Estimation of magnesium sulphate by complexometric titration MCQs With Answer

Estimation of magnesium sulphate by complexometric titration MCQs With Answer

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Introduction: Estimation of magnesium sulphate by complexometric titration is a fundamental pharmaceutical analysis technique for B.Pharm students. This method uses EDTA (ethylenediaminetetraacetic acid) as the titrant to form a stable 1:1 chelate with Mg2+ under controlled pH (usually ammonia buffer around pH 10). Key aspects include choice of indicator (e.g., Eriochrome Black T or Calmagite), buffer composition, masking agents, standardization of EDTA, and calculations based on stoichiometry and stability constants. Understanding interference from other metal ions, endpoint detection, and sample preparation is essential for accurate assay and quality control of magnesium sulphate in formulations. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the fundamental principle behind the complexometric titration used for estimation of magnesium sulphate?

  • Redox reaction between magnesium and EDTA
  • Formation of a 1:1 complex between Mg2+ and EDTA
  • Precipitation of magnesium as magnesium hydroxide
  • Acid-base neutralization between MgSO4 and EDTA

Correct Answer: Formation of a 1:1 complex between Mg2+ and EDTA

Q2. Which form of EDTA is most commonly used as the titrant in routine complexometric assays for magnesium sulphate?

  • Free acid EDTA (H4EDTA)
  • Disodium EDTA (Na2EDTA)
  • Calcium EDTA
  • Potassium EDTA

Correct Answer: Disodium EDTA (Na2EDTA)

Q3. Which indicator is most frequently used for the titration of magnesium in the presence of EDTA?

  • Phenolphthalein
  • Methyl orange
  • Eriochrome Black T
  • Bromothymol blue

Correct Answer: Eriochrome Black T

Q4. What is the typical buffer pH used when titrating magnesium with EDTA to obtain a sharp endpoint with Eriochrome Black T?

  • pH 3.0 (acetate buffer)
  • pH 7.0 (phosphate buffer)
  • pH 10.0 (ammonia/ammonium buffer)
  • pH 12.5 (strongly alkaline)

Correct Answer: pH 10.0 (ammonia/ammonium buffer)

Q5. What is the denticity of EDTA when it binds to a metal ion like Mg2+?

  • Monodentate
  • Bidendate
  • Tetradentate
  • Hexadentate

Correct Answer: Hexadentate

Q6. What color change indicates the endpoint when titrating magnesium with EDTA using Eriochrome Black T?

  • Blue to colorless
  • Yellow to pink
  • Wine-red to blue
  • Green to red

Correct Answer: Wine-red to blue

Q7. Which stoichiometric ratio between Mg2+ and EDTA is assumed in direct complexometric titration?

  • 1 Mg2+ : 2 EDTA
  • 2 Mg2+ : 1 EDTA
  • 1 Mg2+ : 1 EDTA
  • 1 Mg2+ : 3 EDTA

Correct Answer: 1 Mg2+ : 1 EDTA

Q8. For standardizing an EDTA solution used in magnesium assays, which of the following is commonly used as a primary or reference standard?

  • Standard calcium solution prepared from primary standard CaCO3
  • Commercial EDTA of unknown purity
  • Untreated magnesium sulphate sample
  • Phenolphthalein

Correct Answer: Standard calcium solution prepared from primary standard CaCO3

Q9. Which ion among the following generally does NOT interfere in EDTA titration of magnesium at pH 10?

  • Calcium (Ca2+)
  • Iron (Fe3+)
  • Potassium (K+)
  • Copper (Cu2+)

Correct Answer: Potassium (K+)

Q10. In a titration, 25.0 mL of magnesium sulphate solution required 16.5 mL of 0.01 M EDTA. What is the molarity of Mg2+ in the sample?

  • 0.00066 M
  • 0.0066 M
  • 0.066 M
  • 0.000166 M

Correct Answer: 0.0066 M

Q11. Which of the following statements about stability (formation) constants for metal-EDTA complexes is correct?

  • Higher stability constant means weaker metal-EDTA complex
  • Stability constant is independent of pH
  • Mg-EDTA complex has a moderate stability constant compared to transition metals
  • EDTA forms stronger complexes with alkali metals than with Mg2+

Correct Answer: Mg-EDTA complex has a moderate stability constant compared to transition metals

Q12. Which buffering system is typically used to maintain pH 10 during Mg-EDTA titration?

  • Acetate buffer (pH 4–5)
  • Phosphate buffer (pH 7)
  • Ammonia/ammonium chloride buffer
  • Carbonate buffer (pH 9.5)

Correct Answer: Ammonia/ammonium chloride buffer

Q13. When analyzing a sample containing both Ca2+ and Mg2+, which approach is commonly used to determine Mg2+ selectively by EDTA titration?

  • Use of higher pH alone to complex Mg2+ selectively
  • Mask calcium with a suitable masking agent or separate Ca first
  • Ignore calcium; it has no effect
  • Use phenolphthalein as indicator to distinguish metals

Correct Answer: Mask calcium with a suitable masking agent or separate Ca first

Q14. Which reagent can be used to mask iron and other transition metal interferences during magnesium estimation by EDTA?

  • Hydrochloric acid (strong acid)
  • Potassium ferrocyanide
  • Thioglycolic acid
  • Magnesium oxide

Correct Answer: Potassium ferrocyanide

Q15. Why is it important to standardize EDTA solution before routine titration of magnesium sulphate?

  • EDTA is volatile and loses concentration quickly
  • EDTA solutions may have impurities and concentration errors; standardization ensures accurate titrant strength
  • Standardization changes the denticity of EDTA
  • To convert EDTA to a colored form for endpoint detection

Correct Answer: EDTA solutions may have impurities and concentration errors; standardization ensures accurate titrant strength

Q16. In complexometric titration, what is the role of a masking agent?

  • To enhance the color change of the indicator
  • To convert EDTA into an inactive form
  • To selectively bind interfering metal ions so they do not react with EDTA
  • To adjust the pH to the required value

Correct Answer: To selectively bind interfering metal ions so they do not react with EDTA

Q17. Which of the following describes a back titration approach used in magnesium estimation?

  • Add excess EDTA to the sample, then titrate the unreacted EDTA with a standard metal solution
  • Titrate EDTA directly against Mg2+ until a precipitate forms
  • Use pH 3 buffer and phenolphthalein to detect endpoint
  • Evaporate the sample and weigh the residue

Correct Answer: Add excess EDTA to the sample, then titrate the unreacted EDTA with a standard metal solution

Q18. During titration, a persistent faint pink color remains after adding EDTA and indicator. What is the likely cause?

  • Complete complexation of Mg2+
  • Excess EDTA present
  • Presence of transition metal impurities forming colored complexes with the indicator
  • Buffer pH is exactly correct

Correct Answer: Presence of transition metal impurities forming colored complexes with the indicator

Q19. Which of the following will most strongly interfere with the EDTA titration of magnesium if not masked or removed?

  • Sodium (Na+)
  • Calcium (Ca2+)
  • Chloride (Cl−)
  • Sulfate (SO4 2−)

Correct Answer: Calcium (Ca2+)

Q20. How does ionic strength of the solution affect complexometric titrations?

  • Ionic strength has no effect on equilibrium or activity
  • Changes in ionic strength affect activity coefficients and can shift apparent stability constants
  • Ionic strength converts EDTA into a stronger acid
  • Ionic strength increases the color intensity of indicators

Correct Answer: Changes in ionic strength affect activity coefficients and can shift apparent stability constants

Q21. Which measurement is directly obtained at the equivalence point of a Mg2+–EDTA titration?

  • Number of moles of EDTA added equals number of moles of Mg2+ present
  • The pH of the solution becomes neutral
  • All sulphate ions are precipitated
  • The mass of magnesium metal in the sample

Correct Answer: Number of moles of EDTA added equals number of moles of Mg2+ present

Q22. When performing pharmacopeial assay of magnesium sulphate, which sample preparation step is commonly required?

  • Evaporate to dryness and weigh residue directly
  • Neutralize sample, dissolve in water, and filter to remove insolubles before titration
  • Directly titrate solid without dissolution
  • Boil with concentrated sulfuric acid

Correct Answer: Neutralize sample, dissolve in water, and filter to remove insolubles before titration

Q23. Which of the following best explains why ammonia buffer is used at pH 10 for Mg-EDTA titrations?

  • To precipitate magnesium as hydroxide
  • To keep metal ions in the proper ionic form and ensure optimal EDTA complex stability and indicator response
  • To oxidize interfering metals
  • To neutralize the EDTA

Correct Answer: To keep metal ions in the proper ionic form and ensure optimal EDTA complex stability and indicator response

Q24. If a tablet formulation contains magnesium sulphate and calcium carbonate, which practical step helps obtain an accurate Mg assay?

  • Use no buffer and perform titration at ambient pH
  • Mask or separate calcium before titrating magnesium
  • Add extra indicator to overcome interference
  • Ignore calcium as it does not react with EDTA

Correct Answer: Mask or separate calcium before titrating magnesium

Q25. Which statement about the endpoint detection using Eriochrome Black T is true?

  • Indicator is red when free and blue when complexed with metal
  • Indicator is blue when free and wine-red when bound to metal
  • Indicator does not change color during titration
  • Indicator works only at pH below 4

Correct Answer: Indicator is blue when free and wine-red when bound to metal

Q26. A method requires that 0.250 g of magnesium sulphate sample is dissolved and titrated. If the titration finds 0.00150 mol Mg2+, what is the percentage of magnesium (Mg atom, atomic weight 24.305) in the sample? (Use conceptual understanding: find mass of Mg and percent)

  • Approximately 14.6%
  • Approximately 3.66%
  • Approximately 29.2%
  • Approximately 0.0366%

Correct Answer: Approximately 14.6%

Q27. Which procedural step improves accuracy in EDTA titration of magnesium sulphate?

  • Perform titration rapidly without stirring
  • Use freshly prepared buffer and standardize EDTA frequently
  • Skip filtration to retain all solids
  • Use an indicator incompatible with chosen pH

Correct Answer: Use freshly prepared buffer and standardize EDTA frequently

Q28. Why might a pharmacist choose back titration rather than direct titration for a magnesium assay?

  • Direct titration always gives wrong results
  • When the reaction with EDTA is too slow or when the sample matrix interferes with direct endpoint detection
  • Because back titration uses less reagent
  • To avoid using any indicator

Correct Answer: When the reaction with EDTA is too slow or when the sample matrix interferes with direct endpoint detection

Q29. Which laboratory practice reduces systematic error during complexometric titration of magnesium?

  • Not rinsing the burette before filling
  • Standardizing the EDTA using a reliable metal standard and performing replicate titrations
  • Using expired reagents to save cost
  • Performing only a single titration and reporting the result

Correct Answer: Standardizing the EDTA using a reliable metal standard and performing replicate titrations

Q30. In pharmaceutical quality control, why is understanding conditional stability constants important for Mg-EDTA titration?

  • Because they determine the color of the indicator
  • Because conditional constants account for pH and ionic strength, affecting complex formation and endpoint accuracy
  • Because they allow EDTA to be used without buffering
  • Because they convert magnesium to a volatile form for assay

Correct Answer: Because conditional constants account for pH and ionic strength, affecting complex formation and endpoint accuracy

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