pH determination – calorimetric method MCQs With Answer

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pH determination – calorimetric method MCQs With Answer is an essential topic for B.Pharm students learning analytical techniques. This SEO-focused introduction explains how calorimetric (colorimetric/spectrophotometric) pH determination uses pH-sensitive dyes, Beer–Lambert law, and calibration curves to convert absorbance changes into accurate pH values. Topics covered include indicator selection, wavelength choice, isosbestic points, ratiometric calculations, temperature and ionic strength effects, instrument selection, and common interferences. These MCQs emphasize both theory and practical lab considerations to strengthen your understanding of spectrophotometric pH analysis in pharmaceutical contexts. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the basic principle of calorimetric pH determination?

  • Measuring electrical potential generated by hydrogen ions
  • Measuring change in absorbance of a pH-sensitive dye
  • Measuring temperature change during acid–base reactions
  • Measuring mass loss due to volatile acids

Correct Answer: Measuring change in absorbance of a pH-sensitive dye

Q2. Which law relates absorbance to concentration in calorimetric pH methods?

  • Henderson–Hasselbalch law
  • Beer–Lambert law
  • Raoult’s law
  • Henry’s law

Correct Answer: Beer–Lambert law

Q3. In a two-form indicator (HA ⇌ A−), the absorbance change with pH arises because:

  • HA and A− have identical molar absorptivities at all wavelengths
  • HA and A− have different molar absorptivities at measurement wavelengths
  • The indicator concentration changes with temperature only
  • The path length of cuvette changes with pH

Correct Answer: HA and A− have different molar absorptivities at measurement wavelengths

Q4. For reliable calorimetric pH measurement, indicator concentration should generally be:

  • Very high to maximize absorbance
  • Moderate to high to change solution color dramatically
  • Low enough to avoid self-association and inner-filter effects
  • Irrelevant; concentration does not matter

Correct Answer: Low enough to avoid self-association and inner-filter effects

Q5. What is an isosbestic point in spectrophotometric pH analysis?

  • A wavelength where absorbance varies linearly with pH
  • A wavelength where absorbance is maximal for HA only
  • A wavelength where total absorbance is independent of pH for two interconverting species
  • A wavelength where instrument noise is highest

Correct Answer: A wavelength where total absorbance is independent of pH for two interconverting species

Q6. Which method reduces errors from dye concentration and path length variations?

  • Single-wavelength absorbance method
  • Ratiometric (two-wavelength) method
  • pH titration with strong base
  • Gravimetric analysis

Correct Answer: Ratiometric (two-wavelength) method

Q7. Which instrument provides the highest spectral resolution for calorimetric pH measurement?

  • Colorimeter with single filter
  • Naked-eye color comparison
  • UV–Vis spectrophotometer
  • pH paper

Correct Answer: UV–Vis spectrophotometer

Q8. The Henderson–Hasselbalch equation is used in calorimetric pH analysis to relate:

  • Path length to absorbance
  • pH to ratio of base and acid species concentrations
  • Temperature to wavelength shift
  • Buffer capacity to ionic strength

Correct Answer: pH to ratio of base and acid species concentrations

Q9. When selecting an indicator dye for a given pH range, you should choose one with pKa:

  • Far outside the target pH range
  • Approximately equal to target pH (within ±1 unit)
  • Exactly zero
  • Greater than 14

Correct Answer: Approximately equal to target pH (within ±1 unit)

Q10. Which factor most directly shifts the pKa of an indicator and thus affects calorimetric pH readings?

  • Path length of cuvette only
  • Temperature and ionic strength
  • Brand of spectrophotometer
  • Sample color alone

Correct Answer: Temperature and ionic strength

Q11. What is the main purpose of constructing a calibration curve in colorimetric pH determination?

  • To determine molecular weight of the indicator
  • To relate absorbance (or absorbance ratio) to known pH values
  • To calibrate the cuvette path length
  • To measure the viscosity of the buffer

Correct Answer: To relate absorbance (or absorbance ratio) to known pH values

Q12. Which interference is most likely to cause erroneous pH values in calorimetric methods?

  • Presence of a non-absorbing salt at low concentration
  • Colloidal turbidity or particulate scattering
  • Use of a matched cuvette for blank
  • Stable temperature control

Correct Answer: Colloidal turbidity or particulate scattering

Q13. Why is the isosbestic point useful in validating an indicator system for pH analysis?

  • It indicates presence of more than two absorbing species
  • It confirms that only two interconverting species are involved
  • It maximizes noise in the measurement
  • It prevents calibration

Correct Answer: It confirms that only two interconverting species are involved

Q14. If εacid and εbase are molar absorptivities at a wavelength, the absorbance A for total indicator concentration Ct and path length l is:

  • A = l·Ct·(εacid + εbase)
  • A = l·[εacid·[HA] + εbase·[A−]], where [HA]+[A−]=Ct
  • A = εacid/εbase
  • A = Ct/(l·εacid)

Correct Answer: A = l·[εacid·[HA] + εbase·[A−]], where [HA]+[A−]=Ct

Q15. Compared to potentiometric pH measurement, calorimetric pH determination is particularly advantageous when:

  • Direct hydrogen ion activity measurement is required
  • Small sample volumes and high-throughput plate assays are needed
  • Electrode maintenance is easy
  • Absolute pH traceability is mandatory

Correct Answer: Small sample volumes and high-throughput plate assays are needed

Q16. Which wavelength selection strategy maximizes sensitivity for pH-induced spectral change?

  • Choose wavelengths where HA and A− have identical absorbance
  • Choose wavelengths at the absorbance maxima of HA and A− respectively for ratiometric measurement
  • Always measure at UV cutoff of the solvent
  • Measure at random visible wavelength

Correct Answer: Choose wavelengths at the absorbance maxima of HA and A− respectively for ratiometric measurement

Q17. A single-wavelength absorbance approach is most sensitive to errors caused by:

  • Variations in temperature only
  • Variations in indicator concentration and path length
  • Availability of buffer standards
  • Use of matched quartz cuvettes

Correct Answer: Variations in indicator concentration and path length

Q18. In ratiometric pH measurement using two wavelengths, the ratio A1/A2 primarily compensates for:

  • Changes in instrument lamp intensity only
  • Sample scattering but not concentration
  • Variations in dye concentration and path length
  • Buffer composition changes only

Correct Answer: Variations in dye concentration and path length

Q19. Which of the following best describes the role of ionic strength in calorimetric pH determination?

  • Ionic strength has no effect on pKa or spectral properties
  • Ionic strength can alter activity coefficients and shift apparent pKa
  • Ionic strength only affects the cuvette path length
  • Ionic strength always improves accuracy

Correct Answer: Ionic strength can alter activity coefficients and shift apparent pKa

Q20. For a dye with pKa = 7.0, which pH corresponds roughly to equal concentrations of HA and A−?

  • pH 5.0
  • pH 6.0
  • pH 7.0
  • pH 9.0

Correct Answer: pH 7.0

Q21. Which preparation step improves accuracy of colorimetric pH analysis for pharmaceutical samples?

  • Leaving particulate matter in sample
  • Filtering or centrifuging to remove turbidity
  • Increasing indicator concentration to saturation
  • Using uncalibrated cuvettes of unknown path length

Correct Answer: Filtering or centrifuging to remove turbidity

Q22. When deriving pH from absorbance ratio using known molar absorptivities, you must know:

  • Only the instrument model
  • The molar absorptivities of HA and A− at the chosen wavelengths
  • The molecular weight of the indicator
  • Only the total ionic strength

Correct Answer: The molar absorptivities of HA and A− at the chosen wavelengths

Q23. Which of the following is a practical limitation of calorimetric pH methods in pharmaceutical QC?

  • High sensitivity to electrode drift
  • Interference by colored or absorbing excipients
  • Requirement for large sample volumes (>100 mL)
  • Absolute immunity to temperature changes

Correct Answer: Interference by colored or absorbing excipients

Q24. A cuvette with path length 1.0 cm, indicator concentration 1.0×10−5 M, and molar absorptivity 20000 L·mol−1·cm−1 gives absorbance A = ?

  • 0.002
  • 0.20
  • 2.0
  • 20.0

Correct Answer: 0.20

Q25. Which calibration approach yields the best accuracy across a wide pH range for calorimetric measurement?

  • Single-point calibration at neutral pH
  • Multi-point calibration using several buffer standards across the range
  • Calibration using pure water only
  • No calibration is needed

Correct Answer: Multi-point calibration using several buffer standards across the range

Q26. The term “apparent pKa” in spectrophotometric pH analysis refers to:

  • The intrinsic pKa at infinite dilution only
  • The pKa value observed under the specific experimental conditions
  • The pKa of the solvent
  • The pKa of an unrelated compound

Correct Answer: The pKa value observed under the specific experimental conditions

Q27. Which of the following best improves limit of detection in colorimetric pH assays?

  • Use of high-noise light source
  • Longer path length cuvette and optimized wavelength selection
  • Random wavelength selection
  • Increasing sample turbidity

Correct Answer: Longer path length cuvette and optimized wavelength selection

Q28. In multiwell plate calorimetric pH assays, a common advantage is:

  • Ability to measure single sample only
  • High-throughput simultaneous analysis of many samples
  • No need for blank correction
  • Complete elimination of temperature effects

Correct Answer: High-throughput simultaneous analysis of many samples

Q29. Which of the following indicates that a chosen indicator system may be contaminated or degraded?

  • Presence of a clear isosbestic point
  • Absence of expected spectral features and distorted isosbestic behavior
  • Reproducible calibration across all buffers
  • Stable baseline with matched blank

Correct Answer: Absence of expected spectral features and distorted isosbestic behavior

Q30. The major benefit of using a spectrophotometer over a simple colorimeter for pH determination is:

  • Lower maintenance only
  • Ability to scan full spectra and select optimal wavelengths
  • It eliminates the need for calibration
  • It directly measures hydrogen ion activity

Correct Answer: Ability to scan full spectra and select optimal wavelengths

Q31. Which step is essential before measuring sample absorbance for pH analysis?

  • Measure without blanking
  • Zero the instrument with an appropriate blank (solvent + indicator without analyte)
  • Heat the sample to boiling
  • Expose sample to light for extended periods

Correct Answer: Zero the instrument with an appropriate blank (solvent + indicator without analyte)

Q32. In a ratiometric method using wavelengths λ1 and λ2, an observed stable ratio across buffer standards suggests:

  • Instrument malfunction
  • The selected wavelengths are not pH-sensitive for that indicator
  • Perfect indicator response to pH
  • Excessive turbidity

Correct Answer: The selected wavelengths are not pH-sensitive for that indicator

Q33. Which practice helps minimize temperature-related pKa shifts during calorimetric pH measurements?

  • Perform measurements at uncontrolled room temperature
  • Use temperature-controlled cuvette holder and equilibrate samples
  • Vary temperature between standards and samples
  • Ignore temperature because dyes are temperature-insensitive

Correct Answer: Use temperature-controlled cuvette holder and equilibrate samples

Q34. If a pharmaceutical excipient absorbs strongly at the measurement wavelength, best corrective action is:

  • Ignore the interference and report result
  • Choose alternate wavelengths or use extraction/separation before measurement
  • Increase indicator concentration to overpower excipient absorbance
  • Use a shorter path length without recalibration

Correct Answer: Choose alternate wavelengths or use extraction/separation before measurement

Q35. Which calculation step converts measured absorbances at two wavelengths into pH using known ε values?

  • Directly averaging the two absorbances
  • Solving simultaneous equations for [HA] and [A−], then applying Henderson–Hasselbalch
  • Dividing absorbance by path length only
  • Multiplying absorbances to obtain pH

Correct Answer: Solving simultaneous equations for [HA] and [A−], then applying Henderson–Hasselbalch

Q36. Which statement about colorimetric pH determination sensitivity is true?

  • Sensitivity is independent of molar absorptivity
  • Higher molar absorptivity of indicator increases sensitivity
  • Lower path length always increases sensitivity
  • Indicator choice does not affect sensitivity

Correct Answer: Higher molar absorptivity of indicator increases sensitivity

Q37. For quality control assays of buffer formulations, calorimetric pH methods are particularly useful for:

  • Rapid screening of pH in multiple samples simultaneously
  • Absolute traceable pH measurements without calibration
  • Measuring pH of non-aqueous solvents only
  • Replacing all potentiometric measurements

Correct Answer: Rapid screening of pH in multiple samples simultaneously

Q38. During method validation of a calorimetric pH assay, which parameter is essential to assess?

  • Only the brand of cuvettes
  • Linearity, accuracy, precision, limit of detection, and robustness
  • Only the color of the indicator
  • Only the operator’s experience

Correct Answer: Linearity, accuracy, precision, limit of detection, and robustness

Q39. Which property of an indicator dye ensures minimal interference from dissolved oxygen?

  • Indicator that changes fluorescence on oxidation
  • Indicator that relies on acid–base equilibrium and not on redox chemistry
  • Indicator that is easily oxidized
  • Indicator that aggregates in presence of oxygen

Correct Answer: Indicator that relies on acid–base equilibrium and not on redox chemistry

Q40. If a sample’s measured absorbance ratio corresponds to pH 4.5 using your calibration, but potentiometric pH meter reads 4.8, likely causes include:

  • Calibration differences, ionic strength effects, temperature differences, or interfering absorbance
  • Complete method equivalence—no investigation needed
  • Only mechanical failure of spectrophotometer
  • Potentiometric meter is always wrong

Correct Answer: Calibration differences, ionic strength effects, temperature differences, or interfering absorbance

Q41. The ratiometric formula for pH often eliminates dependence on indicator concentration because:

  • Both absorbances include the same linear factor of concentration that cancels in the ratio
  • Indicator concentration is squared in the numerator only
  • Path length changes with concentration
  • pH is independent of any optical measurement

Correct Answer: Both absorbances include the same linear factor of concentration that cancels in the ratio

Q42. Which type of cuvette material is preferred for visible-region calorimetric pH measurements with common dyes?

  • Plastic cuvettes that absorb in visible region
  • Quartz cuvettes are required for visible only measurements
  • Glass or optical-grade plastic suitable for visible wavelengths
  • Metal cuvettes

Correct Answer: Glass or optical-grade plastic suitable for visible wavelengths

Q43. When using multiple indicators to cover a wide pH range, what is a key consideration?

  • Indicators must have overlapping useful ranges and non-interfering spectra
  • All indicators should have identical spectra
  • Indicators must be mixed at very high concentrations
  • Use as many indicators as possible irrespective of overlap

Correct Answer: Indicators must have overlapping useful ranges and non-interfering spectra

Q44. Which is a common mathematical approach to convert absorbance data to pH in two-wavelength methods?

  • Using linear regression of absorbance ratio vs pH from standards
  • Directly equating absorbance to pH without calibration
  • Using gravimetric conversion factors
  • Assuming pH equals absorbance at λ1

Correct Answer: Using linear regression of absorbance ratio vs pH from standards

Q45. For spectrophotometric pH assays, absorbance readings above ~2.0 are typically avoided because:

  • Instrumentation is most accurate at very high absorbance
  • Nonlinear deviations from Beer–Lambert law and high noise occur
  • Higher absorbance reduces sensitivity
  • It guarantees linear response

Correct Answer: Nonlinear deviations from Beer–Lambert law and high noise occur

Q46. In preparing calibration standards for colorimetric pH, which buffer property should be matched between standards and samples?

  • Only color of buffer
  • Ionic strength and solvent composition as closely as possible
  • Brand of container used
  • Exact same nominal pH without considering ionic strength

Correct Answer: Ionic strength and solvent composition as closely as possible

Q47. Which application best suits calorimetric pH determination in pharmaceutical research?

  • Real-time monitoring of pH during dissolution or stability testing with many samples
  • Replacement of all titrimetric assays
  • Measuring solid-state crystalline pH
  • Determining elemental impurities

Correct Answer: Real-time monitoring of pH during dissolution or stability testing with many samples

Q48. If an indicator exhibits aggregation at the working concentration, the expected spectral effect is:

  • Pure isosbestic behavior unaffected
  • New spectral bands, baseline drift, and nonlinearity with concentration
  • Improved linearity with Beer–Lambert law
  • Complete disappearance of all absorbance

Correct Answer: New spectral bands, baseline drift, and nonlinearity with concentration

Q49. Which quality control check can verify spectrophotometer performance before calorimetric pH runs?

  • Ignoring instrument and proceeding with measurements
  • Measuring a certified neutral density filter or standard solution and checking baseline noise
  • Using an uncalibrated lamp intensity setting only
  • Only checking the external appearance of the instrument

Correct Answer: Measuring a certified neutral density filter or standard solution and checking baseline noise

Q50. Which summary statement best describes calorimetric pH determination for B.Pharm students?

  • It is an obsolete method with no current relevance
  • It is a versatile analytical approach using dye absorbance, calibration, and spectral methods to measure pH with attention to interferences, temperature, and validation
  • It requires no understanding of spectroscopy or buffers
  • It provides absolute pH without calibration or controls

Correct Answer: It is a versatile analytical approach using dye absorbance, calibration, and spectral methods to measure pH with attention to interferences, temperature, and validation

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