Potentiometry is a versatile electroanalytical technique widely used in pharmaceutical analysis for pH measurement, ion quantification and potentiometric titrations. B. Pharm students must master principles such as the Nernst equation, ion-selective electrodes (ISEs), reference electrodes, calibration, ionic strength adjustment and troubleshooting. Practical applications include assay of ionic drugs, dissolution testing, stability studies, impurity monitoring and quality control of formulations. Understanding electrode response, selectivity, detection limits, temperature effects and sample preparation strengthens laboratory proficiency and regulatory compliance. This concise, keyword-rich overview prepares you for exam questions and practical lab work. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the fundamental principle of potentiometry?
- Measurement of current flow through a cell under load
- Measurement of potential difference between two electrodes without drawing appreciable current
- Generation of a constant current to force a redox reaction
- Measurement of light absorption to infer ion concentration
Correct Answer: Measurement of potential difference between two electrodes without drawing appreciable current
Q2. Which equation quantitatively relates electrode potential to ion activity?
- Henderson-Hasselbalch equation
- Beer-Lambert law
- Nernst equation
- Arrhenius equation
Correct Answer: Nernst equation
Q3. At 25°C, the theoretical Nernstian slope for a monovalent ion (per decade change in activity) is approximately:
- 29.58 mV
- 59.16 mV
- 118.3 mV
- 9.87 mV
Correct Answer: 59.16 mV
Q4. Ion-selective electrodes (ISEs) respond primarily to:
- Total concentration of all species in solution
- Activity of the primary ion
- Optical properties of the solution
- The temperature only
Correct Answer: Activity of the primary ion
Q5. Which membrane material is typical for a pH (glass) electrode?
- Polyethylene membrane
- Silicate glass membrane
- Gold metal foil
- Cellulose acetate
Correct Answer: Silicate glass membrane
Q6. Best practice for calibrating a pH electrode before pharmaceutical assays is to:
- Use a single buffer at pH 7 only
- Perform two-point calibration with buffers that bracket the expected sample pH
- Rinse with distilled water and assume zero error
- Calibrate at the sample temperature using tap water
Correct Answer: Perform two-point calibration with buffers that bracket the expected sample pH
Q7. Which of the following is a common pharmaceutical application of potentiometry?
- Assay of ionic active pharmaceutical ingredients by potentiometric titration
- Measuring tablet hardness using force sensors
- Determining particle size by laser diffraction
- Measuring UV absorbance for non-ionic compounds only
Correct Answer: Assay of ionic active pharmaceutical ingredients by potentiometric titration
Q8. In a potentiometric titration, the endpoint is most reliably detected by:
- Maximum current flow
- Sharp inflection in potential vs titrant volume plot
- Color change of the solution only
- When the solution becomes cloudy
Correct Answer: Sharp inflection in potential vs titrant volume plot
Q9. Selectivity of an ion-selective electrode is primarily determined by:
- The volume of the sample
- The composition of the membrane or ionophore
- The color of the electrode body
- How long the reference electrode has been used
Correct Answer: The composition of the membrane or ionophore
Q10. A common cause of non-Nernstian (deviated) slope in practice is:
- Perfect electrode conditioning
- Membrane fouling or poor electrode conditioning
- Using high-purity standard buffers
- Measuring at exactly 25°C always
Correct Answer: Membrane fouling or poor electrode conditioning
Q11. What is the purpose of adding an ionic strength adjuster (ISA) in ISE measurements?
- To change the color of the solution
- To maintain constant ionic strength and stabilize activity coefficients
- To precipitate the analyte
- To increase solution viscosity
Correct Answer: To maintain constant ionic strength and stabilize activity coefficients
Q12. Which strategy improves selectivity of an ISE for a target ion in complex pharmaceutical matrices?
- Using a non-specific membrane
- Choosing an ionophore with high affinity for the target ion
- Storing the electrode dry for long periods
- Removing calibration steps
Correct Answer: Choosing an ionophore with high affinity for the target ion
Q13. Which electrode type is most suitable for direct potentiometric determination of chloride in formulations?
- Glass pH electrode
- Silver/silver chloride ion-selective electrode
- Platinum redox electrode
- Carbon paste electrode for non-ionic species
Correct Answer: Silver/silver chloride ion-selective electrode
Q14. A true statement about the reference electrode used in potentiometry is:
- Its potential changes with sample concentration and is unreliable
- It provides a stable, well-defined potential against which the indicator electrode is measured
- It must be identical to the indicator electrode
- It generates the analyte ion in solution
Correct Answer: It provides a stable, well-defined potential against which the indicator electrode is measured
Q15. Why must essentially no current flow during a potentiometric measurement?
- Current increases selectivity undesirably
- Current perturbs the electrode equilibria and changes the measured potential
- Current improves electrode lifespan
- Current stabilizes the Nernst slope
Correct Answer: Current perturbs the electrode equilibria and changes the measured potential
Q16. How does temperature affect the Nernstian slope for an ion-selective electrode?
- Slope decreases as temperature increases
- Slope is independent of temperature
- Slope increases with temperature (proportional to T)
- Slope becomes zero at higher temperatures
Correct Answer: Slope increases with temperature (proportional to T)
Q17. Proper storage of a glass pH electrode between uses typically requires:
- Storing dry on the bench
- Soaking in a suitable storage solution such as KCl or pH buffer
- Heating the electrode to 60°C
- Immersing in organic solvent
Correct Answer: Soaking in a suitable storage solution such as KCl or pH buffer
Q18. Which electrode is commonly used as an indicator in redox potentiometric titrations?
- Glass pH electrode
- Platinum (inert) electrode
- Silver/silver chloride electrode
- Ion-selective electrode for anions
Correct Answer: Platinum (inert) electrode
Q19. In pharmaceutical analysis of small-volume samples, what is an advantage of using microelectrodes?
- They require larger sample volumes
- They provide faster response and allow measurements in small sample volumes
- They eliminate the need for calibration
- They are insensitive to ionic composition
Correct Answer: They provide faster response and allow measurements in small sample volumes
Q20. A lipophilic interfering ion in a sample typically affects an ISE by:
- Improving the Nernstian slope
- Causing false potentiometric response due to membrane partitioning
- Changing the color of the membrane only
- Having no effect on ISE performance
Correct Answer: Causing false potentiometric response due to membrane partitioning
Q21. For dissolution testing of an ionic drug, potentiometry can most directly monitor:
- Solid-state polymorphism
- Concentration of the ionic drug released as a function of time
- Tablet disintegration force
- Particle shape distribution
Correct Answer: Concentration of the ionic drug released as a function of time
Q22. During stability studies, potentiometry is useful for detecting:
- Changes in the drug’s optical rotation
- Acid–base degradation and changes in ionic assay value
- Molecular weight by mass spectrometry
- Colorimetric impurities only
Correct Answer: Acid–base degradation and changes in ionic assay value
Q23. Liquid junction potential arises at the interface between two different electrolytes and can be minimized by:
- Using a high-resistance glass electrode
- Employing a salt bridge filled with concentrated KCl
- Heating the sample to 80°C
- Using deionized water as bridge solution
Correct Answer: Employing a salt bridge filled with concentrated KCl
Q24. Electrode response time is mainly influenced by:
- Diffusion rates and membrane thickness
- Ambient light intensity
- Type of plastic housing for the electrode
- Brand of buffer solutions only
Correct Answer: Diffusion rates and membrane thickness
Q25. To overcome matrix effects in complex pharmaceutical samples when using an ISE, which method is commonly applied?
- Standard addition method
- Ignoring calibration
- Using tap water instead of buffers
- Diluting with organic solvent only
Correct Answer: Standard addition method
Q26. A satisfactory Nernstian slope for a glass pH electrode at 25°C is approximately:
- 10 mV per pH unit
- Approximately 59 mV per pH unit
- 200 mV per pH unit
- 0.5 mV per pH unit
Correct Answer: Approximately 59 mV per pH unit
Q27. Which ionophore is commonly used in potassium-selective electrodes?
- Valinomycin
- Crown ether for sodium only
- Hemoglobin
- Activated carbon
Correct Answer: Valinomycin
Q28. For potentiometric titration of a weak acid in a tablet, the indicator electrode is usually a:
- Redox electrode
- pH (glass) electrode
- Conductivity probe only
- Thermistor
Correct Answer: pH (glass) electrode
Q29. If a drug forms a complex with a counter ion in solution, how does this generally affect ISE response for the free ion?
- Response increases because complexed ion gives stronger signal
- Response decreases because free ion concentration is reduced by complexation
- Complexation has no effect on ISE
- Response becomes independent of ion activity
Correct Answer: Response decreases because free ion concentration is reduced by complexation
Q30. Which of the following is NOT an advantage of potentiometric methods in pharmaceutical QC?
- High selectivity for specific ions using ISEs
- Low sample consumption and rapid measurements
- Ability to analyze non-ionic compounds without modification
- Feasibility for automation and online monitoring
Correct Answer: Ability to analyze non-ionic compounds without modification
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.
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