(Official January 1, 2007)

Assay preparation— Transfer an accurately weighed portion of Ointment, equivalent to about 2 g of ichthammol, to a 250-mL beaker, and add about 70 mL of boiling water. Mix with a glass rod, heat on a steam bath, with frequent agitation, for 10 minutes, cover with a watch glass without removing the stirring rod, and allow to stand at room temperature for 15 to 20 minutes. Place in a refrigerator to cause the upper layer to congeal, form a second opening through the congealed layer with the glass rod, and transfer the dark-colored aqueous extract to a funnel containing a pledget of cotton, collecting the filtrate in a 500-mL volumetric flask. Repeat the extraction of the portion of the Ointment several times in the same manner until the aqueous extract is practically colorless, passing each extract through the same cotton filter into the flask containing the main extract. Dilute with water to volume, and mix.

Procedure for ammonia— Transfer 100.0 mL of the Assay preparation to a suitable distillation flask, add 3 g of paraffin, and add 20 mL of sodium hydroxide solution (4 in 10). Connect the flask to a condenser by means of a spray trap, and immerse the lower outlet tube of the condenser in 30.0 mL of 0.05 N sulfuric acid VS. Distill slowly, collect about 50 mL of distillate, and then titrate the excess acid with 0.05 N sodium hydroxide VS, using methyl red TS as the indicator. Perform a blank determination, and make any necessary correction. Each mL of 0.05 N sulfuric acid is equivalent to 0.8515 mg of NH3.