Thimerosal Tincture »Thimerosal Tincture contains,in each 100mL,not less than 90mg and not more than 110mg of C9H9HgNaO2S. NOTE—Thimerosal Tincture 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— Heat 25mLon a steam bath until the odors of alcohol and acetone are no longer perceptible.Cool and pass hydrogen sulfide through the solution:no black discoloration or black precipitate is formed.Evaporate 50mLof Tincture on a steam bath to a volume of approximately 20mL,cool,and add 3or 4drops of bromine.Add 5mLof 3Nhydrochloric acid,filter,and pass hydrogen sulfide through the filtrate:a black precipitate is formed. Alcohol content,Method IIá611ñ: between 45.0%and 55.0%of C2H5OH. 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 Tincture 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 the vapor from 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 Tincture 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