Assay preparation— Transfer an accurately weighed portion of Ointment,equivalent to about 2g of ichthammol,to a 250-mLbeaker,and add about 70mLof boiling water.Mix with a glass rod,heat on a steam bath,with frequent agitation,for 10minutes,cover with a watch glass without removing the stirring rod,and allow to stand at room temperature for 15to 20minutes.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-mLvolumetric 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 100.0mLof the Assay preparationto a suitable distillation flask,add 3g of paraffin,and add 20mLof sodium hydroxide solution (4in 10).Connect the flask to a condenser by means of a spray trap,and immerse the lower outlet tube of the condenser in 30.0mLof 0.05Nsulfuric acid VS.Distill slowly,collect about 50mLof distillate,and then titrate the excess acid with 0.05Nsodium hydroxide VS,using methyl red TSas the indicator.Perform a blank determination,and make any necessary correction.Each mLof 0.05Nsulfuric acid is equivalent to 0.8515mg of NH3.