» Thimerosal Tincture contains, in each 100 mL, not less than 90 mg and not more than 110 mg of C9H9HgNaO2S.
[noteThimerosal Tincture is sensitive to some metals. ]
Packaging and storage Preserve in tight, light-resistant containers, and avoid exposure to excessive heat.
USP Reference standards 11
Identification Heat 25 mL on 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 50 mL of Tincture on a steam bath to a volume of approximately 20 mL, cool, and add 3 or 4 drops of bromine. Add 5 mL of 3 N hydrochloric acid, filter, and pass hydrogen sulfide through the filtrate: a black precipitate is formed.
Alcohol content, Method II 611: between 45.0% and 55.0% of C2H5OH.
Assay [noteThe 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 50 g of stannous chloride in 100 mL of hydrochloric acid on a steam bath, cool, dilute with water to 500 mL, and mix. Use within 3 months.
Standard solutions Prepare aqueous solutions of USP Thimerosal RS of known concentrations of about 1.8, 2.0, and 2.2 µg per mL.
Standard preparations Pipet 20 mL of each Standard solution into separate 100-mL volumetric flasks, and treat each flask as follows. Add 5 mL of sulfuric acid, cool, add 3 mL of nitric acid, and mix. Add potassium permanganate crystals, while mixing, until the purple color persists for not less than 15 minutes. Add about 200 mg of potassium persulfate, mix, and heat on a steam bath for 2 hours. Cool, dilute with water to volume, and mix.
Assay solution Pipet 2 mL of Tincture into a 1000-mL volumetric flask, dilute with water to volume, and mix.
Assay preparation Pipet 20 mL of Assay solution into a 100-mL volumetric flask, and proceed as directed for Standard preparations, beginning with Add 5 mL of sulfuric acid.
Blank preparation Pipet 20 mL of water into a 100-mL volumetric flask, and proceed as directed for Standard preparation, beginning with Add 5 mL of sulfuric acid.
Procedure Proceed with each of the Standard preparations, the Assay preparation, and the Blank preparation as follows: Pipet 3 mL into 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 150 mL, and add hydroxylamine hydrochloride solution (1 in 10) just to reduce the excess permanganate. Add 5 mL of 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 15 seconds. Use the absorbance of the Blank preparation to correct the absorbances of the Standard preparations and 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 C9H9HgNaO2S in each 100 mL of Tincture taken by the formula:
50Cin which the terms are as defined therein.
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USP35NF30 Page 4843