Introduction: A conductivity cell is a sensor used to measure the electrical conductivity of ionic solutions, essential in quality control and formulation analysis for B.Pharm students. Understanding conductivity cell construction—electrode material, electrode area, electrode spacing, cell geometry and cell constant—ensures accurate conductivity measurements. Key concepts include two-electrode and four-electrode designs, polarization avoidance, AC excitation, temperature control, calibration with standard KCl solutions, and maintenance to prevent contamination or coating. Mastery of these topics helps interpret conductance versus conductivity, perform cell constant determination, and apply temperature corrections in pharmaceutical analyses. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the definition of the cell constant (k) for a conductivity cell?
- The ratio of electrode area to electrode spacing
- The product of solution conductivity and cell conductance
- The distance between electrodes divided by electrode area
- The conductance of the empty cell
Correct Answer: The distance between electrodes divided by electrode area
Q2. Which equation relates conductivity (κ), cell constant (k) and measured conductance (G)?
- κ = G / k
- κ = k × G
- G = κ / k
- k = κ + G
Correct Answer: κ = k × G
Q3. Why are AC signals used instead of DC in conductivity measurements?
- AC reduces electrode polarization and avoids electrolysis
- AC increases electrode polarization for better sensitivity
- DC is easier to generate but less accurate
- DC measurements are faster than AC
Correct Answer: AC reduces electrode polarization and avoids electrolysis
Q4. Which electrode material is commonly used for conductivity cells due to inertness?
- Iron
- Silver chloride
- Platinum
- Copper
Correct Answer: Platinum
Q5. What is the primary purpose of a diaphragm or junction in some conductivity cells?
- To add capacitance to the circuit
- To separate reference and measurement compartments and reduce contamination
- To increase solution conductivity
- To physically support the electrodes only
Correct Answer: To separate reference and measurement compartments and reduce contamination
Q6. In a four-electrode conductivity cell, the outer electrodes are used for:
- Measuring potential drop across the solution
- Applying current to minimize polarization effects
- Temperature sensing
- Holding the cell constant
Correct Answer: Applying current to minimize polarization effects
Q7. How is the cell constant typically determined experimentally?
- By measuring conductance of a standard solution of known conductivity
- By measuring the electrode weight before and after use
- By measuring DC current at fixed voltage in pure water
- By measuring electrode resistance in air
Correct Answer: By measuring conductance of a standard solution of known conductivity
Q8. Which standard solution is most commonly used to calibrate conductivity cells in laboratories?
- Sodium chloride (NaCl) solution
- Potassium chloride (KCl) solution
- Glucose solution
- Sulfuric acid solution
Correct Answer: Potassium chloride (KCl) solution
Q9. What effect does increasing temperature have on the conductivity of an ionic solution?
- Conductivity decreases with increasing temperature
- Conductivity remains constant with temperature
- Conductivity increases with increasing temperature
- Temperature only changes cell constant, not conductivity
Correct Answer: Conductivity increases with increasing temperature
Q10. Which factor does NOT directly affect the cell constant?
- Distance between electrodes
- Electrode area
- Solution ionic strength
- Cell geometry
Correct Answer: Solution ionic strength
Q11. Why is regular cleaning and maintenance of conductivity cell electrodes important?
- To increase electrode spacing
- To prevent coating that alters effective area and causes measurement errors
- To change the cell constant permanently
- To make the electrode reactive with the sample
Correct Answer: To prevent coating that alters effective area and causes measurement errors
Q12. What is the main disadvantage of a simple two-electrode conductivity cell at low conductivities?
- Excessive heating of the solution
- Electrode polarization and interfacial impedance affecting accuracy
- Too large cell constant for measurements
- Inability to use AC excitation
Correct Answer: Electrode polarization and interfacial impedance affecting accuracy
Q13. In cell constant expression k = d/A, what do d and A represent?
- d = electrode diameter, A = electrode area
- d = distance between electrodes, A = electrode surface area perpendicular to current
- d = diffusion coefficient, A = activity coefficient
- d = displacement, A = applied voltage
Correct Answer: d = distance between electrodes, A = electrode surface area perpendicular to current
Q14. Which design feature helps reduce stray capacitance and fringe effects in a conductivity cell?
- Using very thin electrodes
- Using guard electrodes and appropriate shielding
- Placing electrodes very close to container walls
- Using DC measurement
Correct Answer: Using guard electrodes and appropriate shielding
Q15. How does electrode surface roughness affect conductivity measurements?
- Roughness does not affect measurements
- Increases effective surface area leading to altered cell constant if not accounted for
- Makes electrodes chemically inert
- Reduces temperature dependence of conductivity
Correct Answer: Increases effective surface area leading to altered cell constant if not accounted for
Q16. For accurate pharmaceutical conductivity measurements, why is temperature control or compensation required?
- Temperature affects electrode color only
- Temperature changes solution conductivity and must be corrected to a reference temperature
- Temperature only affects the cell constant, not conductivity
- Temperature increases ionic charge of species
Correct Answer: Temperature changes solution conductivity and must be corrected to a reference temperature
Q17. Which unit is used for conductivity (κ) in the SI system commonly reported in labs?
- Mho per centimeter (S/cm)
- Ohm
- Siemens meter (S·m)
- Siemens per meter (S/m)
Correct Answer: Siemens per meter (S/m)
Q18. What is the typical purpose of calibrating a conductivity meter with multiple KCl standards?
- To determine the electrode color response
- To create a calibration curve across the measurement range and confirm linearity
- To measure pH simultaneously
- To clean the electrodes only
Correct Answer: To create a calibration curve across the measurement range and confirm linearity
Q19. Which measurement artifact results from gas bubble formation on electrode surfaces?
- Reduced polarization
- Increased effective electrode area
- Erratic conductance readings due to insulation of electrode surface
- Lower solution temperature
Correct Answer: Erratic conductance readings due to insulation of electrode surface
Q20. When selecting electrode spacing for a conductivity cell, which consideration is correct?
- Wider spacing always gives lower sensitivity
- Narrower spacing reduces cell constant always
- Spacing should be chosen to match anticipated conductivity range and minimize polarization
- Spacing is irrelevant if using AC
Correct Answer: Spacing should be chosen to match anticipated conductivity range and minimize polarization
Q21. What is the effect of ionic strength on measured conductivity for a given ion concentration?
- Ionic strength only affects pH, not conductivity
- Higher ionic strength increases inter-ionic interactions reducing mobility and altering conductivity non-linearly
- Ionic strength linearly increases conductivity regardless of ion type
- Ionic strength decreases cell constant
Correct Answer: Higher ionic strength increases inter-ionic interactions reducing mobility and altering conductivity non-linearly
Q22. In pharmaceutical analysis, conductivity measurements are commonly used to:
- Determine molecular weight of drugs
- Monitor ionic impurities, buffer strength and water quality
- Measure viscosity of solutions
- Identify organic functional groups
Correct Answer: Monitor ionic impurities, buffer strength and water quality
Q23. What is the main reason to use platinum black or platinized electrodes?
- To make electrodes magnetic
- To increase electrode surface area and lower contact resistance for better sensitivity
- To decrease electrode chemical stability
- To reduce electrode spacing permanently
Correct Answer: To increase electrode surface area and lower contact resistance for better sensitivity
Q24. Which frequency range is typically used for AC conductivity measurements to avoid polarization?
- Very low frequency (< 1 Hz)
- Intermediate frequency (kHz range)
- Extremely high frequency (GHz range)
- DC only
Correct Answer: Intermediate frequency (kHz range)
Q25. How does cell geometry influence the cell constant?
- Only electrode material matters for cell constant
- Geometry determines distance and area relationships, directly affecting k
- Geometry affects only temperature dependence
- Geometry is irrelevant if calibration is done
Correct Answer: Geometry determines distance and area relationships, directly affecting k
Q26. What precaution is important when storing conductivity cells between uses?
- Store dry to prevent corrosion unless specified wet storage is required
- Always immerse in strong acid
- Store at high temperature to dry quickly
- Keep electrodes coated with sample residues
Correct Answer: Store dry to prevent corrosion unless specified wet storage is required
Q27. Why might a four-electrode cell be preferred for low-conductivity pharmaceutical samples?
- It is cheaper to manufacture
- It separates current-supplying and potential-sensing electrodes, reducing polarization and improving accuracy at low conductance
- It increases temperature sensitivity
- It eliminates the need for calibration
Correct Answer: It separates current-supplying and potential-sensing electrodes, reducing polarization and improving accuracy at low conductance
Q28. Which practice improves reproducibility of conductivity measurements in a lab?
- Varying sample volume between measurements
- Not rinsing electrodes between samples
- Using consistent temperature, calibration, electrode cleaning and sample handling
- Changing cell constant frequently without recalibration
Correct Answer: Using consistent temperature, calibration, electrode cleaning and sample handling
Q29. What does a sudden drift in conductivity readings during a run most likely indicate?
- Stable electrode surface
- Temperature stabilization
- Contamination, bubble formation, electrode fouling or instrument instability
- Correct calibration
Correct Answer: Contamination, bubble formation, electrode fouling or instrument instability
Q30. When reporting conductivity results for pharmaceutical standards, what should always be included?
- Only the raw conductance value
- The conductivity value, temperature at which measured, cell constant and calibration details
- Only the cell constant
- The electrode serial number only
Correct Answer: The conductivity value, temperature at which measured, cell constant and calibration details
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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