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— Weigh accurately a filled Topical Aerosol container, and record the weight. Place the container in a dry ice-alcohol bath, and cool for 60 minutes. Remove the container from the bath, and carefully remove the spray cap with wire cutters, taking precautions to save all pieces of the spray head and cap. With the aid of three 5-mL portions of water, transfer the contents of the container to a beaker previously chilled in the bath. Dry the rinsed empty container and all of its parts in an oven at 105 for 2 hours, cool, and weigh. Calculate the weight of the container contents. Add a few boiling chips to the beaker, and carefully stir to help evaporate the propellant. After the bulk of the propellant has evaporated, place the beaker on a steam bath, evaporate the volatile solvents, and cool. Transfer the residual liquid with the aid of 35.0 mL of alcohol to a 50-mL volumetric flask, dilute with water to volume, and mix. Dilute quantitatively with water a volume, v mL, of this solution to V mL of Assay solution containing about 2 µg of thimerosal per mL.

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 the weight of the container contents taken by the formula: in which the terms are as defined therein.