Packaging and storage— Preserve in tight containers.

Identification— Digest about 2g with 5mLof nitric acid over gentle heat for 1hour,dilute with water to about 50mL,and filter:the solution responds to Identificationtest Aunder Selenium Sulfide,when tested as directed,beginning with “to 10mLof the filtrate add 5mLof water.”

pHá791ñ: between 2.0and 6.0.

Assay— Place a portion of well-mixed Topical Suspension,equivalent to about 100mg of selenium sulfide and accurately weighed,in a suitable flask.Cautiously digest with 25mLof fuming nitric acid over gentle heat for 2hours,and proceed as directed in the Assayunder Selenium Sulfide,beginning with “Cool,transfer the solution to a 250-mLvolumetric flask.”Each mLof 0.05Nsodium thiosulfate is equivalent to 1.789mg of SeS2.Where the Topical Suspension is labeled in terms of percentage (w/v)or of the amount of SeS2in a given volume of Topical Suspension,determine the density of the Topical Suspension as follows:Using a tared,100-mLvolumetric flask,weigh 100mLof Topical Suspension that previously has been shaken to ensure homogeneity,allowed to stand until the entrapped air rises,and finally inverted carefully just prior to transfer to the volumetric flask.From the observed weight of 100mLof the Topical Suspension,calculate the quantity of SeS2in each 100mL.USP28

(Official July 1,2007)

Expert Committee:(PA6)Pharmaceutical Analysis 6

USP28–NF23Page 1764

Phone Number:1-301-816-8389