Introduction of dissolution error as a result of different openings in vessel covers

Vessel covers are required in dissolution studies to retard solvent evaporation. Sufficient openings on the cover are required by the current United States Pharmacopeia (USP) "to allow ready insertion of a thermometer and for withdrawal of specimens/samples." However, the area of the opening has not been regulated by the USP. The purpose of this work was to measure the loss of the dissolution solvent as a result of openings in vessel covers and to investigate errors caused by the solvent loss. The method used in the study involved measurements of the opening areas in three commercial vessel covers and determinations of solvent loss under a typical dissolution condition. The results showed that the openings in the three covers varied from 75 mm2 to 2285 mm2; this was 1-30% of the total vessel open area for a USP standard vessel. The volume of the dissolution solvent was reduced by 0.6-3.4% for an 8-hr dissolution study, and 1.7-9.7% for a 24-hr study when the three different vessel covers were used. The rate of solvent loss exhibited a linear relationship with the area of the opening in the covers. A slower rate of reduction was found for a smaller initial volume of the solvent. The conclusion was that higher dissolution results were produced if the solvent loss was not considered.