Dissolution media volume of 900mL is specified in United States Pharmacopeia (USP) dissolution test methods to achieve sink conditions ensuring proper discrimination between fast-dissolving immediate release and extended-release products.

Sink conditions are maintained when the concentration of the drug in the dissolution medium remains below 10-20% of its saturation solubility throughout the dissolution test. This volume ensures sufficient drug dissolution without saturation, providing reliable drug release profiles and accurate assessment of product performance.