Thimerosal Topical Solution »Thimerosal Topical Solution contains,in each 100mL,not less than 95mg and not more than 105mg of C9H9HgNaO2S. NOTE—Thimerosal Topical Solution is sensitive to some metals. Packaging and storage— Preserve in tight,light-resistant containers,and avoid exposure to excessive heat. USP Reference standards á11ñ USP Thimerosal RS. Identification— A: Pass hydrogen sulfide through 50mLof it:no black discoloration or black precipitate is formed.To 50mLof Topical Solution add 3or 4drops of bromine,mix,and warm on a steam bath to expel the excess bromine.Add 5mLof 3Nhydrochloric acid,filter,and pass hydrogen sulfide through the filtrate:a black precipitate is formed. B: To 1mLof Topical Solution add 9mLof water,mix,and add 1mLof cupric sulfate TS:a green color is produced immediately and is followed by the gradual precipitation of flocculent,greenish brown particles. pHá791ñ: between 9.6and 10.2. Assay— [NOTE—The Standard preparations and Assay preparation may be diluted quantitatively with water,if necessary,to yield solutions,of suitable concentration,adaptable to the linear or working range of the instrument.] Stannous chloride solution— Dissolve 50g of stannous chloride in 100mLof hydrochloric acid on a steam bath,cool,dilute with water to 500mL,and mix.Use within 3months. Standard solutions— Prepare aqueous solutions of USP Thimerosal RSof known concentrations of about 1.8,2.0,and 2.2µg per mL. Standard preparations— Pipet 20mLof each Standard solutioninto separate 100-mLvolumetric flasks,and treat each flask as follows.Add 5mLof sulfuric acid,cool,add 3mLof nitric acid,and mix.Add potassium permanganate crystals,while mixing,until the purple color persists for not less than 15minutes.Add about 200mg of potassium persulfate,mix,and heat on a steam bath for 2hours.Cool,dilute with water to volume,and mix. Assay solution— Pipet 2mLof Topical Solution into a 1000-mLvolumetric flask,dilute with water to volume,and mix. Assay preparation— Pipet 20mLof Assay solutioninto a 100-mLvolumetric flask,and proceed as directed for Standard preparations,beginning with “Add 5mLof sulfuric acid.” Blank preparation— Pipet 20mLof water into a 100-mLvolumetric flask,and proceed as directed for Standard preparation,beginning with “Add 5mLof sulfuric acid.” Procedure— Proceed with each of the Standard preparations,the Assay preparation,and the Blank preparationas follows:Pipet 3mLinto the scrubbing chamber of a suitable system designed for determination of mercury by flameless atomic absorption,using a mercury hollow-cathode lamp,dilute with water to about 150mL,and add hydroxylamine hydrochloride solution (1in 10)just to reduce the excess permanganate.Add 5mLof Stannous chloride solution,and immediately attach the scrubbing chamber to the system.Concomitantly determine the absorbance of each solution at an integration time of 15seconds.Use the absorbance of the Blank preparationto correct the absorbances of the Standard preparationsand the Assay preparation.Plot the corrected absorbances of the standards versus the respective concentrations of the Standard solutions,in µg per mL,and from the curve so obtained determine the concentration,C,in µg per mL,of the Assay solution.Calculate the quantity,in mg,of C9H9HgNaO2Sin each 100mLof Topical Solution taken by the formula: 50C, in which the terms are as defined therein. Auxiliary Information— Staff Liaison:Behnam Davani,Ph.D.,MBA,Senior Scientist Expert Committee:(PA7)Pharmaceutical Analysis 7 USP28–NF23Page 1913 Phone Number:1-301-816-8394