Introduction: Surface area determination methods are essential in B. Pharm for understanding drug particle behavior, formulation performance, and predicting dissolution and bioavailability. Key techniques include gas adsorption (BET and Langmuir), mercury intrusion porosimetry, BJH pore analysis, inverse gas chromatography and dye adsorption tests. Proper surface area analysis requires attention to sample degassing, adsorbate choice (nitrogen or krypton), adsorption isotherms and Brunauer classifications. Accurate specific surface area (m2/g) and pore-size distribution data guide particle engineering, stability assessment and dissolution enhancement strategies. This introduction highlights practical principles and method selection criteria for pharmaceutical solids. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary pharmaceutical relevance of measuring specific surface area (SSA) of drug powders?
- Estimating tablet hardness directly
- Predicting chemical stability at high temperature
- Assessing dissolution rate and bioavailability
- Determining color and odor of the powder
Correct Answer: Assessing dissolution rate and bioavailability
Q2. Which technique is most commonly used to determine specific surface area by physical adsorption?
- Mercury intrusion porosimetry
- Brunauer–Emmett–Teller (BET) gas adsorption
- Atomic force microscopy (AFM)
- Laser diffraction particle sizing
Correct Answer: Brunauer–Emmett–Teller (BET) gas adsorption
Q3. Which adsorbate and temperature are standard for BET surface area measurements?
- Helium at 25 °C
- Nitrogen at 77 K (liquid nitrogen temperature)
- Water vapor at 37 °C
- Carbon dioxide at 298 K
Correct Answer: Nitrogen at 77 K (liquid nitrogen temperature)
Q4. For samples with very low surface area, which gas is commonly used instead of nitrogen to improve sensitivity?
- Oxygen
- Helium
- Krypton
- Hydrogen
Correct Answer: Krypton
Q5. The BET model assumes which of the following about adsorption layers?
- Only monolayer adsorption occurs
- Layers form with decreasing enthalpy for each successive layer and multilayer adsorption is possible
- Covalent bonding between adsorbate and surface dominates
- Adsorbate molecules penetrate the bulk solid
Correct Answer: Layers form with decreasing enthalpy for each successive layer and multilayer adsorption is possible
Q6. Which p/p0 range is typically used to calculate BET surface area from the linear BET plot?
- 0.90–0.99
- 0.05–0.30
- 0.30–0.60
- 0–0.01
Correct Answer: 0.05–0.30
Q7. In the BET analysis, the monolayer capacity (Vm) is obtained from which aspect of the linearized BET plot?
- The intercept only
- The slope only
- Both slope and intercept
- The maximum adsorbed volume at p/p0 = 1
Correct Answer: Both slope and intercept
Q8. Which Brunauer isotherm type is typical for nonporous or macroporous materials with unrestricted multilayer adsorption?
- Type I
- Type II
- Type III
- Type IV
Correct Answer: Type II
Q9. Which technique is most appropriate to characterize pore size distribution in the mesopore range (2–50 nm)?
- BET surface area only
- Mercury intrusion porosimetry
- BJH (Barrett–Joyner–Halenda) analysis applied to nitrogen adsorption data
- Optical microscopy
Correct Answer: BJH (Barrett–Joyner–Halenda) analysis applied to nitrogen adsorption data
strongQ10. Mercury intrusion porosimetry is best suited for measuring which pore size region and has what important limitation?
- Micropores (<2 nm); it underestimates macropores
- Mesopores (2–50 nm); it cannot be used for powders
- Macropores (>50 nm); high pressures may damage fragile structures and it measures throat size not internal diameter
- All pore sizes equally; it is non-destructive
Correct Answer: Macropores (>50 nm); high pressures may damage fragile structures and it measures throat size not internal diameter
(Note: The question above Q10 had a formatting error in the “strong” tag; correct as below)
Q10. Mercury intrusion porosimetry is best suited for measuring which pore size region and has what important limitation?
- Micropores (<2 nm); it underestimates macropores
- Mesopores (2–50 nm); it cannot be used for powders
- Macropores (>50 nm); high pressures may damage fragile structures and it measures throat size not internal diameter
- All pore sizes equally; it is non-destructive
Correct Answer: Macropores (>50 nm); high pressures may damage fragile structures and it measures throat size not internal diameter
Q11. Why is sample degassing (outgassing) performed prior to gas adsorption analysis?
- To increase particle size
- To remove adsorbed moisture and contaminants that would block adsorption sites
- To chemically modify the surface for stronger binding
- To sterilize the sample
Correct Answer: To remove adsorbed moisture and contaminants that would block adsorption sites
strongQ12. Which of the following indicates the presence of hysteresis in adsorption-desorption isotherms and suggests capillary condensation in mesopores?
- A perfectly reversible isotherm
- A hysteresis loop between adsorption and desorption branches
- Only monolayer adsorption is observed
- Zero adsorption at all relative pressures
Correct Answer: A hysteresis loop between adsorption and desorption branches
(Formatting fixed:)
Q12. Which of the following indicates the presence of hysteresis in adsorption-desorption isotherms and suggests capillary condensation in mesopores?
- A perfectly reversible isotherm
- A hysteresis loop between adsorption and desorption branches
- Only monolayer adsorption is observed
- Zero adsorption at all relative pressures
Correct Answer: A hysteresis loop between adsorption and desorption branches
Q13. Inverse gas chromatography (IGC) is particularly useful for measuring which surface property of pharmaceutical powders?
- Specific surface area in m2/g by monolayer adsorption
- Surface energy and acid-base character
- Pore size below 1 nm
- Bulk density only
Correct Answer: Surface energy and acid-base character
Q14. Dye adsorption methods (e.g., methylene blue) are often used as a quick estimate of what?
- Total porosity by mercury porosimetry
- Accessible surface area or external surface related to adsorption sites
- True particle density
- Crystalline polymorph identity
Correct Answer: Accessible surface area or external surface related to adsorption sites
Q15. Which pore size classification correctly matches micropores, mesopores and macropores?
- Micropores >50 nm, Mesopores 2–50 nm, Macropores <2 nm
- Micropores <2 nm, Mesopores 2–50 nm, Macropores >50 nm
- Micropores 2–50 nm, Mesopores <2 nm, Macropores >50 nm
- Micropores 2–10 nm, Mesopores 10–100 nm, Macropores >100 nm
Correct Answer: Micropores <2 nm, Mesopores 2–50 nm, Macropores >50 nm
Q16. Which assumption in BET analysis may lead to errors when applied to chemisorbing surfaces?
- Assumption of a fixed monolayer molecule area
- Assumption of purely physical physisorption with relatively weak van der Waals forces
- Assumption that pores are cylindrical
- Assumption that mercury is non-wetting
Correct Answer: Assumption of purely physical physisorption with relatively weak van der Waals forces
Q17. How does increasing the specific surface area of a drug powder typically affect its dissolution rate, assuming chemical form and solubility are unchanged?
- Decrease dissolution rate due to aggregation
- Increase dissolution rate due to greater available surface for solvent interaction
- No effect on dissolution rate
- Convert the drug to a different polymorph
Correct Answer: Increase dissolution rate due to greater available surface for solvent interaction
Q18. Which experimental detail is critical to report when publishing BET surface area data for reproducibility?
- Color of the powder
- Degassing temperature and time, adsorbate, and p/p0 range used for BET fit
- Manufacturer of the sample vial
- Humidity in the laboratory on a random day
Correct Answer: Degassing temperature and time, adsorbate, and p/p0 range used for BET fit
Q19. Which measurement gives the best estimate of internal surface area including micropores?
- Nitrogen BET only at high p/p0
- Carbon dioxide adsorption at 273 K or specialized micropore analysis methods
- Mercury intrusion porosimetry at low pressure
- Optical microscopy
Correct Answer: Carbon dioxide adsorption at 273 K or specialized micropore analysis methods
Q20. When calculating SSA from an average particle diameter assuming smooth spheres, which additional property is required?
- Bulk electrical conductivity
- True particle density
- Melting point
- Color index
Correct Answer: True particle density
Q21. The t-plot method is primarily used to:
- Determine true particle density from adsorption data
- Separate external surface area from micropore surface area and estimate micropore volume
- Measure macroscopic tablet porosity
- Calculate tablet disintegration time
Correct Answer: Separate external surface area from micropore surface area and estimate micropore volume
Q22. Which is a common practical limitation of BET surface area values for agglomerated nanoparticle powders?
- BET overestimates SSA due to ignoring porosity
- BET measures only external surface and cannot account for inaccessible surfaces within agglomerates without proper dispersion
- BET cannot be used for nanoparticles at all
- BET always gives identical values irrespective of sample history
Correct Answer: BET measures only external surface and cannot account for inaccessible surfaces within agglomerates without proper dispersion
strongQ23. Which of the following indicates that BET data may not be reliable for a given sample?
- High linearity in the BET plot between 0.05 and 0.30
- Negative intercept or unrealistic C constant values
- Stable baseline and reproducible isotherm
- Agreement with complementary techniques
Correct Answer: Negative intercept or unrealistic C constant values
(Formatting fixed:)
Q23. Which of the following indicates that BET data may not be reliable for a given sample?
- High linearity in the BET plot between 0.05 and 0.30
- Negative intercept or unrealistic C constant values
- Stable baseline and reproducible isotherm
- Agreement with complementary techniques
Correct Answer: Negative intercept or unrealistic C constant values
Q24. Which statement correctly contrasts Langmuir and BET adsorption models?
- Langmuir describes multilayer adsorption; BET describes only monolayer adsorption
- Langmuir assumes a homogeneous surface and monolayer coverage; BET extends to multilayer adsorption on a uniform surface
- Both models assume chemisorption only
- BET requires chemical reactions to form surface complexes while Langmuir does not
Correct Answer: Langmuir assumes a homogeneous surface and monolayer coverage; BET extends to multilayer adsorption on a uniform surface
Q25. Which surface area method can also provide information on surface energetic heterogeneity useful for predicting powder blending behavior?
- Nitrogen BET area only
- Inverse gas chromatography (IGC)
- Mercury intrusion porosimetry only
- Optical density measurement
Correct Answer: Inverse gas chromatography (IGC)
Q26. Which factor can lead to underestimation of surface area by gas adsorption?
- Inadequate degassing leaving adsorbed water blocking sites
- Using krypton for very low area samples
- Correct choice of p/p0 range for BET
- Measuring at cryogenic temperatures
Correct Answer: Inadequate degassing leaving adsorbed water blocking sites
Q27. An adsorption isotherm showing a sharp rise at very low relative pressures (p/p0 <0.1) is most indicative of which feature?
- Large macropores only
- Significant microporosity
- Completely nonporous smooth surface
- Instrument malfunction
Correct Answer: Significant microporosity
Q28. Which experimental approach helps access internal surface area of strongly agglomerated particles before BET measurement?
- Skip degassing to preserve structure
- Apply careful dispersion methods such as mild sonication or controlled milling and validated pre-treatment
- Use mercury intrusion in place of gas adsorption without dispersion
- Measure only particle size by laser diffraction
Correct Answer: Apply careful dispersion methods such as mild sonication or controlled milling and validated pre-treatment
Q29. Which description correctly states a limitation of microscopy (SEM/AFM) for quantitative SSA measurement compared with adsorption methods?
- Microscopy measures total internal pore volume directly
- Microscopy typically measures external topography on limited areas and may miss nanoscale internal porosity that contributes to SSA measured by adsorption
- Microscopy always overestimates BET area
- Microscopy gives identical m2/g values to BET for all samples
Correct Answer: Microscopy typically measures external topography on limited areas and may miss nanoscale internal porosity that contributes to SSA measured by adsorption
Q30. When selecting a surface area method for a pharmaceutical solid, which combination of considerations is most appropriate?
- Only cost matters; choose the cheapest method
- Consider target pore-size range, sample fragility, required sensitivity (micropore detection), and how results relate to dissolution/bioavailability
- Always choose mercury intrusion regardless of sample type
- Choose methods that provide the highest numerical surface area regardless of relevance
Correct Answer: Consider target pore-size range, sample fragility, required sensitivity (micropore detection), and how results relate to dissolution/bioavailability
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