Hydrodynamics of Compendial Dissolution Apparatus

Various dissolution test systems have been developed and several of them now enjoy compendial status in pharmacopeias, for example the reciprocating cylinder (United States Pharmacopeia Apparatus 3), the flow-through apparatus [European Pharmacopoeia (Pharm. Eur.) 2.9.3], or the apparatus for transdermal delivery systems, such as the paddle over disc. Hydrodynamic properties of these and other apparatus have been described only sparingly. The paucity of quantitative data related to hydrodynamics of pharmacopeial dissolution testers is lamentable, since well-controllable hydrodynamics are essential to both biopharmaceutical simulations and quality control. Here, we focus the discussion on the paddle and the basket apparatus, since these are the most important and widely used for oral solid dosage forms. A brief treatise on the hydrodynamics of the flow-through apparatus completes this section. 

Methods Used for the Investigation of Flow Patterns and Flow Rates 

Flow patterns of hydrodynamic systems like the compendial dissolution apparatus may be qualitatively characterized by means of dilute dye injection (e.g., methylene blue) or by techniques using particulate materials such as aluminum powders or polystyrene particles. Flow patterns may be also visualized by taking advantage of density or pH differences within the fluid stream. The Schlieren method, for instance, is based on refraction index measurement. Hot wire anemo-metry is an appropriate method to quantitatively characterize flow rates. The flow rate is proportional to the cooling rate of a thin hot wire presented to the stream. Using laser Doppler 

Flow Rate as a Function of Stirring Rate for Paddle and Basket 

Recently, studies were performed to quantitatively examine the hydrodynamics of the two most common in vitro dissolution testers. Rotational (tangential) fluid velocities were corre- 

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