Indicator electrodes – metal and glass electrodes MCQs With Answer

Indicator electrodes – metal and glass electrodes MCQs With Answer

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Indicator electrodes – metal and glass electrodes MCQs With Answer

This concise introduction covers indicator electrodes—metal and glass electrodes—tailored for B. Pharm students. You will learn core concepts such as potentiometry, ion-selective electrodes, the Nernst equation, pH electrode construction and response, reference electrodes (Ag/AgCl, SCE), liquid junction potentials, selectivity coefficients (Nikolskii–Eisenman), alkaline/acid errors, calibration, conditioning and maintenance. Emphasis is on practical pharmaceutical applications: titrations, drug analysis, and quality control. The language is simple yet technically accurate to build conceptual depth and exam readiness. Focus keywords: indicator electrodes, metal electrodes, glass electrodes, pH electrode, potentiometry, selectivity, calibration, reference electrode. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the fundamental principle of an indicator electrode in potentiometry?

  • Measures current proportional to ion concentration
  • Measures potential related to the activity of a specific ion following the Nernst equation
  • Measures mass change due to ion adsorption
  • Measures optical absorbance of ion complexes

Correct Answer: Measures potential related to the activity of a specific ion following the Nernst equation

Q2. How does a glass pH electrode generate its potential?

  • Redox reaction at a metal surface
  • Diffusion of OH- through a porous membrane
  • H+ ion exchange at a hydrated glass membrane creating a membrane potential
  • Optical fluorescence from a dye impregnated in the glass

Correct Answer: H+ ion exchange at a hydrated glass membrane creating a membrane potential

Q3. What is the theoretical Nernstian slope for a monovalent ion at 25°C?

  • 29.58 mV per decade
  • 59.16 mV per decade
  • 118.32 mV per decade
  • 9.85 mV per decade

Correct Answer: 59.16 mV per decade

Q4. What is the expected Nernstian slope for a divalent ion (z = 2) at 25°C?

  • 59.16 mV per decade
  • 29.58 mV per decade
  • 19.72 mV per decade
  • 118.32 mV per decade

Correct Answer: 29.58 mV per decade

Q5. Which of the following is a commonly used reference electrode in potentiometric measurements?

  • Platinum hydrogen electrode
  • Silver/silver chloride (Ag/AgCl) reference electrode
  • Glass membrane reference electrode
  • Gold microelectrode

Correct Answer: Silver/silver chloride (Ag/AgCl) reference electrode

Q6. What causes a liquid junction potential in a reference electrode system?

  • Temperature equilibration only
  • Different mobilities of ions across the salt bridge interface
  • Incomplete hydration of the glass membrane
  • Electrode metal oxidation

Correct Answer: Different mobilities of ions across the salt bridge interface

Q7. Which electrode type directly follows the metal/metal-ion equilibrium and is used as an indicator for metal ions?

  • Glass pH electrode
  • Metal/metal ion electrode (e.g., Ag/Ag+, Cu/Cu2+)
  • Ion-selective polymer membrane electrode for anions
  • Optical ion sensor

Correct Answer: Metal/metal ion electrode (e.g., Ag/Ag+, Cu/Cu2+)

Q8. What is the recommended storage/conditioning medium for many reference electrodes to maintain potential stability?

  • Dry air at room temperature
  • 3 M KCl solution or manufacturer-specified storage solution
  • Distilled water only
  • Concentrated NaOH

Correct Answer: 3 M KCl solution or manufacturer-specified storage solution

Q9. What is the primary cause of the alkaline error in glass pH electrodes?

  • Loss of hydration layer at low pH
  • Sodium ion (Na+) interfering with H+ response at high pH
  • Oxidation of the internal reference silver wire
  • Presence of organic solvents

Correct Answer: Sodium ion (Na+) interfering with H+ response at high pH

Q10. What is the acid error observed with glass pH electrodes at very low pH values?

  • An increase in slope beyond Nernstian value
  • Saturation of exchange sites leading to lower-than-expected potential change (acid error)
  • Complete loss of response
  • Contamination of reference electrode only

Correct Answer: Saturation of exchange sites leading to lower-than-expected potential change (acid error)

Q11. Which equation generalizes the Nernst response to include interfering ions and selectivity coefficients for ion-selective electrodes?

  • Henderson–Hasselbalch equation
  • Nernst–Planck equation
  • Nikolskii–Eisenman equation
  • Butler–Volmer equation

Correct Answer: Nikolskii–Eisenman equation

Q12. How does the magnitude of the selectivity coefficient (kij) relate to electrode selectivity?

  • Larger kij means more selective for the primary ion
  • Smaller kij means more selective for the primary ion
  • kij has no relation to selectivity
  • kij only applies to gas electrodes

Correct Answer: Smaller kij means more selective for the primary ion

Q13. Which factor is least likely to cause non-Nernstian slope in an indicator electrode calibration?

  • Membrane fouling or coating
  • Temperature changes not accounted for
  • Proper two-point calibration with fresh buffers
  • Significant liquid junction potential drift

Correct Answer: Proper two-point calibration with fresh buffers

Q14. How is the detection limit of an ion-selective electrode typically defined?

  • The concentration at which electrode physically dissolves
  • The lowest activity at which electrode gives a reproducible potential distinguishable from background
  • The concentration corresponding to the maximum slope
  • Twice the standard error of the calibration slope

Correct Answer: The lowest activity at which electrode gives a reproducible potential distinguishable from background

Q15. Which reference electrode contains elemental mercury and a mercury chloride salt?

  • Silver/silver chloride electrode
  • Saturated calomel electrode (SCE)
  • Glass reference electrode
  • Platinum hydrogen electrode

Correct Answer: Saturated calomel electrode (SCE)

Q16. Which factors primarily determine the response time of an ion-selective electrode?

  • Solution pH only
  • Membrane thickness, diffusion coefficients, and conditioning
  • Color of the sample
  • Manufacturer brand name only

Correct Answer: Membrane thickness, diffusion coefficients, and conditioning

Q17. Which electrode is typically considered non-polarizable and suitable as a stable reference?

  • Platinum working electrode
  • Saturated calomel electrode (SCE)
  • Dry glass pH electrode
  • Gold microelectrode

Correct Answer: Saturated calomel electrode (SCE)

Q18. What quantity does a glass pH electrode measure directly?

  • Concentration of H+ in mol/L
  • Activity of H+ ions
  • Total acidity including weak acids
  • Optical density related to pH

Correct Answer: Activity of H+ ions

Q19. How does temperature affect the theoretical Nernst slope of an ion-selective electrode?

  • Slope decreases with increasing temperature
  • Slope increases with increasing temperature (proportional to T)
  • Temperature has no effect
  • Slope oscillates unpredictably with temperature

Correct Answer: Slope increases with increasing temperature (proportional to T)

Q20. Which interfering ion is most responsible for errors in glass pH electrodes at high pH values?

  • Calcium (Ca2+)
  • Nitrate (NO3-)
  • Sodium (Na+)
  • Chloride (Cl-)

Correct Answer: Sodium (Na+)

Q21. What is the theoretical slope per decade for a Cu2+ selective electrode at 25°C?

  • 59.16 mV per decade
  • 29.58 mV per decade
  • 19.72 mV per decade
  • 118.32 mV per decade

Correct Answer: 29.58 mV per decade

Q22. Why is the glass membrane of a pH electrode hydrated before use?

  • To sterilize the glass surface
  • Hydration forms a thin hydrated gel layer enabling selective H+ exchange
  • To remove alkali metals permanently
  • To deposit reference electrolyte on the membrane

Correct Answer: Hydration forms a thin hydrated gel layer enabling selective H+ exchange

Q23. Which indicator electrode is most often used in potentiometric titration of chloride with silver nitrate?

  • Glass pH electrode
  • Silver indicator electrode (Ag/AgCl or silver wire)
  • Potassium ion-selective electrode
  • Platinum redox electrode

Correct Answer: Silver indicator electrode (Ag/AgCl or silver wire)

Q24. How can liquid junction potential be minimized in practical electrode setups?

  • Use a concentrated KCl salt bridge and match ionic mobility where possible
  • Use distilled water in the bridge
  • Use low ionic strength sample solutions only
  • Store electrode overnight in air

Correct Answer: Use a concentrated KCl salt bridge and match ionic mobility where possible

Q25. What is the recommended calibration method for routine pH measurement in pharmaceutical analysis?

  • Single-point calibration at pH 7 only
  • Two-point calibration using standard buffer solutions bracketing the expected pH
  • Calibrate using distilled water and sample
  • No calibration needed if electrode is new

Correct Answer: Two-point calibration using standard buffer solutions bracketing the expected pH

Q26. In the context of glass pH electrodes, what is meant by the term “alkali error” (sodium error)?

  • The electrode reads lower pH than true at high pH due to Na+ interference
  • The electrode cannot measure pH below 2
  • The reference electrode becomes alkaline
  • The electrode produces a larger slope than theoretical

Correct Answer: The electrode reads lower pH than true at high pH due to Na+ interference

Q27. For a metal/metal-ion indicator electrode, which expression correctly represents the potential dependence?

  • E = E° + (2.303 RT/F) log [H+]
  • E = E° + (RT/nF) ln a_metal ion
  • E = k·[ion] + constant
  • E = E° − (RT/F) ln T

Correct Answer: E = E° + (RT/nF) ln a_metal ion

Q28. Which maintenance practice helps prolong the life and accuracy of glass pH electrodes?

  • Allowing the bulb to dry between measurements
  • Regularly storing the electrode in appropriate storage solution (e.g., KCl) and periodic cleaning
  • Rinsing with acetone after each use
  • Heating the electrode to 60°C daily

Correct Answer: Regularly storing the electrode in appropriate storage solution (e.g., KCl) and periodic cleaning

Q29. What is the ideal slope (magnitude) of a glass pH electrode per pH unit at 25°C?

  • 29.58 mV per pH unit
  • 59.16 mV per pH unit
  • 118.32 mV per pH unit
  • 9.85 mV per pH unit

Correct Answer: 59.16 mV per pH unit

Q30. How is the detection limit of an ion-selective electrode often determined experimentally from a calibration plot?

  • As the highest concentration showing a linear response
  • By extrapolating to infinite dilution
  • From the intersection of linear segments of the calibration curve (Nernstian and baseline regions)
  • As the concentration used to condition the electrode

Correct Answer: From the intersection of linear segments of the calibration curve (Nernstian and baseline regions)

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