Introduction
Understanding measurement and control of pH is essential for M.Pharm students involved in bioprocess engineering and pharmaceutical manufacturing. Accurate pH measurement influences enzyme activity, microbial growth, drug stability and product quality. This blog provides focused multiple-choice questions with answers to sharpen your knowledge of pH sensors, electrode theory, calibration, temperature effects, sensor maintenance, and control strategies used in bioreactors and downstream processes. Questions are designed to test both theoretical understanding (Nernst equation, buffer capacity, junction potentials) and practical skills (probe storage, in-line vs. off-line measurement, PID tuning, sterilization). Use these MCQs to prepare for exams and real-world process troubleshooting.
Q1. What is the theoretical electrode slope (mV per pH unit) of a perfect glass pH electrode at 25°C according to the Nernst equation?
- 29.58 mV/pH
- 59.16 mV/pH
- 118.32 mV/pH
- 0 mV/pH
Correct Answer: 59.16 mV/pH
Q2. Which reference electrode is most commonly used in laboratory pH probes for bioprocess applications?
- Platinum reference electrode
- Silver/silver chloride (Ag/AgCl) reference electrode
- Zinc reference electrode
- Mercury/mercurous chloride (calomel) is the only option
Correct Answer: Silver/silver chloride (Ag/AgCl) reference electrode
Q3. Automatic temperature compensation (ATC) in modern pH meters primarily corrects for which effect?
- Temperature-dependent changes in buffer pKa only
- Temperature-dependent change in electrode slope (Nernst response)
- Electrical noise caused by temperature fluctuations
- Evaporation of the sample at higher temperatures
Correct Answer: Temperature-dependent change in electrode slope (Nernst response)
Q4. During calibration a glass electrode shows a slope of 48 mV/pH at 25°C. This most likely indicates:
- Electrode is exhibiting ideal Nernst behavior
- Electrode slope is lower than ideal, indicating ageing or contamination
- Electrode is functioning better than theoretical expectation
- Temperature compensation is set too high
Correct Answer: Electrode slope is lower than ideal, indicating ageing or contamination
Q5. For accurate pH measurement in low ionic strength solutions, which issue is most likely to cause error?
- Excessively fast electrode response time
- Liquid junction potential and measurement instability
- Overcompensation by the pH meter
- Too high buffer capacity
Correct Answer: Liquid junction potential and measurement instability
Q6. Which storage practice is recommended for conventional glass pH electrodes between uses?
- Store dry on the bench
- Store immersed in 3M KCl or manufacturer’s storage solution
- Store in distilled or deionized water
- Store in concentrated acid to keep the membrane hydrated
Correct Answer: Store immersed in 3M KCl or manufacturer’s storage solution
Q7. Which sensor type is best described as solid-state, fast-response, and less fragile than glass for in-line pH monitoring in fermenters?
- Glass membrane electrode
- ISFET (ion-sensitive field-effect transistor) pH sensor
- Calomel reference probe
- Platinum redox electrode
Correct Answer: ISFET (ion-sensitive field-effect transistor) pH sensor
Q8. Buffer capacity is highest when:
- pH is equal to the buffer’s pKa
- pH is two units away from pKa
- Buffer concentration is minimal
- Temperature is 0°C
Correct Answer: pH is equal to the buffer’s pKa
Q9. When performing a two-point pH calibration for general lab use, which pair of buffer pH values is most commonly selected?
- pH 2.0 and pH 4.0
- pH 4.0 and pH 7.0
- pH 7.0 and pH 9.0
- pH 1.0 and pH 13.0
Correct Answer: pH 4.0 and pH 7.0
Q10. Which of the following is NOT a typical source of pH electrode drift in bioprocess measurements?
- Fouling or protein adsorption on the glass membrane
- Reference junction clogging
- Proper storage in KCl solution
- Chemical poisoning by sulfide or heavy metals
Correct Answer: Proper storage in KCl solution
Q11. In a closed-loop pH control of a bioreactor, the most common manipulated variable is:
- Bioreactor agitation speed
- Addition rate of acid or base
- Inlet air flow rate
- Temperature setpoint
Correct Answer: Addition rate of acid or base
Q12. A PID controller tuned for pH control tends to require which of the following characteristics compared to temperature control?
- Faster integral action because pH disturbances are slow
- Slower integral action and careful anti-windup due to aggressive valve action and nonlinearity
- No derivative action ever
- No need for tuning because pH loops are linear
Correct Answer: Slower integral action and careful anti-windup due to aggressive valve action and nonlinearity
Q13. Which statement about in-line optical (spectrophotometric) pH sensors is TRUE?
- They rely on a glass membrane and need KCl storage
- They measure pH based on absorbance changes of pH-sensitive dyes
- They are more affected by ionic strength than glass electrodes
- They are the standard reference electrodes in most labs
Correct Answer: They measure pH based on absorbance changes of pH-sensitive dyes
Q14. Liquid junction potentials arise primarily from:
- Differences in mobility of ions between sample and reference electrolyte
- Mechanical vibration of the electrode
- Electrode temperature sensor failure
- High buffer capacity of the sample
Correct Answer: Differences in mobility of ions between sample and reference electrolyte
Q15. Which practice is most appropriate to reduce measurement error when measuring pH in viscous or high-protein fermentation broths?
- Use the same electrode without cleaning to maintain continuity
- Use a flow-through sample conditioner or in-line sensor with cleaning/steam-sterilisable capability
- Always dilute the sample with distilled water before measurement
- Measure pH at room temperature without compensation
Correct Answer: Use a flow-through sample conditioner or in-line sensor with cleaning/steam-sterilisable capability
Q16. Which of the following describes the Henderson–Hasselbalch equation application to pH control?
- It predicts electrode impedance at different pH
- It relates pH to the ratio of conjugate base to acid and helps design buffer systems
- It determines the slope of the Nernst equation
- It is used to calibrate Ag/AgCl reference electrodes
Correct Answer: It relates pH to the ratio of conjugate base to acid and helps design buffer systems
Q17. Which of the following is true regarding sterilization of conventional glass pH probes?
- All glass electrodes are fully autoclave-proof and can be repeatedly sterilized at 121°C without damage
- Many conventional glass electrodes are not autoclaveable; sterilizable probes or protective assemblies are used for in-situ sterilization
- Sterilization has no impact on reference junction composition
- Submerging in ethanol is equivalent to autoclaving for sterilization
Correct Answer: Many conventional glass electrodes are not autoclaveable; sterilizable probes or protective assemblies are used for in-situ sterilization
Q18. What is the primary reason to include temperature measurement alongside pH during bioprocess monitoring?
- To enable automatic temperature compensation and account for temperature effects on electrode response and buffer pKa
- Temperature reading is required to calibrate the oxygen probe
- Temperature has no influence on pH measurements
- To allow the controller to switch to pH-neutral mode
Correct Answer: To enable automatic temperature compensation and account for temperature effects on electrode response and buffer pKa
Q19. If a pH probe has very high input impedance requirements, what minimum input impedance is typically recommended for the pH meter to avoid loading errors?
- 10^3 ohm
- 10^6 ohm
- 10^9 ohm
- 10^12 ohm or higher
Correct Answer: 10^12 ohm or higher
Q20. In advanced pH control for fed-batch fermentation, feed-forward control can be used to:
- Replace the need for a feedback pH controller entirely
- Anticipate pH disturbances caused by planned substrate feeds and reduce corrective action needed by the feedback loop
- Eliminate electrode calibration requirements
- Create artificial junction potentials to stabilize readings
Correct Answer: Anticipate pH disturbances caused by planned substrate feeds and reduce corrective action needed by the feedback loop
