Spectrophotometric titrations and quantitative analysis MCQs With Answer

Spectrophotometric titrations and quantitative analysis MCQs With Answer

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Spectrophotometric titrations and quantitative analysis are essential for B.Pharm students, merging titrimetric accuracy with spectroscopic detection to quantify drugs, impurities, and excipients. This introduction covers core keywords and concepts: spectrophotometric titrations, quantitative analysis, Beer–Lambert law, absorbance, molar absorptivity, path length, calibration curve, limits of detection (LOD/LOQ), linearity, instrument components (light source, monochromator, detector), sample preparation, blanks, cuvettes, pH effects, and common interferences. Understanding these principles supports method validation, accuracy, precision, and reliable endpoint determination in pharmaceutical assays. Each MCQ includes answers to reinforce learning and exam preparation. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the Beer–Lambert law expressed in its common form for absorbance?

  • A = ε × b × c
  • A = ε / (b × c)
  • A = b / (ε × c)
  • A = ε + b + c

Correct Answer: A = ε × b × c

Q2. Which factor does NOT directly cause deviation from Beer–Lambert law at high concentrations?

  • Molecular association or dissociation
  • Stray light from the instrument
  • Polychromatic light source
  • Using a quartz cuvette instead of glass

Correct Answer: Using a quartz cuvette instead of glass

Q3. In a spectrophotometric titration, the equivalence point is best defined as:

  • The point where the indicator first shows any color
  • The theoretical stoichiometric completion of the reaction between analyte and titrant
  • The last measurable absorbance before dilution
  • The volume where the solution becomes neutral

Correct Answer: The theoretical stoichiometric completion of the reaction between analyte and titrant

Q4. Which instrumental component is primarily responsible for selecting a narrow wavelength from a polychromatic source?

  • Detector
  • Monochromator
  • Cuvette
  • Light source

Correct Answer: Monochromator

Q5. Molar absorptivity (ε) has which units in the Beer–Lambert equation?

  • mol L⁻¹ cm⁻¹
  • L mol⁻¹ cm⁻¹
  • cm L mol⁻¹
  • unitless

Correct Answer: L mol⁻¹ cm⁻¹

Q6. Why is the absorbance measured at λmax preferred for quantitative analysis?

  • It minimizes instrument noise only
  • It provides maximum sensitivity and steeper calibration slope
  • It reduces path length effects
  • It eliminates the need for blanks

Correct Answer: It provides maximum sensitivity and steeper calibration slope

Q7. In spectrophotometric titrations, which approach corrects for matrix effects by adding known standard to the sample?

  • External calibration
  • Standard addition method
  • Internal standardization with a fused standard
  • Blank subtraction only

Correct Answer: Standard addition method

Q8. What is the primary purpose of running a reagent blank in spectrophotometry?

  • To calibrate the monochromator wavelength scale
  • To correct for absorbance due to solvents and reagents
  • To increase sample concentration
  • To measure path length of cuvettes

Correct Answer: To correct for absorbance due to solvents and reagents

Q9. Which detector is commonly used in UV-Visible spectrophotometers for high sensitivity?

  • Flame ionization detector
  • Photomultiplier tube (PMT)
  • Thermocouple
  • Electrochemical sensor

Correct Answer: Photomultiplier tube (PMT)

Q10. In a titration monitored by absorbance, the endpoint is typically identified by:

  • The theoretical equivalence volume without measurement
  • Observation of minimum or maximum in the absorbance vs volume plot
  • Only color change of an external indicator, ignoring absorbance
  • Measuring pH only

Correct Answer: Observation of minimum or maximum in the absorbance vs volume plot

Q11. What effect does stray light have on measured absorbance values?

  • Causes apparent absorbance to increase linearly
  • Leads to lower apparent absorbance at high true absorbance (negative bias)
  • Has no effect if path length is short
  • Only affects fluorescence measurements

Correct Answer: Leads to lower apparent absorbance at high true absorbance (negative bias)

Q12. Isosbestic points are useful because they indicate:

  • Instrument wavelength calibration error
  • Presence of two interconverting species with constant total absorbance at that wavelength
  • Complete degradation of analyte
  • That Beer’s law is invalid

Correct Answer: Presence of two interconverting species with constant total absorbance at that wavelength

Q13. Which statement about calibration curves in spectrophotometry is correct?

  • A perfect calibration curve always passes through the origin
  • Linearity range must be established; slope equals ε×b if Beer’s law holds
  • Only one standard is required for calibration
  • R² value is not relevant for method validation

Correct Answer: Linearity range must be established; slope equals ε×b if Beer’s law holds

Q14. When analyzing mixtures with overlapping spectra, which technique can improve selectivity?

  • Use of wider spectral bandwidth and no baseline correction
  • Derivative spectrophotometry or multicomponent calibration
  • Ignoring the overlap and measuring at any wavelength
  • Using only visual titration

Correct Answer: Derivative spectrophotometry or multicomponent calibration

Q15. In complexometric titrations monitored spectrophotometrically, metallochromic indicators are used because they:

  • Change pH at equivalence
  • Form colored complexes with metal ions shifting absorbance
  • Increase ionic strength only
  • React irreversibly with titrant

Correct Answer: Form colored complexes with metal ions shifting absorbance

Q16. Limit of detection (LOD) in spectrophotometric methods is best described as:

  • The lowest concentration that can be measured with acceptable precision
  • The concentration producing a signal three times the standard deviation of the blank
  • The highest concentration without deviation from Beer’s law
  • The point where absorbance equals zero

Correct Answer: The concentration producing a signal three times the standard deviation of the blank

Q17. Which cuvette material is required for reliable measurements in the UV region below 300 nm?

  • Plastic
  • Glass
  • Quartz
  • Polystyrene

Correct Answer: Quartz

Q18. Additivity of absorbance applies when:

  • Species absorb at the same wavelength and interact strongly
  • Different absorbing species do not chemically interact and obey Beer’s law
  • Solutions are turbid and scattering is significant
  • Concentrations are extremely high causing aggregation

Correct Answer: Different absorbing species do not chemically interact and obey Beer’s law

Q19. Which practice improves accuracy when preparing a series of standards for a calibration curve?

  • Use a single volumetric flask for all standards
  • Prepare standards gravimetrically or with class A volumetric flasks and pipettes
  • Rinse glassware only once with distilled water
  • Make serial dilutions without mixing thoroughly

Correct Answer: Prepare standards gravimetrically or with class A volumetric flasks and pipettes

Q20. During a spectrophotometric titration of a weak acid with strong base, why does pH control matter?

  • It affects the molar absorptivity by changing the protonation state of analyte
  • pH control is irrelevant if wavelength is fixed
  • Only ionic strength matters, not pH
  • It prevents evaporation of solvent

Correct Answer: It affects the molar absorptivity by changing the protonation state of analyte

Q21. Which parameter is obtained from the slope of a calibration curve in spectrophotometry (absorbance vs concentration)?

  • Path length only
  • Molar absorptivity times path length (ε × b)
  • Blank absorbance
  • Concentration squared

Correct Answer: Molar absorptivity times path length (ε × b)

Q22. Which is a sign of improper baseline or drift during spectrophotometric titration?

  • Stable absorbance for multiple blank measurements
  • Gradual change in absorbance over time unrelated to titrant volume
  • Sharp endpoint with reproducible replicate results
  • Zero absorbance for all samples

Correct Answer: Gradual change in absorbance over time unrelated to titrant volume

Q23. For a photometric titration using an indicator that forms a colored complex, which choice ensures a sharp endpoint?

  • Low indicator concentration that does not alter equivalence
  • High indicator concentration that overwhelms analyte
  • No stirring during titration
  • Performing titration at extremely low temperature only

Correct Answer: Low indicator concentration that does not alter equivalence

Q24. Which approach minimizes interference from scattering (turbidity) in spectrophotometric assays?

  • Measure at very short path lengths only
  • Use background subtraction, centrifugation/filtration, or measure at wavelengths where scattering is negligible
  • Ignore turbidity if absorbance is low
  • Add more solvent to increase turbidity

Correct Answer: Use background subtraction, centrifugation/filtration, or measure at wavelengths where scattering is negligible

Q25. In a back-titration monitored spectrophotometrically, the measured absorbance corresponds to:

  • The analyte directly
  • The excess reagent remaining after reaction with analyte
  • The color of the burette
  • The pH of the titrant only

Correct Answer: The excess reagent remaining after reaction with analyte

Q26. Which spectral feature indicates a complex formation between analyte and reagent?

  • No change in spectral shape but decreased absorbance only
  • Appearance of a new absorbance band or shift in λmax
  • Complete disappearance of all absorbance
  • Only baseline noise increases

Correct Answer: Appearance of a new absorbance band or shift in λmax

Q27. Which data treatment can extend linearity for slightly deviating Beer’s law data?

  • Ignoring outliers without evaluation
  • Using polynomial or weighted regression and limiting concentration range
  • Multiplying absorbance by concentration
  • Discarding the calibration curve entirely

Correct Answer: Using polynomial or weighted regression and limiting concentration range

Q28. Why is temperature control important during spectrophotometric titration?

  • Temperature only affects pH meters, not absorbance
  • Temperature changes can alter reaction equilibria and molar absorptivity
  • It prevents cuvette breakage only
  • It is only relevant for fluorescence methods

Correct Answer: Temperature changes can alter reaction equilibria and molar absorptivity

Q29. Which of the following best describes the standard addition method advantage?

  • It is faster than external calibration in all cases
  • It compensates for matrix effects by spiking sample with known amounts of standard
  • It requires no knowledge of instrument response
  • It eliminates the need for instrument calibration

Correct Answer: It compensates for matrix effects by spiking sample with known amounts of standard

Q30. A spectrophotometric titration curve shows a sigmoid absorbance change with a steep inflection. The steep region corresponds to:

  • The titrant’s stock concentration
  • The equivalence region where analyte is rapidly converted
  • The blank measurement only
  • Instrument malfunction region

Correct Answer: The equivalence region where analyte is rapidly converted

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