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 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 C
9H
9HgNaO
2S in each 100 mL of Tincture taken by the formula:
50C,
in which the terms are as defined therein.
467
:
meets the requirements.