Zinc Oxide and Salicylic Acid Paste »Zinc Oxide and Salicylic Acid Paste contains not less than 23.5percent and not more than 25.5percent of zinc oxide (ZnO),and not less than 1.9percent and not more than 2.1percent of salicylic acid (C7H6O3). It may be prepared as follows: Salicylic Acid,in fine powder 20g Zinc Oxide Paste,a sufficient quantity to make 1000g Thoroughly triturate the Salicylic Acid with a portion of the paste,then add the remaining paste,and triturate until a smooth mixture is obtained. Packaging and storage— Preserve in well-closed containers. Identification— A: The residue obtained in the Assay for zinc oxideis yellow when hot and white when cool. B: Shake 1g of it with 10mLof water,and filter.To the filtrate add 1mLof ferric chloride TS:an intense reddish violet color is produced.To this solution add 1mLof acetic acid:the color is not dispersed.To this solution add 2mLof 2Nhydrochloric acid:the color is dispersed and a white crystalline precipitate is formed. Minimum fill á755ñ: meets the requirements. Assay for zinc oxide— Weigh accurately in a tared porcelain crucible about 500mg of Paste,heat gently until melted,and continue the heating,gradually raising the temperature until the mass is thoroughly charred.Ignite the mass strongly until all of the carbonaceous material has been dissipated,the residue is uniformly yellow,and the weight of the cooled residue is constant.The weight of the residue represents the quantity of ZnOin the weight of the Paste taken for the assay. Assay for salicylic acid— Transfer to a suitable beaker about 5g of Paste,accurately weighed.Add 40mLof alcohol,previously neutralized with 0.1Nsodium hydroxide to a phenol red endpoint,and heat on a water bath for 5minutes,with frequent swirling.While still hot,add phenol red TS,and titrate with 0.1Nsodium hydroxide VSto a red endpoint.Each mLof 0.1Nsodium hydroxide is equivalent to 13.81mg of salicylic acid (C7H6O3). 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