The 900 ml volume is commonly used in dissolution testing primarily to accommodate the volume displacement caused by the dosage form (tablet, capsule, etc.) and the stirring apparatus within the dissolution vessel, while ensuring sufficient media for accurate measurement.
Here's a more detailed breakdown:
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Volume Displacement: When a solid dosage form is introduced into the dissolution vessel, it occupies a certain volume. The same is true for the dissolution paddle or basket shaft. If the vessel were filled to the brim (e.g., 1000 ml), this displacement could cause overflow and invalidate the test. The 900 ml starting volume provides a buffer to prevent this.
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Ensuring Adequate Media for Dissolution: While displacement is a factor, the final volume must still be sufficient to maintain sink conditions for the drug being tested. Sink conditions ensure that the concentration of the dissolved drug remains below a certain level (typically 1/3 of the saturation concentration), allowing for optimal and reproducible dissolution. Using a smaller initial volume might compromise sink conditions for highly soluble drugs.
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Standardization and Historical Context: The 900 mL volume is a standardized volume in many pharmacopeias (like USP and EP) and has been used for many years. This historical precedent and standardization promote consistency and comparability across different drug products and laboratories. While other volumes (e.g., 500 mL) are sometimes used, 900 mL is a common and well-established standard.
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Accuracy and Precision: The larger volume can also improve the accuracy and precision of the analytical measurements used to determine the amount of drug dissolved. A higher concentration of dissolved drug (within the constraints of sink conditions) can often lead to more reliable measurements.
In summary, the selection of 900 mL as a standard volume for dissolution testing is a compromise between accommodating volume displacement, maintaining sink conditions, ensuring accurate measurement, and adhering to established regulatory guidelines and historical precedent.
