Principles of complexometric titration MCQs With Answer

Principles of complexometric titration MCQs With Answer — Complexometric titration, especially EDTA titration, is a vital analytical technique for B. Pharm students to quantify metal ions in formulations, raw materials and water. This introduction reviews chelation chemistry, stoichiometry, stability (formation) constants, conditional constants, buffer and pH control, metallochromic indicators, masking and demasking agents, back-titration, and titration curves. Understanding factors like ligand strength, complex stability, and endpoint detection is essential for accurate assay of Ca2+, Mg2+, Zn2+, Fe3+ and other pharmaceutically relevant ions. Practice with MCQs reinforces concepts and problem-solving skills needed for lab standardization and quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is a complexometric titration?

• A titration based on precipitation of an insoluble salt
• A titration based on the formation of a coordination complex between a metal ion and a ligand such as EDTA
• A redox titration using potassium permanganate
• An acid-base titration measuring pH changes

Correct Answer: A titration based on the formation of a coordination complex between a metal ion and a ligand such as EDTA

Q2. What is the typical stoichiometry of complex formation between EDTA and most divalent metal ions?

• 1 metal : 2 EDTA
• 2 metal : 1 EDTA
• 1 metal : 1 EDTA
• 3 metal : 1 EDTA

Correct Answer: 1 metal : 1 EDTA

Q3. What is the denticity of EDTA when fully coordinated to a metal ion?

• Monodentate
• Bidentate
• Tetradentate
• Hexadentate

Correct Answer: Hexadentate

Q4. What is the formal charge of fully deprotonated EDTA (Y) at high pH?

• 0
• -2
• -3
• -4

Correct Answer: -4

Q5. Which metallochromic indicator is most commonly used for the EDTA titration of calcium and magnesium (water hardness)?

• Phenolphthalein
• Eriochrome Black T
• Methylene blue
• Potassium permanganate

Correct Answer: Eriochrome Black T

Q6. What is the typical pH and buffer used for EDTA titration of Ca2+ and Mg2+?

• pH 2–3, hydrochloric acid
• pH 4–5, acetate buffer
• pH 7, phosphate buffer
• pH ~10, ammonia-ammonium buffer

Correct Answer: pH ~10, ammonia-ammonium buffer

Q7. During a Ca/Mg titration with Eriochrome Black T, what color change indicates the endpoint?

• Blue to green
• Red to yellow
• Wine-red (indicator-metal complex) to blue (free indicator)
• Colorless to pink

Correct Answer: Wine-red (indicator-metal complex) to blue (free indicator)

Q8. What does the conditional formation constant (K’) describe?

• The stability constant measured only at 25°C
• The effective formation constant of a metal–ligand complex under specific experimental conditions such as pH and ionic strength
• The dissociation constant of water
• The redox potential of the metal

Correct Answer: The effective formation constant of a metal–ligand complex under specific experimental conditions such as pH and ionic strength

Q9. What is meant by the term pM in complexometric titration?

• pH of the metal solution
• -log[free metal ion concentration]
• The partition coefficient of the metal
• -log[EDTA concentration]

Correct Answer: -log[free metal ion concentration]

Q10. What is the main purpose of adding a masking agent during complexometric titration?

• To accelerate the titration reaction
• To convert metal ions to their elemental state
• To selectively prevent interfering ions from reacting by forming stronger complexes with them
• To change the color of the indicator

Correct Answer: To selectively prevent interfering ions from reacting by forming stronger complexes with them

Q11. Which of the following is commonly used as a masking agent for copper(II) in EDTA titrations?

• Thiourea
• Sodium hydroxide
• Hydrochloric acid
• Potassium permanganate

Correct Answer: Thiourea

Q12. When is back titration typically used in complexometric analysis?

• When the analyte reacts instantly and a direct titration is simple
• When the endpoint color change is very vivid and easy to detect
• When direct titration is impractical due to slow kinetics, insoluble analyte, or indistinct endpoint
• Only for redox-active metals

Correct Answer: When direct titration is impractical due to slow kinetics, insoluble analyte, or indistinct endpoint

Q13. Which metal ion is frequently used as a primary standard to standardize EDTA solutions in routine laboratory practice?

• Lead(II)
• Calcium(II)
• Gold(III)
• Mercury(II)

Correct Answer: Calcium(II)

Q14. How does a metallochromic indicator indicate the endpoint in an EDTA titration?

• The indicator oxidizes at the endpoint producing a new color
• The indicator binds strongly to EDTA directly causing a color change
• The indicator forms a colored complex with the metal; when EDTA binds the metal, the indicator is released and changes to its free color
• The indicator decomposes at the equivalence point

Correct Answer: The indicator forms a colored complex with the metal; when EDTA binds the metal, the indicator is released and changes to its free color

Q15. What is the principal analytical advantage of complexometric titration in pharmaceutical quality control?

• It ignores metal ion concentration
• It provides rapid, selective and quantitative determination of metal ions with high accuracy
• It measures organic impurities directly
• It replaces the need for all instrumental methods

Correct Answer: It provides rapid, selective and quantitative determination of metal ions with high accuracy

Q16. Which factors influence conditional formation constants and therefore must be controlled during titrations?

• pH only
• Ionic strength only
• Temperature only
• pH, ionic strength and temperature

Correct Answer: pH, ionic strength and temperature

Q17. Which of the following is a metallochromic indicator used in complexometric titrations?

• Eriochrome Black T
• Phenol red
• Methyl orange
• Starch

Correct Answer: Eriochrome Black T

Q18. What is the effect of lowering the pH on the chelating ability of EDTA?

• EDTA chelating ability increases because more donor sites are available
• EDTA chelating ability decreases because protonation of EDTA reduces available donor sites
• pH has no effect on EDTA chelation
• EDTA becomes oxidized at low pH

Correct Answer: EDTA chelating ability decreases because protonation of EDTA reduces available donor sites

Q19. Why is control of ionic strength important in complexometric titrations?

• Ionic strength changes the color of indicators
• Ionic strength influences activity coefficients and thus apparent stability constants, affecting reproducibility and accuracy
• Ionic strength alters the valence of metal ions
• Ionic strength speeds up the titration

Correct Answer: Ionic strength influences activity coefficients and thus apparent stability constants, affecting reproducibility and accuracy

Q20. What defines the equivalence point in an EDTA complexometric titration?

• The point at which indicator starts to change color slightly
• The point where stoichiometric amount of EDTA has been added to complex all free analyte metal ions
• The first addition of EDTA to the solution
• The point where pH becomes 7

Correct Answer: The point where stoichiometric amount of EDTA has been added to complex all free analyte metal ions

Q21. Which of the following is a common experimental error in complexometric titrations that leads to inaccurate results?

• Using the correct pH buffer
• Using freshly prepared indicator
• Failing to mask interfering ions or using wrong pH
• Standardizing EDTA before use

Correct Answer: Failing to mask interfering ions or using wrong pH

Q22. Which method is commonly used to determine the stoichiometry of a metal–ligand complex prior to titration?

• Chromatography of the complex
• Job’s method of continuous variations
• Gravimetric precipitation
• Flame photometry

Correct Answer: Job’s method of continuous variations

Q23. Which buffer is typically used for EDTA titration of zinc(II) ions?

• Ammonia-ammonium buffer at pH 10
• Acetate buffer at pH 5–6
• Strong acid (HCl) at pH 1
• Phenolphthalein buffer at pH 9

Correct Answer: Acetate buffer at pH 5–6

Q24. In determining total hardness of water by EDTA titration, results are commonly reported as:

• mg/L of NaCl
• ppm as CaCO3
• percent w/v of EDTA
• Molarity of ammonia

Correct Answer: ppm as CaCO3

Q25. What does “masking” an ion mean in the context of complexometric titration?

• Removing the ion by precipitation
• Adding a reagent that forms a more stable complex with the interfering ion so it does not react with EDTA
• Evaporating the sample to dryness
• Heating the sample to change color

Correct Answer: Adding a reagent that forms a more stable complex with the interfering ion so it does not react with EDTA

Q26. What is “demasking” in complexometric titration?

• Using a stronger ligand to remove the analyte
• Destroying or removing the masking complex to release the masked ion for subsequent determination
• Neutralizing the titration mixture after endpoint
• Filtering the solution prior to titration

Correct Answer: Destroying or removing the masking complex to release the masked ion for subsequent determination

Q27. Which detection technique can be used as an alternative to color indicators for complexometric titrations in colored or turbid samples?

• Visual color matching only
• Potentiometric detection using metal-ion selective electrodes or a pH/mV change
• Mass spectrometry
• Fluorescence spectroscopy exclusively

Correct Answer: Potentiometric detection using metal-ion selective electrodes or a pH/mV change

Q28. Which statement about EDTA titration of Fe3+ in pharmaceutical samples is most appropriate?

• Fe3+ always gives a sharp endpoint with Eriochrome Black T at pH 10
• Fe3+ often requires reduction to Fe2+ or special pH control because of hydrolysis and complex speciation
• Fe3+ cannot form complexes with EDTA
• Fe3+ measurement needs no buffering and is independent of pH

Correct Answer: Fe3+ often requires reduction to Fe2+ or special pH control because of hydrolysis and complex speciation

Q29. In pharmaceutical analysis, which practical parameter is most directly improved by standardizing the EDTA solution before use?

• Color of the titration solution
• Accuracy of metal ion concentration determinations
• The ionic radius of metal ions
• The dielectric constant of the solvent

Correct Answer: Accuracy of metal ion concentration determinations

Q30. Which pharmaceutical quality control application commonly uses complexometric titration with EDTA?

• Determination of moisture content in tablets
• Quantification of residual solvents by GC
• Determination of trace and major metal ion impurities (e.g., Ca, Mg, Zn) in raw materials and formulations
• Assay of organic active pharmaceutical ingredient by titration with strong acid

Correct Answer: Determination of trace and major metal ion impurities (e.g., Ca, Mg, Zn) in raw materials and formulations

Download