What does Cs represent in the Noyes-Whitney dissolution rate equation dM/dt = D·A/h·(Cs − Cb)?

• A. Drug solubility in the dissolution medium
• B. Diffusion coefficient
• C. Boundary layer thickness
• D. Bulk drug concentration

Answer: (A)

Dissolution rate is driven by the difference between how much drug can be in solution at equilibrium and how much is actually present near the dissolving surface. In the Noyes-Whitney equation, Cs is the saturation solubility of the drug in the dissolution medium at the given temperature—it’s the maximum soluble concentration possible when the solid and solution are in equilibrium. This value sets the driving force for dissolution through the term (Cs − Cb): if the bulk concentration near the surface (Cb) is well below Cs, dissolution proceeds quickly; as Cb approaches Cs, the driving force shrinks and the rate drops toward zero. The remaining factors in the equation (diffusion coefficient D, surface area A, and boundary-layer thickness h) modulate how fast the drug can diffuse from the surface into the bulk, but Cs specifically represents solubility.