Loss on drying 
731
—
Dry about 100 mg, accurately weighed, in a capillary-stoppered bottle in vacuum at a pressure not exceeding 5 mm of mercury at 60
for 3 hours: it loses not more than 2.0% of its weight.
Assay for oxytetracycline—
Transfer a suitable, accurately weighed quantity of Topical Powder to a glass blender jar containing a sufficient, accurately measured volume of 0.1 N hydrochloric acid to yield a stock solution containing not less than 150 µg of oxytetracycline per mL, and blend at high speed for 3 to 5 minutes. Proceed as directed for oxytetracycline under
Antibiotics—Microbial Assays 81, using an accurately measured volume of this solution diluted quantitatively and stepwise with water to obtain a
Test Dilution having a concentration of oxytetracycline assumed to be equal to the median dose level of the Standard.
Assay for polymyxin B—
Transfer an accurately weighed quantity of Topical Powder, equivalent to about 10,000 USP Polymyxin B Units, to a 50-mL centrifuge tube, add 15 mL of acetone and 0.05 mL of hydrochloric acid, and stir. Add 20 mL of acetone, and centrifuge for 10 minutes. Decant and discard the clear liquid. Add 15 mL of acetone and 0.05 mL of hydrochloric acid to the residue, stir, add 20 mL of acetone, and centrifuge for 10 minutes, decanting and discarding the clear liquid.
[NOTE—Take care not to discard any of the residue with the clear liquid.
] Add 5 mL of
Buffer No. 6 to the residue, and mix. Transfer the mixture to a 100-mL volumetric flask with the aid of
Buffer No. 6, dilute with the same solvent to volume, and mix. Proceed as directed for polymyxin B under
Antibiotics—Microbial Assays 81, using an accurately measured volume of this stock solution diluted quantitatively with
Buffer No. 6 to obtain a
Test Dilution having a concentration of polymyxin B assumed to be equal to the median dose level of the Standard.
