Accurate pH determination using the electrometric method is essential in pharmaceutical analysis and quality control. This introduction covers pH meter principles, glass and reference electrodes, calibration with standard buffer solutions, temperature compensation, and key error sources like junction potential, alkaline and acid errors. B.Pharm students should understand the Nernst equation, electrode maintenance, sample preparation, and interpreting millivolt readings for reliable potentiometric measurements. Mastery of these concepts improves assay accuracy for formulations, stability studies, and dissolution testing. Practical knowledge of electrode response, slope, and troubleshooting is crucial for routine laboratory work. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary function of the glass electrode in the electrometric pH determination?
- To act as a reference potential independent of pH
- To selectively respond to hydrogen ion activity and generate a potential
- To measure ionic strength of the solution
- To provide temperature compensation for the pH meter
Correct Answer: To selectively respond to hydrogen ion activity and generate a potential
Q2. Which equation describes the relationship between measured potential and pH for a hydrogen-sensitive electrode?
- Henderson-Hasselbalch equation
- Nernst equation
- Van’t Hoff equation
- Arrhenius equation
Correct Answer: Nernst equation
Q3. During calibration, a two-point calibration of a pH meter typically uses buffers of which pH values?
- 1.00 and 14.00
- 4.01 and 7.00 or 7.00 and 10.01
- 3.00 and 9.00
- 6.00 and 8.00
Correct Answer: 4.01 and 7.00 or 7.00 and 10.01
Q4. What is the typical theoretical slope (mV/pH) at 25°C for an ideal glass electrode according to the Nernst equation?
- 50.0 mV/pH
- 59.16 mV/pH
- 25.0 mV/pH
- 100.0 mV/pH
Correct Answer: 59.16 mV/pH
Q5. Which electrode provides a stable reference potential in common electrometric pH measurements?
- Glass electrode
- Saturated calomel electrode (SCE) or silver/silver chloride (Ag/AgCl) electrode
- Platinum electrode
- Gold electrode
Correct Answer: Saturated calomel electrode (SCE) or silver/silver chloride (Ag/AgCl) electrode
Q6. What causes alkaline error (also called sodium error) in glass electrode measurements?
- Interference from chloride ions at low pH
- Slow response time at low temperature
- Glass electrode responding to high concentrations of sodium ions at high pH
- Reference electrode drift due to salt bridge contamination
Correct Answer: Glass electrode responding to high concentrations of sodium ions at high pH
Q7. Which maintenance step is recommended to prolong the life of a glass pH electrode?
- Store the electrode in distilled water only
- Store the electrode in an appropriate storage solution (e.g., pH 4 buffer or KCl solution)
- Always dry the electrode tip between measurements
- Soak the electrode in strong acid overnight
Correct Answer: Store the electrode in an appropriate storage solution (e.g., pH 4 buffer or KCl solution)
Q8. In potentiometric pH measurement, what is the measurable electrical quantity directly obtained from the instrument?
- Current (amperes)
- Voltage or potential difference (millivolts)
- Resistance (ohms)
- Capacitance (farads)
Correct Answer: Voltage or potential difference (millivolts)
Q9. Why is temperature compensation important in pH measurements?
- Temperature changes the color of indicator dyes
- Temperature affects electrode slope and buffer pH values, altering the measured potential
- Temperature only affects the reference electrode but not the glass electrode
- Temperature prevents electrode wetting
Correct Answer: Temperature affects electrode slope and buffer pH values, altering the measured potential
Q10. What is the role of a salt bridge or junction in a reference electrode?
- To increase solution conductivity
- To provide ionic contact and minimize liquid junction potential between reference and sample
- To generate hydrogen ions
- To mechanically support the electrode
Correct Answer: To provide ionic contact and minimize liquid junction potential between reference and sample
Q11. Which buffer is commonly used as a primary standard for pH calibration around pH 7?
- pH 1.00 hydrochloric acid
- pH 7.00 phosphate buffer
- pH 10.01 carbonate buffer
- pH 4.01 acetic acid buffer
Correct Answer: pH 7.00 phosphate buffer
Q12. What does “combined electrode” refer to in pH measurement?
- An electrode combining redox and pH functions
- An electrode containing both glass sensing and built-in reference electrode in one body
- An electrode used for both pH and conductivity
- An electrode that combines temperature sensing and current output
Correct Answer: An electrode containing both glass sensing and built-in reference electrode in one body
Q13. How is pH related to hydrogen ion activity (aH+)?
- pH = log10(aH+)
- pH = -log10(aH+)
- pH = ln(aH+)
- pH = -ln(aH+)
Correct Answer: pH = -log10(aH+)
Q14. Which of the following can cause slow electrode response or hysteresis?
- Clean electrode surface and fresh fill solution
- Clogged junction, protein fouling, or inadequate hydration of the glass membrane
- Calibration with fresh buffers
- Using a combined electrode
Correct Answer: Clogged junction, protein fouling, or inadequate hydration of the glass membrane
Q15. What effect does low ionic strength in a sample have on electrometric pH measurements?
- No effect; pH is independent of ionic strength
- Can cause unstable readings and increased liquid junction potential, reducing accuracy
- Always increases measured pH by one unit
- Improves electrode response and accuracy
Correct Answer: Can cause unstable readings and increased liquid junction potential, reducing accuracy
Q16. How can ionic strength effects be minimized when measuring pH of dilute pharmaceutical samples?
- By heating the sample to 80°C
- By adding an appropriate ionic strength adjuster (ISA), such as KCl
- By diluting the sample further
- By using a non-aqueous solvent
Correct Answer: By adding an appropriate ionic strength adjuster (ISA), such as KCl
Q17. What is the main disadvantage of measuring pH in non-aqueous solvents using a standard glass electrode?
- There is no difference; standard electrodes work equally well
- Glass electrodes can respond differently due to solvent effects and require specialized electrodes or calibration
- Non-aqueous solvents always yield lower pH values
- Reference electrodes dissolve in organic solvents
Correct Answer: Glass electrodes can respond differently due to solvent effects and require specialized electrodes or calibration
Q18. In potentiometric titration using a pH electrode, what indicates the titration end point?
- A constant temperature
- A sudden large change in measured pH or potential (inflection point)
- The electrode resistance becomes zero
- The solution becomes colorless
Correct Answer: A sudden large change in measured pH or potential (inflection point)
Q19. What is liquid junction potential and why is it important?
- The potential created by electrode heating; it is irrelevant to pH
- A potential difference at the interface between different electrolyte solutions that can introduce systematic error in pH measurements
- A beneficial voltage that stabilizes readings
- Potential caused by stirring the sample
Correct Answer: A potential difference at the interface between different electrolyte solutions that can introduce systematic error in pH measurements
Q20. Which corrective action is appropriate when a pH electrode shows a slope much lower than theoretical?
- Replace batteries in the pH meter
- Clean the electrode, rehydrate the glass membrane, and recalibrate; if persists, replace the electrode
- Increase sample temperature to 60°C
- Use distilled water as a buffer
Correct Answer: Clean the electrode, rehydrate the glass membrane, and recalibrate; if persists, replace the electrode
Q21. What is the typical storage solution for Ag/AgCl reference electrodes?
- Deionized water only
- 3 M KCl or a specific KCl-based storage solution
- Concentrated sulfuric acid
- Absolute ethanol
Correct Answer: 3 M KCl or a specific KCl-based storage solution
Q22. Which parameter can be directly derived from measured millivolts if electrode slope and intercept are known?
- Conductivity of the solution
- pH value of the solution
- Sample viscosity
- Temperature of the solution
Correct Answer: pH value of the solution
Q23. When performing pH measurements on viscous pharmaceutical formulations, what is a recommended practice?
- Use the same electrode as for aqueous solutions without modification
- Use an appropriate electrode (e.g., spear tip or flat surface) and ensure mechanical stirring or homogenization for uniform contact
- Heat the sample to reduce viscosity
- Filter the sample through a 0.1 μm membrane before measurement
Correct Answer: Use an appropriate electrode (e.g., spear tip or flat surface) and ensure mechanical stirring or homogenization for uniform contact
Q24. What is the primary advantage of using a double-junction reference electrode in pharmaceutical samples?
- Increases the electrical resistance dramatically
- Reduces contamination of the sample by reference electrolytes and minimizes junction potential with complex matrices
- Eliminates the need for calibration
- Makes the reference electrode pH-sensitive
Correct Answer: Reduces contamination of the sample by reference electrolytes and minimizes junction potential with complex matrices
Q25. How often should pH electrodes typically be recalibrated in a routine pharmaceutical QC lab?
- Once a year
- Before each analytical session or daily, depending on usage and required accuracy
- Only when readings look incorrect
- Never; factory calibration is sufficient
Correct Answer: Before each analytical session or daily, depending on usage and required accuracy
Q26. Which phenomenon explains why the electrode response deviates from ideal at very low pH (acid error)?
- Hydration layer of the glass becomes conductive
- Metal ions in the sample bind to the glass, altering response
- Excess hydrogen causes non-linearity and potential interferences at extreme pH values
- Reference electrode dissolves
Correct Answer: Excess hydrogen causes non-linearity and potential interferences at extreme pH values
Q27. For accurate pH measurement, which sample preparation step is often necessary for carbonated solutions?
- Measure immediately without any treatment
- Degas the sample (e.g., by sonication or gentle stirring) to remove dissolved CO2 before measurement
- Add strong acid to neutralize CO2
- Cool the sample to 0°C
Correct Answer: Degas the sample (e.g., by sonication or gentle stirring) to remove dissolved CO2 before measurement
Q28. What indicates that a pH electrode is properly hydrated and ready for accurate measurements?
- The electrode tip is dry and shiny
- The electrode exhibits a near-theoretical slope after calibration and stable readings
- The electrode produces a constant high resistance
- The reference junction is blocked
Correct Answer: The electrode exhibits a near-theoretical slope after calibration and stable readings
Q29. Which factor is NOT typically a source of error in electrometric pH determination?
- Electrode fouling
- Improper calibration with standard buffers
- Ambient light intensity in the lab
- Temperature differences between buffer and sample
Correct Answer: Ambient light intensity in the lab
Q30. What is the purpose of using a buffer solution of known pH during calibration?
- To flush impurities out of the electrode
- To establish the electrode slope and intercept so measured potentials can be converted to pH
- To sterilize the electrode
- To change the electrode reference potential permanently
Correct Answer: To establish the electrode slope and intercept so measured potentials can be converted to pH
Q31. What effect does protein or surfactant adsorption on the glass membrane have on pH measurement?
- It improves the electrode slope
- It causes fouling, slow response, and inaccurate readings
- It sterilizes the electrode
- It reduces sample ionic strength
Correct Answer: It causes fouling, slow response, and inaccurate readings
Q32. When converting a measured EMF (in mV) to pH, which instrument parameters are required?
- Only the temperature
- The electrode slope (mV/pH), offset/intercept, and measured EMF
- The sample color and viscosity
- The serial number of the pH meter
Correct Answer: The electrode slope (mV/pH), offset/intercept, and measured EMF
Q33. What is the effect of using contaminated buffer solutions for calibration?
- Calibration will remain accurate
- It introduces systematic errors leading to inaccurate pH measurements
- It sterilizes the electrode
- It extends electrode life
Correct Answer: It introduces systematic errors leading to inaccurate pH measurements
Q34. Which electrode design is preferred for measuring pH in semi-solid or paste formulations?
- Standard bulb glass electrode
- Spear-tip or flat-surface electrode designed for solids and semi-solids
- Metallic redox electrode
- Reference electrode alone
Correct Answer: Spear-tip or flat-surface electrode designed for solids and semi-solids
Q35. How does the isoelectric point (pI) of a protein influence pH measurement in protein formulations?
- At pI, protein is most soluble and does not affect pH
- Near pI, protein may precipitate or adsorb to electrode surfaces, affecting measurements
- pI has no relevance to pH measurement
- At pI, electrode slope doubles automatically
Correct Answer: Near pI, protein may precipitate or adsorb to electrode surfaces, affecting measurements
Q36. Which cleaning solution is commonly used to remove protein or organic fouling from the glass membrane?
- 1% SDS followed by thorough rinsing, or mild enzymatic cleaners designed for electrodes
- Concentrated nitric acid
- Absolute ethanol only
- Dry heat sterilization
Correct Answer: 1% SDS followed by thorough rinsing, or mild enzymatic cleaners designed for electrodes
Q37. What is meant by the term “electrode slope” in pH measurement?
- The physical slope of the electrode tip
- The change in potential (mV) per unit change in pH, ideally near 59.16 mV/pH at 25°C
- The angle at which electrode must be held
- The slope of the electrode calibration curve vs. temperature only
Correct Answer: The change in potential (mV) per unit change in pH, ideally near 59.16 mV/pH at 25°C
Q38. Which quality control practice ensures traceability of pH measurements in a pharmaceutical lab?
- Using unverified home-made buffers
- Using certified reference buffer solutions and recording calibration logs with dates and electrode details
- Calibrating only when needed
- Changing electrodes without documentation
Correct Answer: Using certified reference buffer solutions and recording calibration logs with dates and electrode details
Q39. What typically happens to the response time of a pH electrode as it ages?
- Response time improves continuously
- Response time usually increases (slower) due to glass aging and junction fouling
- Response time becomes zero
- Age has no effect on response time
Correct Answer: Response time usually increases (slower) due to glass aging and junction fouling
Q40. How should electrodes be rinsed between measurements to avoid cross-contamination?
- Rinse briefly with distilled or deionized water and blot gently with lint-free tissue; avoid aggressive drying
- Wipe with dry tissue forcefully
- Immerse in acetone
- Rinse with strong acid each time
Correct Answer: Rinse briefly with distilled or deionized water and blot gently with lint-free tissue; avoid aggressive drying
Q41. Why are multiple-point calibrations (three-point) sometimes preferred over two-point calibrations?
- They are faster
- They improve accuracy across a wider pH range and help detect non-linearity
- They reduce electrode lifespan
- They eliminate the need for temperature compensation
Correct Answer: They improve accuracy across a wider pH range and help detect non-linearity
Q42. Which of the following is an acceptable practice when measuring pH of highly acidic samples for accurate results?
- Use a standard glass electrode without considering acid error
- Consider using specialized electrodes or apply corrections, and calibrate near the sample pH
- Heat the sample to boiling
- Dilute with strong base before measurement
Correct Answer: Consider using specialized electrodes or apply corrections, and calibrate near the sample pH
Q43. What is the effect of high dissolved solids (high ionic strength) on the reference junction potential?
- High ionic strength eliminates junction potential
- It can alter the junction potential and introduce measurement errors if junction composition differs from sample
- It has no effect on electrometric measurements
- It makes the electrode slope ideal
Correct Answer: It can alter the junction potential and introduce measurement errors if junction composition differs from sample
Q44. Which parameter is directly influenced by the glass membrane composition of a pH electrode?
- Electrode color only
- Ion selectivity, response time, and susceptibility to alkali/acid errors
- Battery life of the pH meter
- Sample boiling point
Correct Answer: Ion selectivity, response time, and susceptibility to alkali/acid errors
Q45. For mV to pH conversion, which sign convention is used for pH electrodes in the Nernst relation?
- Potential increases with increasing pH for a typical glass electrode
- Potential decreases with increasing pH for a typical glass electrode (negative slope of approx. -59.16 mV/pH at 25°C)
- Potential is independent of pH
- Potential oscillates with pH
Correct Answer: Potential decreases with increasing pH for a typical glass electrode (negative slope of approx. -59.16 mV/pH at 25°C)
Q46. What precaution is recommended when measuring pH of samples containing organic solvents?
- Use the same calibration buffers without consideration
- Use electrodes designed for organic solvents and calibrate with appropriate solvent-compatible standards
- Always dilute sample with ethanol
- Measure at elevated pressure
Correct Answer: Use electrodes designed for organic solvents and calibrate with appropriate solvent-compatible standards
Q47. Which measurement is most suitable to verify the integrity of a reference electrode?
- Measure its color
- Measure its potential against a stable reference (e.g., measure EMF vs. a standard reference) and check for stability
- Shake it vigorously and watch bubbles
- Measure its weight
Correct Answer: Measure its potential against a stable reference (e.g., measure EMF vs. a standard reference) and check for stability
Q48. What is the main reason for performing pH measurements in pharmaceutical stability studies?
- To determine color changes only
- To monitor chemical stability, degradation pathways, and product performance, since pH affects solubility and degradation rates
- To replace microbiological testing
- To avoid the need for dissolution testing
Correct Answer: To monitor chemical stability, degradation pathways, and product performance, since pH affects solubility and degradation rates
Q49. In routine lab practice, what log should be maintained to support pH measurements and compliance?
- No log is necessary
- Calibration log with buffer lot numbers, electrode IDs, calibration date/time, slope and offset values, and operator initials
- Only the final pH value needs to be recorded
- A log of room décor changes
Correct Answer: Calibration log with buffer lot numbers, electrode IDs, calibration date/time, slope and offset values, and operator initials
Q50. Which troubleshooting step should be taken if pH readings drift slowly over time during measurement?
- Ignore the drift and report initial reading
- Check electrode condition, junction clogging, reference fill solution, recondition or replace electrode, and re-calibrate
- Immediately replace the pH meter
- Measure in complete darkness
Correct Answer: Check electrode condition, junction clogging, reference fill solution, recondition or replace electrode, and re-calibrate
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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