Principles of potentiometry MCQs With Answer

Principles of potentiometry MCQs With Answer

Introduction: Potentiometry is a core electroanalytical technique in pharmaceutical analysis, focusing on measuring electrical potential differences between electrodes to determine ion concentrations. This topic covers electrochemical cells, reference and indicator electrodes, ion-selective electrodes (ISE), the Nernst equation, electrode response, calibration, detection limits, selectivity coefficients, junction potentials, and practical applications in B. Pharm such as pH measurement, dissolution testing, and assay of ionic drugs. Understanding instrumentation, temperature effects, and sample matrix control is essential for accurate potentiometric assays. This collection of focused MCQs will reinforce key concepts and problem-solving skills for pharmacy students. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is potentiometry primarily used to measure in pharmaceutical analysis?

• Mass of a drug molecule
• Electrical current through a solution
• Potential difference between electrodes related to ion activity
• Absorbance of a solution at a wavelength

Correct Answer: Potential difference between electrodes related to ion activity

Q2. Which electrode is typically used as the stable reference in potentiometric measurements?

• Glass electrode
• Calomel electrode (SCE)
• Platinum indicator electrode
• Mercury drop electrode

Correct Answer: Calomel electrode (SCE)

Q3. The Nernst equation relates electrode potential to which of the following?

• Temperature and solution viscosity
• Ion concentration or activity and temperature
• Current density and electrode area
• pH and optical density

Correct Answer: Ion concentration or activity and temperature

Q4. For a monovalent cation-selective electrode at 25°C, the theoretical Nernstian slope is approximately:

• 59.16 mV per decade
• 29.58 mV per decade
• 118.3 mV per decade
• 10.0 mV per decade

Correct Answer: 59.16 mV per decade

Q5. Which type of electrode is most commonly used for pH measurement in pharmaceutical labs?

• Ion-selective electrode for sodium
• Glass electrode
• Amperometric electrode
• Carbon paste electrode

Correct Answer: Glass electrode

Q6. What is the main purpose of a salt bridge or junction in a potentiometric cell?

• To generate current for the cell
• To maintain electrical contact and minimize liquid junction potential
• To selectively bind analyte ions
• To change the pH of the sample

Correct Answer: To maintain electrical contact and minimize liquid junction potential

Q7. In potentiometric titration, the endpoint is detected by:

• A sudden change in current
• A sudden change in measured potential
• An increase in solution turbidity
• A color change of an indicator dye

Correct Answer: A sudden change in measured potential

Q8. Selectivity coefficient of an ion-selective electrode indicates:

• Rate of electrode drift over time
• Relative response to interfering ions versus primary ion
• Electrical resistance of the membrane
• Temperature dependence of the electrode

Correct Answer: Relative response to interfering ions versus primary ion

Q9. Which reference electrode contains silver and silver chloride?

• Standard hydrogen electrode
• Saturated calomel electrode (SCE)
• Silver/silver chloride (Ag/AgCl) electrode
• Lead/lead oxide electrode

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

Q10. Why is ionic strength adjustment important in potentiometric measurements?

• It increases sample viscosity
• It stabilizes activity coefficients and improves reproducibility
• It changes the color of the solution for visual endpoints
• It prevents electrode dissolution by organic solvents

Correct Answer: It stabilizes activity coefficients and improves reproducibility

Q11. Which factor most directly causes deviation from ideal Nernstian behavior?

• Perfectly maintained reference electrode
• Interferences, membrane defects, or non-Nernstian ion transport
• Using pure water as solvent
• Constant temperature at 25°C

Correct Answer: Interferences, membrane defects, or non-Nernstian ion transport

Q12. The detection limit of an ion-selective electrode is defined as:

• The highest concentration measurable linearly
• The lowest concentration giving a measurable potential above noise
• The time required to reach 95% of the final potential
• The slope of the calibration curve at high concentrations

Correct Answer: The lowest concentration giving a measurable potential above noise

Q13. Which membrane type is commonly used in ion-selective electrodes for neutral or lipophilic ionophores?

• Glass membrane
• Polymeric PVC membrane with ionophore
• Platinum metal membrane
• Quartz crystal membrane

Correct Answer: Polymeric PVC membrane with ionophore

Q14. Drift in potentiometric response is usually caused by:

• Stable reference potential
• Membrane changes, fouling, or reference junction issues
• Optimal ionic strength
• Precise temperature control

Correct Answer: Membrane changes, fouling, or reference junction issues

Q15. In pharmaceutical dissolution studies, potentiometry can be used to:

• Measure particle size distribution
• Monitor release of ionic drugs (e.g., acidity/basicity) in real time
• Detect endotoxins in solution
• Measure optical rotation of chiral drugs

Correct Answer: Monitor release of ionic drugs (e.g., acidity/basicity) in real time

Q16. Temperature affects potentiometric measurements by altering:

• Only the electrode materials, not the potential
• The Nernstian slope and ion activity coefficients
• Visual appearance of the electrode
• The pH of the reference solution without changing potential

Correct Answer: The Nernstian slope and ion activity coefficients

Q17. Which practice improves accuracy of pH measurements with a glass electrode?

• Not calibrating the electrode
• Calibrating with at least two standard buffers near sample pH
• Using organic solvents without checking compatibility
• Storing the electrode dry at all times

Correct Answer: Calibrating with at least two standard buffers near sample pH

Q18. A non-zero liquid junction potential arises from:

• Equal mobility of all ions across the junction
• Different mobilities of ions in the bridge and sample
• Absence of ions in the sample
• Perfectly matched ionic composition between solutions

Correct Answer: Different mobilities of ions in the bridge and sample

Q19. Which measurement is considered potentiometric rather than voltammetric?

• Applying a varying potential and measuring current
• Measuring open-circuit potential with negligible current flow
• Measuring current at a fixed potential
• Measuring charge passed during electrolysis

Correct Answer: Measuring open-circuit potential with negligible current flow

Q20. For a divalent ion, the ideal Nernstian slope at 25°C per decade is approximately:

• 59.16 mV
• 29.58 mV
• 118.3 mV
• 14.79 mV

Correct Answer: 29.58 mV

Q21. Which of the following is a common maintenance step for reference electrodes?

• Filling reference junction with appropriate electrolyte and periodic cleaning
• Heating the electrode to dry it
• Storing the electrode in concentrated acid permanently
• Removing the junction to prevent contamination

Correct Answer: Filling reference junction with appropriate electrolyte and periodic cleaning

Q22. What is the main advantage of ion-selective electrodes in pharmaceutical analysis?

• They provide qualitative colorimetric data
• They allow rapid, selective, and direct measurement of specific ions
• They require large sample volumes and long preparation
• They measure molecular weight directly

Correct Answer: They allow rapid, selective, and direct measurement of specific ions

Q23. Which electrode would you choose to measure fluoride ion concentration in a formulation?

• Glass pH electrode
• Fluoride ion-selective electrode with lanthanum fluoride membrane
• Platinum redox electrode
• Silver electrode without membrane

Correct Answer: Fluoride ion-selective electrode with lanthanum fluoride membrane

Q24. Calibration of an ion-selective electrode is best performed using:

• Single standard very far from sample concentration
• At least two standards bracketing the expected sample concentration
• Only deionized water as a standard
• Any random solutions without known concentrations

Correct Answer: At least two standards bracketing the expected sample concentration

Q25. In redox potentiometry, the measured potential depends on:

• Ratio of oxidized to reduced species according to Nernst equation
• Only the total concentration of both species combined
• The color of the solution
• The pressure of the system

Correct Answer: Ratio of oxidized to reduced species according to Nernst equation

Q26. Which interfering effect can be mitigated by using a suitable ion exchanger or masking agent?

• Temperature fluctuations
• Specific ionic interferences affecting selectivity
• Electrode mechanical breakage
• Power supply noise

Correct Answer: Specific ionic interferences affecting selectivity

Q27. The term “slope” in electrode calibration refers to:

• The time taken to reach equilibrium
• The change in potential per decade change in ion activity
• The electrode surface area
• The impedance of the measuring instrument

Correct Answer: The change in potential per decade change in ion activity

Q28. Which parameter is most important when choosing an electrode for trace-level ion analysis?

• Detection limit and selectivity
• Color of the electrode body
• Price only
• Only the membrane thickness

Correct Answer: Detection limit and selectivity

Q29. During potentiometric titration of a weak acid with a strong base, the equivalence point potential is influenced by:

• Only the ionic strength, not pKa
• The pKa of the weak acid and ionic strength of the medium
• The molecular weight of the acid exclusively
• The color of the titrant

Correct Answer: The pKa of the weak acid and ionic strength of the medium

Q30. A common cause of slow electrode response time in ISEs is:

• Optimal stirring and fresh membrane
• Membrane fouling, slow diffusion, or poor conditioning
• Using fresh calibration standards
• Low ionic strength that enhances response speed

Correct Answer: Membrane fouling, slow diffusion, or poor conditioning

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com