying the sample
• Performing acid-base titration in solvents other than water to modify acidity/basicity and detect end-point
• Chromatographic separation followed by UV detection

Correct Answer: Performing acid-base titration in solvents other than water to modify acidity/basicity and detect end-point

Q2. Which solvent system is commonly used for non-aqueous titration of organic acid salts like sodium benzoate?

• Pure water
• Glacial acetic acid often mixed with methanol or dioxane
• Concentrated sulfuric acid only
• Hexane

Correct Answer: Glacial acetic acid often mixed with methanol or dioxane

Q3. Which titrant is typically used to determine acidic drugs (or liberated acids) in non-aqueous titration?

• Perchloric acid in glacial acetic acid
• Sodium methoxide in methanol
• Sodium chloride solution
• Silver nitrate in ethanol

Correct Answer: Sodium methoxide in methanol

Q4. For titration of weak bases in non-aqueous media, which titrant is most commonly used?

• Potassium permanganate
• Perchloric acid in glacial acetic acid
• Sodium hydroxide in water
• Hydrogen peroxide

Correct Answer: Perchloric acid in glacial acetic acid

Q5. Which indicator is widely recommended for visual end-point detection in perchloric acid non-aqueous titrations?

• Crystal violet
• Methylene blue
• Bromothymol blue
• Starch

Correct Answer: Crystal violet

Q6. What is the approximate pKa of benzoic acid relevant to converting sodium benzoate into benzoic acid?

• pKa ≈ 2.0
• pKa ≈ 4.20
• pKa ≈ 7.0
• pKa ≈ 10.5

Correct Answer: pKa ≈ 4.20

Q7. Why is a non-aqueous medium used instead of water for certain titrations?

• It always gives faster reactions than water
• It suppresses water ionization and alters apparent acidity/basicity of analytes for a sharper end-point
• It increases the conductivity for potentiometric endpoints
• It is cheaper than distilled water

Correct Answer: It suppresses water ionization and alters apparent acidity/basicity of analytes for a sharper end-point

Q8. What sample preparation step is commonly performed before titrating sodium benzoate by non-aqueous methods?

• Oxidizing the sample with potassium dichromate
• Converting the sodium benzoate to benzoic acid by acidification and appropriate extraction
• Directly drying and weighing without dissolution
• Addition of sodium chloride to precipitate impurities

Correct Answer: Converting the sodium benzoate to benzoic acid by acidification and appropriate extraction

Q9. Which method often gives the most objective detection of the equivalence point in non-aqueous titrations?

• Visual color change only
• Weighing precipitate
• Potentiometric titration using a suitable electrode
• Smell test

Correct Answer: Potentiometric titration using a suitable electrode

Q10. Why might aqueous titration be unsuitable for direct estimation of sodium benzoate?

• Sodium benzoate is completely insoluble in water
• Water masks weak acidity/basicity and common excipients may interfere, giving poor end-points
• Water reacts violently with sodium benzoate
• Aqueous titration is illegal for preservatives

Correct Answer: Water masks weak acidity/basicity and common excipients may interfere, giving poor end-points

Q11. If 25.0 mL of 0.010 N perchloric acid is consumed in titrating a solution containing benzoic acid equivalent from sodium benzoate, what mass of sodium benzoate was present? (Molecular weight ≈ 144.11 g/mol)

• 3.60 mg
• 36.03 mg
• 144.11 mg
• 360.3 mg

Correct Answer: 36.03 mg

Q12. Why must titrants used in non-aqueous titration be standardized before assay?

• To save reagent costs
• To determine the exact concentration (normality) for accurate stoichiometric calculations
• To change the chemical nature of the titrant
• It is not necessary to standardize titrants

Correct Answer: To determine the exact concentration (normality) for accurate stoichiometric calculations

Q13. What is the likely effect of moisture contamination in a non-aqueous titrant bottle?

• No effect at all
• It will change the titrant strength and reduce accuracy
• It will make the titrant glow under UV light
• It will increase the molar mass of the titrant

Correct Answer: It will change the titrant strength and reduce accuracy

Q14. How is the assay result of sodium benzoate usually expressed in pharmaceutical analysis?

• Parts per trillion
• % w/w (percent weight by weight)
• In liters
• As a pH value

Correct Answer: % w/w (percent weight by weight)

Q15. Which excipient is most likely to interfere with the non-aqueous titration of sodium benzoate if not removed?

• Microcrystalline cellulose (inert)
• Citric acid (a co-existing acid)
• Sodium chloride (neutral salt)
• Magnesium stearate at trace levels

Correct Answer: Citric acid (a co-existing acid)

Q16. How does the color change of an indicator in non-aqueous media typically compare to that in aqueous media?

• It is identical for all indicators
• Colors and transition pH ranges may shift, so indicator choice must suit the solvent system
• Indicators cannot be used in non-aqueous media
• Indicators become fluorescent only

Correct Answer: Colors and transition pH ranges may shift, so indicator choice must suit the solvent system

Q17. What is the equivalent weight of sodium benzoate for titrimetric calculations (approximate)?

• 122.12 g/eq
• 144.11 g/eq
• 22.99 g/eq
• 166.0 g/eq

Correct Answer: 144.11 g/eq

Q18. Why are anhydrous solvents recommended for non-aqueous titrations?

• Water is expensive
• To prevent water from altering acidity/basicity and reaction stoichiometry
• Because anhydrous solvents are colorless
• To improve magnetic properties of the sample

Correct Answer: To prevent water from altering acidity/basicity and reaction stoichiometry

Q19. What is a practical guideline for selecting sample size in a titration to ensure accurate measurement?

• Choose a sample that gives a titrant volume between about 10–50 mL
• Always use 1 mg sample irrespective of titrant strength
• Use the entire batch sample for a single titration
• Sample size is irrelevant

Correct Answer: Choose a sample that gives a titrant volume between about 10–50 mL

Q20. What is the stoichiometric ratio between benzoate (as base after conversion) and perchloric acid at the equivalence point?

• 2:1 (acid:base)
• 1:2 (acid:base)
• 1:1
• 3:1

Correct Answer: 1:1

Q21. How should sensitive titrants like perchloric acid be stored in the laboratory?

• In an open container on the bench
• In amber, tightly stoppered bottles away from heat and light
• Mixed with water in large plastic containers
• On the floor near solvents

Correct Answer: In amber, tightly stoppered bottles away from heat and light

Q22. Is phenolphthalein generally suitable as an indicator for non-aqueous titration of benzoic acid derivatives?

• Yes, always
• No, phenolphthalein transition is often inappropriate in many non-aqueous solvents
• Yes, but only in hexane
• Only when mixed with bromothymol blue

Correct Answer: No, phenolphthalein transition is often inappropriate in many non-aqueous solvents

Q23. Which detection method provides the highest objectivity and sensitivity for end-point determination in non-aqueous titrations?

• Smell
• Potentiometric titration using electrodes
• Color comparison by eye only
• Weighing the titrant bottle

Correct Answer: Potentiometric titration using electrodes

Q24. Typical titrant normality used for assay titrations of sodium benzoate in the lab is often around which value?

• 0.0001 N
• 0.1 N
• 10 N
• 100 N

Correct Answer: 0.1 N

Q25. What is the purpose of performing a blank titration in non-aqueous assays?

• To calibrate the pH electrode only
• To measure and correct for reagent and solvent impurities or background titrant consumption
• To warm up the burette
• To check the color of the solvent

Correct Answer: To measure and correct for reagent and solvent impurities or background titrant consumption

Q26. Why might a back titration be used instead of direct titration for sodium benzoate?

• When direct titration is too rapid and easy
• When the analyte is insoluble or reacts slowly in the chosen solvent, so excess reagent is added and the remainder is titrated
• Because back titration uses no reagents
• It reduces the number of steps and increases interference

Correct Answer: When the analyte is insoluble or reacts slowly in the chosen solvent, so excess reagent is added and the remainder is titrated

Q27. Which common procedural error can lead to low recovery of sodium benzoate in non-aqueous titration?

• Complete conversion of sodium benzoate to benzoic acid
• Incomplete conversion or incomplete extraction of benzoic acid from the sample matrix
• Using freshly dried solvents
• Standardizing the titrant before use

Correct Answer: Incomplete conversion or incomplete extraction of benzoic acid from the sample matrix

Q28. At what temperature are most non-aqueous titration assays for routine assay performed?

• Approximately 25°C (room temperature)
• −20°C
• 100°C
• Boiling point of acetic acid

Correct Answer: Approximately 25°C (room temperature)

Q29. What is a major safety concern when working with perchloric acid in non-aqueous titrations?

• It is a mild detergent
• Perchloric acid is a strong oxidizer and can form explosive perchlorates if mishandled
• It is completely inert and safe
• It has no odor and poses no hazards

Correct Answer: Perchloric acid is a strong oxidizer and can form explosive perchlorates if mishandled

Q30. In validation of a non-aqueous titration method for sodium benzoate, what does specificity refer to?

• The ability of the method to quantify only the analyte in the presence of other components
• How fast the titration can be completed
• The color intensity of the indicator
• The cost-effectiveness of reagents

Correct Answer: The ability of the method to quantify only the analyte in the presence of other components

G S Sachin

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.

Mail- Sachin@pharmacyfreak.com