Stability problems in suspensions and remedies MCQs With Answer

Introduction: Stability problems in suspensions and remedies are core to B. Pharm pharmaceutics. Key issues include sedimentation, caking, flocculation vs. deflocculation, zeta potential control, and crystal growth (Ostwald ripening). Understanding Stokes’ law (effects of particle size, density difference, viscosity), DLVO theory (van der Waals attraction and electrostatic repulsion), wetting agents, flocculating agents (electrolytes/polymers), structured vehicles, thixotropy, and pH/ionic strength adjustments helps design robust, easily redispersible systems. Critical metrics include sedimentation volume (F), degree of flocculation, rheology (pseudoplasticity), and redispersibility tests. Remedies span controlled flocculation, viscosity and density modification, particle size distribution optimization, growth inhibitors, and proper preservatives. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. The major long-term instability in a deflocculated suspension is:

• Caking (formation of a hard, compacted sediment)
• Creaming
• Phase inversion
• Coalescence

Correct Answer: Caking (formation of a hard, compacted sediment)

Q2. According to Stokes’ law, which change will decrease the sedimentation rate of particles in a suspension?

• Increasing particle diameter
• Increasing the density difference between particle and medium
• Decreasing the viscosity of the medium
• Increasing the viscosity of the medium

Correct Answer: Increasing the viscosity of the medium

Q3. The goal of controlled flocculation in pharmaceutical suspensions is to produce:

• Slow sedimentation with a dense, non-redispersible cake
• Rapid sedimentation with a hard cake
• A loose, voluminous sediment that is easily redispersed
• No sedimentation at all

Correct Answer: A loose, voluminous sediment that is easily redispersed

Q4. Which parameter directly expresses the ratio of sediment volume to the original suspension volume?

• Degree of flocculation
• Sedimentation volume (F)
• Zeta potential
• Apparent viscosity

Correct Answer: Sedimentation volume (F)

Q5. Sedimentation volume (F) for a suspension is defined as:

• F = V_o / V_u
• F = V_u / V_o
• F = η / ρ
• F = dV/dt

Correct Answer: F = V_u / V_o

Q6. Reducing the magnitude of zeta potential (toward zero) by adding electrolytes generally promotes:

• Deflocculation due to stronger repulsion
• Flocculation due to lower electrostatic repulsion
• Increased viscosity independent of charge
• Chemical degradation of the drug

Correct Answer: Flocculation due to lower electrostatic repulsion

Q7. DLVO theory explains suspension stability as the balance between:

• Hydrogen bonding and steric hindrance
• Electrostatic repulsion and van der Waals attraction
• Osmotic pressure and capillary forces
• Diffusion and convection

Correct Answer: Electrostatic repulsion and van der Waals attraction

Q8. The most appropriate wetting agent for hydrophobic drug particles in an oral aqueous suspension is:

• Polysorbate 80 (Tween 80)
• Liquid paraffin
• Magnesium stearate
• Talc

Correct Answer: Polysorbate 80 (Tween 80)

Q9. Bridging flocculation typically occurs when:

• Electrolyte concentration is zero
• Polymers adsorb on multiple particles at partial surface coverage
• Particles are completely uncharged
• Viscosity is extremely high

Correct Answer: Polymers adsorb on multiple particles at partial surface coverage

Q10. Which excipient primarily increases viscosity with minimal charge-related interactions?

• Hydroxypropyl methylcellulose (HPMC)
• Sodium chloride
• Sodium citrate
• Polysorbate 80

Correct Answer: Hydroxypropyl methylcellulose (HPMC)

Q11. Thixotropy is desirable in suspensions because it:

• Maintains constant viscosity regardless of shear
• Gives high viscosity at rest and lower viscosity during shaking/pouring
• Prevents all sedimentation
• Increases sedimentation rate

Correct Answer: Gives high viscosity at rest and lower viscosity during shaking/pouring

Q12. The optimal sequence for preparing a flocculated suspension is:

• Add flocculant first, then drug powder, then wetting agent
• Dry blend all excipients, then add water at once
• Wetten drug, disperse in vehicle, add flocculant, then adjust viscosity
• Add preservative last, then filter out flocs

Correct Answer: Wetten drug, disperse in vehicle, add flocculant, then adjust viscosity

Q13. According to the Schulze–Hardy rule, which ions are most effective at inducing flocculation of a negatively charged suspension?

• Monovalent cations at low concentration
• Multivalent cations (higher valency counter-ions)
• Nonionic surfactants
• Anionic polymers

Correct Answer: Multivalent cations (higher valency counter-ions)

Q14. A typical deflocculated suspension shows which behavior over time?

• Large F (~1) with easy redispersion
• Small F that decreases, forming a dense, hard cake
• No sedimentation due to high viscosity
• Increasing zeta potential toward zero

Correct Answer: Small F that decreases, forming a dense, hard cake

Q15. A degree of flocculation (β) greater than 1 indicates:

• Particles are fully deflocculated
• Particles are flocculated relative to the deflocculated state
• No sediment forms
• Viscosity is independent of shear rate

Correct Answer: Particles are flocculated relative to the deflocculated state

Q16. The phenomenon where larger crystals grow at the expense of smaller ones, leading to caking, is called:

• Perikinetic flocculation
• Ostwald ripening
• Orthokinetic flocculation
• Autoxidation

Correct Answer: Ostwald ripening

Q17. A practical remedy to inhibit Ostwald ripening in suspensions is to:

• Increase temperature cycling during storage
• Add a polymeric crystal growth inhibitor (e.g., PVP)
• Reduce vehicle viscosity to promote diffusion
• Add a high level of non-adsorbing polymer

Correct Answer: Add a polymeric crystal growth inhibitor (e.g., PVP)

Q18. Zeta potential measurement primarily informs about:

• Drug assay
• Surface charge and electrostatic stability of dispersed particles
• Viscosity–temperature dependence
• Microbial load

Correct Answer: Surface charge and electrostatic stability of dispersed particles

Q19. A lower contact angle at the solid–liquid interface indicates better wetting. Which range reflects good wetting?

• Greater than 90°
• Exactly 90°
• 0° to 90°
• Exactly 180°

Correct Answer: 0° to 90°

Q20. Which of the following is NOT typically used as a flocculating agent in aqueous suspensions?

• Sodium chloride
• Sodium citrate
• Gelatin
• Sucrose

Correct Answer: Sucrose

Q21. Increasing the density of the continuous phase (e.g., with glycerol or sorbitol) will generally:

• Increase sedimentation rate by increasing Δρ
• Decrease sedimentation rate by reducing Δρ
• Eliminate the need for wetting agents
• Increase zeta potential

Correct Answer: Decrease sedimentation rate by reducing Δρ

Q22. A suspension that sediments quickly but is easily redispersed and shows F ≈ 1 is most likely:

• Deflocculated
• Flocculated
• Coalesced
• Supersaturated

Correct Answer: Flocculated

Q23. Which test best assesses resuspendability after storage?

• pH measurement
• Redispersibility test (number of inversions to redisperse)
• Assay of API
• Optical clarity

Correct Answer: Redispersibility test (number of inversions to redisperse)

Q24. The most desirable flow behavior for oral suspensions is:

• Dilatant (shear-thickening) without thixotropy
• Bingham plastic with high yield stress only
• Pseudoplastic with thixotropy
• Newtonian

Correct Answer: Pseudoplastic with thixotropy

Q25. Adding an anionic surfactant to a cationic drug suspension can cause:

• Enhanced chemical stability with no interactions
• Complexation/precipitation due to ionic interaction
• Immediate sterilization
• Increased zeta potential magnitude of the same sign

Correct Answer: Complexation/precipitation due to ionic interaction

Q26. Which particle size distribution strategy best reduces the risk of caking?

• Very broad distribution with many fines
• Narrow distribution minimizing very fine particles
• Only very large particles
• Random uncontrolled distribution

Correct Answer: Narrow distribution minimizing very fine particles

Q27. If the particle diameter is reduced from 10 μm to 5 μm, the sedimentation rate (v ∝ d²) becomes:

• Twice the original
• Half the original
• One-fourth of the original
• Unchanged

Correct Answer: One-fourth of the original

Q28. Depletion flocculation caused by excess non-adsorbing polymer can be reduced by:

• Increasing the polymer concentration further
• Decreasing the concentration of the non-adsorbing polymer
• Raising the storage temperature
• Eliminating all electrolytes

Correct Answer: Decreasing the concentration of the non-adsorbing polymer

Q29. Which preservative choice is most effective for an acidic oral suspension (pH ~4.5)?

• Sodium benzoate at pH ~4.5
• Benzalkonium chloride at pH ~9
• No preservative is needed
• Sodium borate at pH ~9

Correct Answer: Sodium benzoate at pH ~4.5

Q30. Which storage condition can accelerate crystal growth and caking in suspensions?

• Stable temperature without fluctuation
• Temperature cycling (freeze–thaw)
• Protection from light
• Inert headspace gas

Correct Answer: Temperature cycling (freeze–thaw)

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