731 LOSS ON DRYING
The procedure set forth in this chapter determines the amount of volatile matter of any kind that is driven off under the conditions specified. For substances appearing to contain water as the only volatile constituent, the procedure given in the chapter, Water Determination 921, is appropriate, and is specified in the individual monograph.
Mix and accurately weigh the substance to be tested, and, unless otherwise directed in the individual monograph, conduct the determination on 1 to 2 g. If the test specimen is in the form of large crystals, reduce the particle size to about 2 mm by quickly crushing. Tare a glass-stoppered, shallow weighing bottle that has been dried for 30 minutes under the same conditions to be employed in the determination. Put the test specimen in the bottle, replace the cover, and accurately weigh the bottle and the contents. By gentle, sidewise shaking, distribute the test specimen as evenly as practicable to a depth of about 5 mm generally, and not more than 10 mm in the case of bulky materials. Place the loaded bottle in the drying chamber, removing the stopper and leaving it also in the chamber. Dry the test specimen at the temperature and for the time specified in the monograph. [noteThe temperature specified in the monograph is to be regarded as being within the range of ±2 of the stated figure.] Upon opening the chamber, close the bottle promptly, and allow it to come to room temperature in a desiccator before weighing.
If the substance melts at a lower temperature than that specified for the determination of Loss on drying, maintain the bottle with its contents for 1 to 2 hours at a temperature 5 to 10 below the melting temperature, then dry at the specified temperature.
Where the specimen under test is Capsules, use a portion of the mixed contents of not fewer than 4 capsules.
Where the specimen under test is Tablets, use powder from not fewer than 4 tablets ground to a fine powder.
Where the individual monograph directs that loss on drying be determined by thermogravimetric analysis, a sensitive electrobalance is to be used.
Where drying in vacuum over a desiccant is directed in the individual monograph, a vacuum desiccator or a vacuum drying pistol, or other suitable vacuum drying apparatus, is to be used.
Where drying in a desiccator is specified, exercise particular care to ensure that the desiccant is kept fully effective by frequent replacement.
Where drying in a capillary-stoppered bottle* in vacuum is directed in the individual monograph, use a bottle or tube fitted with a stopper having a 225 ± 25 µm diameter capillary, and maintain the heating chamber at a pressure of 5 mm or less of mercury. At the end of the heating period, admit dry air to the heating chamber, remove the bottle, and with the capillary stopper still in place allow it to cool in a desiccator before weighing.
* Available as an antibiotic moisture content flask from Kontes, 1022 Spruce St., Vineland, NJ 08360-2841.
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USP32NF27 Page 290Pharmacopeial Forum: Volume No. 34(3) Page 760