Salicylic Acid Plaster
» Salicylic Acid Plaster is a uniform mixture of Salicylic Acid in a suitable base, spread on paper, cotton cloth, or other suitable backing material. The plaster mass contains not less than 90.0 percent and not more than 110.0 percent of the labeled amount of C7H6O3.
Packaging and storage Preserve in well-closed containers, preferably at controlled room temperature.
Assay Weigh accurately an amount of Plaster, corresponding to about 500 mg of salicylic acid, cut the portion into small strips, place them in a small flask, add 50 mL of chloroform, and shake the mixture until the plaster mass is disintegrated. Decant the chloroform extract into a 250-mL beaker, and wash the plaster backing with two 25-mL portions of chloroform, receiving the washings in the same beaker. Then wash the backing with 50 mL of alcohol to which has been added 1 mL of 6 N ammonium hydroxide, and add the washing to the chloroform extract. Again wash the backing with 40 mL of alcohol, and add the washing to the chloroform extract. Dry the backing, weigh, and subtract the weight from the weight of Plaster taken for the assay to obtain the weight of plaster mass. Stir the chloroform extract until any coagulum has separated into a compact mass, and filter the extract through purified cotton into a separator. Knead the coagulum, if any, with a glass rod to expel the solvent, and rinse the coagulum and the beaker with 10 mL of alcohol. Pour the rinsing through the cotton, then press the cotton with a glass rod to expel the solvent. Extract the filtrate with three 10-mL portions of 1 N sodium hydroxide, drawing off each portion into a 500-mL volumetric flask, and finally wash with two 25-mL portions of water, receiving the washings in the same flask. Dilute with water to volume, and pipet a 25-mL aliquot into a 500-mL iodine flask. Add 30.0 mL of 0.1 N bromine VS, then add 5 mL of hydrochloric acid, and immediately insert the stopper. Shake the flask repeatedly during 30 minutes, allow it to stand for 15 minutes, add quickly 5 mL of potassium iodide solution (1 in 5), taking precautions against the escape of bromine vapor, and at once insert the stopper in the flask. Shake thoroughly, remove the stopper, and rinse it and the neck of the flask with a small quantity of water, so that the washing flows into the flask. Add 1 mL of chloroform, shake the mixture, and titrate the liberated iodine with 0.1 N sodium thiosulfate VS, adding 3 mL of starch TS as the endpoint is approached. Perform a blank determination (see Residual Titrations under Titrimetry 541). Each mL of 0.1 N bromine is equivalent to 2.302 mg of C7H6O3.