Zinc Oxide Ointment »Zinc Oxide Ointment contains not less than 18.5percent and not more than 21.5percent of ZnO. It may be prepared as follows: Zinc Oxide 200g Mineral Oil 150g White Ointment 650g To make 1000g Levigate the Zinc Oxide with the Mineral Oil to a smooth paste,and then incorporate the White Ointment [see Ointments and Suppositoriesunder Added Substances(Ingredients and Processes)in the General Notices]. Packaging and storage— Preserve in well-closed containers,and avoid prolonged exposure to temperatures exceeding 30. Identification— The residue obtained in the Assayis yellow when hot and white when cool. Minimum fill á755ñ: meets the requirements. Calcium,magnesium,and other foreign substances— Heat about 2g gently until melted,and continue the heating,gradually raising the temperature until the mass is thoroughly charred.Ignite the mass until the residue is uniformly yellow.To the residue add 6mLof 3Nhydrochloric acid:no effervescence occurs.Heat the mixture on a steam bath for 10to 15minutes:not more than a trace of insoluble residue remains.Filter the solution,dilute with water to 10mL,add 6Nammonium hydroxide until the precipitate first formed redissolves,then add 2mLeach of ammonium oxalate TSand dibasic sodium phosphate TS:not more than a slight turbidity is produced in 5minutes. Assay— Weigh accurately in a porcelain crucible about 700mg of Ointment,heat gently until melted,and continue the heating,gradually raising the temperature until the mass is thoroughly charred.Ignite the mass until the residue is uniformly yellow,and cool.Dissolve the residue in 10mLof 2Nsulfuric acid,warming if necessary to effect complete solution,transfer the solution to a beaker,and rinse the crucible with small portions of water until the combined solution and rinsings measure 50mL.Add 15mLof ammonia–ammonium chloride buffer TSand 1mLof eriochrome black TS,and titrate with 0.05Medetate disodium VSuntil the solution is blue in color.Each mLof 0.05Medetate disodium is equivalent to 4.069mg of ZnO. Auxiliary Information— Staff Liaison:Lawrence Evans,III,Ph.D.,Scientist Expert Committee:(PA6)Pharmaceutical Analysis 6 USP28–NF23Page 2055 Phone Number:1-301-816-8389