Reference electrodes are essential components in electrochemical measurements, providing a stable, well-defined potential against which working electrodes are measured. For B.Pharm students, understanding the Standard Hydrogen Electrode (SHE), Silver/Silver Chloride (Ag/AgCl) electrode, and Saturated Calomel Electrode (SCE) is vital for potentiometry, pH measurement, and sensor calibration. This introduction covers construction, electrode reactions, advantages, limitations, salt bridge and junction potentials, maintenance, and temperature dependence, linking theory to pharmaceutical applications such as drug stability testing and biosensor development. Key concepts include electrode potential, standard conditions, and the Nernst equation. Read carefully and apply these concepts. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which electrode is defined as having an arbitrary potential of 0.00 V under standard conditions?
- Silver/Silver Chloride (Ag/AgCl) electrode
- Saturated Calomel Electrode (SCE)
- Standard Hydrogen Electrode (SHE)
- Glass pH electrode
Correct Answer: Standard Hydrogen Electrode (SHE)
Q2. The half-reaction for the Ag/AgCl reference electrode is best written as:
- AgCl(s) + e- ⇌ Ag(s) + Cl-
- Ag+ + e- ⇌ Ag(s)
- 2AgCl(s) ⇌ 2Ag(s) + Cl2(g)
- AgCl(s) ⇌ Ag+ + Cl-
Correct Answer: AgCl(s) + e- ⇌ Ag(s) + Cl-
Q3. A major advantage of the Ag/AgCl electrode over SHE in routine laboratory work is:
- It defines 0.00 V by convention
- It requires bubbling hydrogen gas continuously
- It is easier to use and maintain (no H2 gas needed)
- It is completely insensitive to chloride concentration
Correct Answer: It is easier to use and maintain (no H2 gas needed)
Q4. The saturated calomel electrode (SCE) contains which primary mercury compound?
- Mercury(II) oxide (HgO)
- Mercurous chloride (Hg2Cl2, calomel)
- Mercury(II) chloride (HgCl2)
- Metallic mercury only
Correct Answer: Mercurous chloride (Hg2Cl2, calomel)
Q5. Junction potential arises at the interface between the reference electrode and sample solution mainly because of:
- Temperature differences only
- Unequal ionic mobilities across the junction
- Different electrode metals in contact
- Gas evolution at the junction
Correct Answer: Unequal ionic mobilities across the junction
Q6. Which salt is commonly used in salt bridges for reference electrodes to minimize liquid junction potential?
- NaCl
- KCl
- CaCl2
- MgSO4
Correct Answer: KCl
Q7. How does the potential of an Ag/AgCl electrode change when the chloride concentration is increased (at constant temperature)?
- Potential becomes more positive
- Potential becomes more negative
- Potential remains unchanged
- Potential oscillates unpredictably
Correct Answer: Potential becomes more positive
Q8. Which reference electrode is most suitable when corrosive chloride-rich samples would attack a calomel electrode?
- Saturated Calomel Electrode (SCE)
- Standard Hydrogen Electrode (SHE)
- Silver/Silver Chloride (Ag/AgCl) electrode
- Platinum metal electrode
Correct Answer: Silver/Silver Chloride (Ag/AgCl) electrode
Q9. The SHE half-reaction at 25°C is 2H+ + 2e- ⇌ H2(g). Using the Nernst equation, the electrode potential will shift by approximately how many millivolts per pH unit?
- 59 mV per pH unit
- 1.0 mV per pH unit
- 100 mV per pH unit
- 0 mV per pH unit
Correct Answer: 59 mV per pH unit
Q10. A disadvantage of SCE compared to Ag/AgCl is:
- SCE is more environmentally friendly
- SCE contains mercury, posing disposal and toxicity concerns
- SCE requires continuous hydrogen gas
- SCE cannot be used with high chloride solutions
Correct Answer: SCE contains mercury, posing disposal and toxicity concerns
Q11. For an Ag/AgCl electrode in saturated KCl at 25°C, the commonly quoted potential versus SHE is approximately:
- +0.197 V
- -0.059 V
- 0.000 V
- +0.500 V
Correct Answer: +0.197 V
Q12. When calibrating a pH meter, which reference electrode property is most critical?
- Optical transparency
- Stable and known reference potential
- Ability to generate current
- Magnetic susceptibility
Correct Answer: Stable and known reference potential
Q13. The Nernst equation for the Ag/AgCl electrode shows a dependence on which ion concentration?
- Sodium ion concentration
- Chloride ion concentration
- Hydroxide ion concentration only
- Potassium ion concentration
Correct Answer: Chloride ion concentration
Q14. A leaking reference electrode that contaminates the sample most likely has a damaged:
- Working electrode
- Salt bridge or junction
- Reference metal surface
- Gas inlet port
Correct Answer: Salt bridge or junction
Q15. Which of the following will reduce liquid junction potential during measurement?
- Using a concentrated KCl salt bridge with ions of similar mobility
- Leaving the electrode dry
- Using a porous frit without electrolyte
- Increasing sample stirring speed only
Correct Answer: Using a concentrated KCl salt bridge with ions of similar mobility
Q16. In converting a measured potential from Ag/AgCl (sat KCl) to SHE, you should:
- Subtract approximately 0.197 V from the measured value
- Add approximately 0.197 V to the measured value
- Ignore the conversion because potentials are identical
- Multiply the measured value by 0.197
Correct Answer: Add approximately 0.197 V to the measured value
Q17. The chemical formula of calomel used in SCE is:
- HgCl2
- HgO
- Hg2Cl2
- HgCl
Correct Answer: Hg2Cl2
Q18. Which factor does NOT significantly affect reference electrode potential stability?
- Temperature fluctuations
- Chloride concentration changes (for Ag/AgCl)
- Mechanical vibrations of the lab bench
- Contamination of the junction
Correct Answer: Mechanical vibrations of the lab bench
Q19. For potentiometric titrations in pharmaceutical analysis, a good reference electrode should be:
- Unstable and drifting
- Stable, reproducible, and low impedance
- Reactive with analyte
- Highly temperature-sensitive without compensation
Correct Answer: Stable, reproducible, and low impedance
Q20. If an Ag/AgCl electrode is used in a solution containing very low chloride, the electrode potential will:
- Become independent of chloride concentration
- Shift according to the Nernst equation toward negative values
- Shift toward more positive values regardless of Nernst
- Instantly short-circuit
Correct Answer: Shift according to the Nernst equation toward negative values
Q21. Which electrode is preferred when extremely low junction potential is required and hydrogen handling is feasible?
- Silver/Silver Chloride (Ag/AgCl)
- Saturated Calomel Electrode (SCE)
- Standard Hydrogen Electrode (SHE)
- Glass electrode
Correct Answer: Standard Hydrogen Electrode (SHE)
Q22. Maintenance of a liquid-junction reference electrode typically includes:
- Polishing with abrasive until shiny
- Refilling the electrolyte and replacing the junction frit if clogged
- Heating it to 100°C daily
- Soaking in strong acid overnight
Correct Answer: Refilling the electrolyte and replacing the junction frit if clogged
Q23. What is the main electrochemical reason for using a high concentration of KCl in a salt bridge?
- KCl has the highest solubility of all salts
- K+ and Cl- have similar ionic mobilities, minimizing junction potentials
- KCl reacts with analyte to stabilize it
- KCl increases solution pH drastically
Correct Answer: K+ and Cl- have similar ionic mobilities, minimizing junction potentials
Q24. In a pharmaceutical potentiometric sensor using Ag/AgCl reference, which error source is most likely if measured potentials drift slowly over hours?
- Rapid gas evolution at working electrode
- Evaporation or depletion of reference electrolyte and junction fouling
- Instant electromagnetic interference spikes
- Incorrect glassware dimensions
Correct Answer: Evaporation or depletion of reference electrolyte and junction fouling
Q25. Which statement about SCE and Ag/AgCl is TRUE?
- SCE potential is independent of chloride concentration
- Both rely on equilibrium between metal(s) and chloride ions
- Ag/AgCl contains mercury while SCE does not
- Neither electrode has a liquid junction
Correct Answer: Both rely on equilibrium between metal(s) and chloride ions
Q26. Using the Nernst equation, a one-electron simple electrode at 25°C changes by how many mV per tenfold change in activity of the reacting species?
- 59.16 mV
- 2.303 mV
- 0.05916 mV
- 100 mV
Correct Answer: 59.16 mV
Q27. Which precaution is most important when disposing of a calomel electrode?
- Dispose it as regular glass waste
- Follow hazardous waste protocols due to mercury content
- Burn it in a fume hood
- Neutralize with sodium hydroxide and pour down the drain
Correct Answer: Follow hazardous waste protocols due to mercury content
Q28. A reference electrode with a ceramic frit junction is likely to:
- Have zero resistance
- Provide a reproducible but sometimes slower ion exchange and possible clogging
- Generate hydrogen gas continuously
- Be immune to contamination
Correct Answer: Provide a reproducible but sometimes slower ion exchange and possible clogging
Q29. Which calculation best describes the potential of Ag/AgCl at 25°C given [Cl-] = 0.01 M, where E° (AgCl/Ag, Cl-) ≈ +0.222 V for 1 M Cl-? (Use Nernst concept qualitatively.)
- Potential will be slightly more positive than +0.222 V
- Potential will be roughly unchanged at +0.222 V
- Potential will be more negative than +0.222 V (log dependence on lower [Cl-])
- Potential will become exactly 0 V
Correct Answer: Potential will be more negative than +0.222 V (log dependence on lower [Cl-])
Q30. For accurate electrochemical measurements in pharmaceutical labs, which combination is most appropriate?
- Use an unstable reference and ignore maintenance
- Use a well-maintained Ag/AgCl or SCE with a proper salt bridge and temperature control
- Use only SHE for all routine analyses
- Use any electrode without calibration
Correct Answer: Use a well-maintained Ag/AgCl or SCE with a proper salt bridge and temperature control
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