Dissolution rate and Noyes–Whitney MCQs With Answer

Dissolution rate and Noyes–Whitney MCQs With Answer

This quiz compilation is tailored for M.Pharm students studying Advanced Biopharmaceutics & Pharmacokinetics (MPH 202T). It focuses on the principles governing dissolution rate, the Noyes–Whitney equation and its practical implications in formulation development, testing and IVIVC. Questions cover mechanistic concepts — diffusion layer, saturation solubility, surface area, diffusion coefficient — as well as experimental considerations like intrinsic dissolution, sink conditions, apparatus selection and strategies to improve dissolution of poorly soluble drugs. Each MCQ tests deeper understanding needed for formulation design, dissolution method selection and regulatory applications such as biowaivers. Answers are provided to help self-assessment and revision.

Q1. In the Noyes–Whitney equation dM/dt = (D·A·(Cs − C))/h, what does the term (Cs − C) represent?

• The diffusion coefficient in the stagnant layer
• The diffusion layer thickness
• The concentration difference between the particle surface and the bulk solution (concentration gradient)
• The surface area available for dissolution

Correct Answer: The concentration difference between the particle surface and the bulk solution (concentration gradient)

Q2. Which parameter in the Noyes–Whitney equation is most directly increased by reducing particle size (assuming the same mass)?

• Diffusion coefficient (D)
• Diffusion layer thickness (h)
• Saturation solubility (Cs)
• Surface area (A)

Correct Answer: Surface area (A)

Q3. Which formulation or test condition primarily decreases the diffusion layer thickness (h) and thereby increases dissolution rate?

• Decreasing agitation speed
• Increasing particle size
• Increasing agitation/stirring speed
• Adding non-polar oils to the medium

Correct Answer: Increasing agitation/stirring speed

Q4. What is the commonly accepted definition of ‘sink conditions’ in dissolution testing?

• Bulk concentration equals saturation solubility (C = Cs)
• Bulk concentration is greater than saturation solubility (C > Cs)
• Bulk concentration is less than ~10% of the saturation solubility (C ≤ 0.1·Cs)
• Surface concentration is negligible compared with bulk concentration

Correct Answer: Bulk concentration is less than ~10% of the saturation solubility (C ≤ 0.1·Cs)

Q5. Which technique is standard for measuring intrinsic dissolution rate (IDR) of a drug substance?

• Flow-through cell with crushed powder
• Rotating disk method with constant surface area
• Paddle apparatus with varying tablet sizes
• Diffusion cell using dialysis membrane

Correct Answer: Rotating disk method with constant surface area

Q6. Which strategy will directly increase the effective saturation solubility (Cs) of a weakly acidic drug in aqueous medium?

• Lowering the pH well below the pKa
• Raising the pH to create ionized species
• Increasing particle size
• Decreasing temperature

Correct Answer: Raising the pH to create ionized species

Q7. According to the Noyes–Whitney relationship, dissolution rate is directly proportional to which of the following?

• Diffusion coefficient (D)
• Surface area (A)
• Concentration difference (Cs − C)
• All of the above

Correct Answer: All of the above

Q8. How does an increase in the viscosity of the dissolution medium typically affect dissolution rate, via the Noyes–Whitney parameters?

• Increases diffusion coefficient (D) and increases rate
• Decreases diffusion layer thickness (h) and increases rate
• Decreases diffusion coefficient (D) and/or effectively increases h, decreasing rate
• No effect on dissolution rate

Correct Answer: Decreases diffusion coefficient (D) and/or effectively increases h, decreasing rate

Q9. In the Noyes–Whitney equation, what is the physical meaning of the diffusion coefficient (D)?

• The thickness of the hydrodynamic boundary layer
• The rate at which solvent molecules evaporate
• The proportionality constant that describes how fast molecules diffuse through the stagnant layer
• The maximum amount of drug that can dissolve per unit area

Correct Answer: The proportionality constant that describes how fast molecules diffuse through the stagnant layer

Q10. Which named equation or modification explicitly incorporates the concept of a stagnant diffusion layer (h) into dissolution rate expressions?

• Higuchi equation
• Nernst–Brunner (or Noyes–Whitney/Nernst-Brunner) equation
• Arrhenius equation
• Michaelis–Menten equation

Correct Answer: Nernst–Brunner (or Noyes–Whitney/Nernst-Brunner) equation

Q11. What is the expected effect on dissolution rate if a crystalline drug is converted to its amorphous form while keeping other variables constant?

• Dissolution rate decreases due to lower surface energy
• Dissolution rate remains unchanged
• Dissolution rate increases due to higher apparent solubility and faster molecular mobility
• Dissolution rate becomes zero due to instability

Correct Answer: Dissolution rate increases due to higher apparent solubility and faster molecular mobility

Q12. During an intrinsic dissolution test, which parameter is maintained constant to obtain an intrinsic dissolution rate?

• Hydrodynamic conditions only
• Surface area of the compacted drug disc
• Medium pH only
• Tablet hardness

Correct Answer: Surface area of the compacted drug disc

Q13. For a poorly water-soluble neutral drug, which formulation approach most directly increases the dissolution rate without changing chemistry?

• Salt formation
• Micronization or nanonization to increase surface area
• Adjusting pH to ionize the drug
• Adding a non-ionic oil phase

Correct Answer: Micronization or nanonization to increase surface area

Q14. Under sink conditions where bulk concentration C is negligible compared to Cs, what happens to the dissolution rate if Cs is doubled (other parameters constant)?

• The dissolution rate becomes one-fourth
• The dissolution rate doubles
• The dissolution rate remains unchanged
• The dissolution rate becomes zero

Correct Answer: The dissolution rate doubles

Q15. Which official USP dissolution apparatus uses a paddle (rotating blade) as the hydrodynamic element?

• USP Apparatus I (Basket)
• USP Apparatus II (Paddle)
• USP Apparatus III (Reciprocating Cylinder)
• USP Apparatus IV (Flow-through cell)

Correct Answer: USP Apparatus II (Paddle)

Q16. How does temperature generally affect the diffusion coefficient (D) and therefore dissolution rate?

• Increasing temperature generally decreases D and slows dissolution
• Temperature has no effect on D
• Increasing temperature generally increases D and speeds up dissolution
• Only pressure affects D, not temperature

Correct Answer: Increasing temperature generally increases D and speeds up dissolution

Q17. What is the primary role of surfactants when included in dissolution media or formulations for poorly wetting drugs?

• They increase particle size to slow dissolution
• They reduce wettability to prevent dissolution
• They improve wetting and can increase apparent solubility via micellar solubilization
• They chemically react with drug to form insoluble complexes

Correct Answer: They improve wetting and can increase apparent solubility via micellar solubilization

Q18. Which model is commonly used to describe diffusion-controlled drug release from a homogeneous matrix (particularly for planar systems)?

• Michaelis–Menten model
• Higuchi model
• First-order exponential decay
• Langmuir isotherm

Correct Answer: Higuchi model

Q19. If agitation speed in a paddle apparatus is increased, which Noyes–Whitney parameter is most directly affected and how?

• Diffusion coefficient (D) increases dramatically
• Saturation solubility (Cs) decreases
• Diffusion layer thickness (h) decreases, increasing dissolution rate
• Surface area (A) decreases

Correct Answer: Diffusion layer thickness (h) decreases, increasing dissolution rate

Q20. Which in vitro dissolution approach mimics simultaneous dissolution and absorption by using an aqueous phase with an immiscible organic phase to capture partitioned drug?

• pH-stat dissolution
• Biphasic (two-phase) dissolution test
• Intrinsic dissolution rotating disk without organic phase
• Disk diffusion assay

Correct Answer: Biphasic (two-phase) dissolution test

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