The only limitation for the volume of dissolution medium is that the expected amount of test drug must be freely soluble in the volume at 37 ºC. It is possible that the drug may not be soluble at room temperature, but at 37 ºC it may be freely soluble. In such situations, for dissolution testing purposes, the drug will be considered as freely soluble.
This freely soluble drug requirement/concept is often referred to as a “sink” condition. The sink condition may be any condition where drug is freely soluble. Often, in the literature, it is defined as 2, 3, 5 or more times the volume of the dissolution medium required to dissolve the expected amount of the drug. These choices may be personal preferences, however, in practice the drug should be freely soluble in the medium. Therefore, before conducting a dissolution test and choosing a volume, one must determine the solubility of the drug in the medium at 37 ºC. Once this volume is known, to be on the safe side, one may use 10% extra volume than needed.
It is to be noted that if the volume required is 250 mL, then there is no harm in using larger than the required volume, for example 900 mL. The excess volume should not impact dissolution, however, lower than required should, and may make the dissolution test and results in valid. In the event, the excess volume provides differences in dissolution results, then this would be an indication of lack proper or efficient stirring in the vessel. In such situations, use of such an apparatus may not be appropriate and the results would not reflect true dissolution characteristics of the test product but an interaction of the product and the dissolution apparatus.
Having said that, as 900 mL is the commonly used or suggested volume and there is no harm in following this suggestion or tradition. In fact one should try to use this standardized approach of using 900 mL. In such cases, the analyst must make sure that expected amount of drug is freely soluble in 900 mL. If not then the use of a solublizing agent such as SLS may be considered, to establish that the drug is, indeed, freely soluble in the 900 mL medium.
