Mercury ions—
Iodide reagent—
[NOTE—Prepare fresh daily; keep the stopper in the flask and protect from light.] Dissolve 33.20 g of potassium iodide in 75 mL of water in a 100-mL volumetric flask, dilute with water to volume, and mix.
Standard preparation—
Transfer about 190 mg of mercuric chloride, accurately weighed, to a 200-mL volumetric flask, and dissolve in 100 mL of water. Dilute with water to volume, and mix. Transfer 5.0 mL of this solution to a 50-mL volumetric flask, dilute with water to volume, and mix. The concentration of mercuric chloride in the Standard preparation is about 95 µg per mL.
Test solution—
Transfer about 500 mg of Thimerosal, accurately weighed, to a 100-mL volumetric flask, add water to volume, and mix to dissolve.
Test preparation A—
Transfer 10.0 mL of the Test solution to a 50-mL volumetric flask, dilute with water to volume, and mix.
Test preparation B—
Transfer 10.0 mL of the Test solution to a 50-mL volumetric flask, add 5.0 mL of the Standard preparation, dilute with water to volume, and mix.
Procedure—
[NOTE—Protect all solutions from light prior to determining their absorbances.
] Label five 10-mL volumetric flasks
C,
D,
E,
F, and
R. Transfer 5.0 mL of
Test preparation A to flasks
C and
D, transfer 5.0 mL of
Test preparation B to flasks
E and
F, and transfer 5.0 mL of water to flask
R. Dilute flasks
C and
E with water to volume, and mix. Dilute flasks
D,
F, and
R with the
Iodide reagent to volume, and mix. Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance for the tetraiodomercurate ion at about 323 nm (located from the similarly determined UV spectrum of a solution prepared by mixing 1.0 mL of
Standard preparation with 5.0 mL of
Iodide reagent and diluting with water to 10.0 mL), with a suitable spectrophotometer, using water as the blank. Record the absorbances of the solutions in flasks
C,
D,
E,
F, and
R as
AC,
AD,
AE,
AF, and
AR, respectively. Calculate the percentage of mercury ions by the formula:
(200.59 / 271.50)(5C / W)(AU / AS),
in which 200.59 is the atomic weight of mercury, 271.50 is the molecular weight of mercuric chloride,
AU is the absorbance of the test specimen obtained by the equation:
AU = (
AD 
AR AC),
AS is the absorbance of the Standard obtained by the equation:
C is the concentration, in µg per mL, of mercuric chloride in the
Standard preparation, and
W is the weight, in mg, of Thimerosal taken: the limit is 0.70%.
Assay—
[NOTE—The
Standard preparation and the
Assay preparation may be diluted quantitatively with water, if necessary, to obtain solutions of suitable concentrations adaptable to the linear or working range of the instrument.
]
Standard preparation—
Dissolve an accurately weighed quantity of
USP Thimerosal RS in water, and dilute quantitatively with water to obtain a solution having a known concentration of about 1 mg per mL.
Assay preparation—
Transfer about 100 mg of Thimerosal, accurately weighed, to a 100-mL volumetric flask, add 25 mL of water, and mix to dissolve. Dilute with water to volume, and mix.
Procedure—
Concomitantly determine the absorbances of the
Standard preparation and the
Assay preparation, at the mercury resonance line of 254 nm, with a suitable atomic absorption spectrophotometer (see
Spectrophotometry and Light-Scattering 
851
), equipped with a mercury hollow-cathode lamp and an air–acetylene flame, using water as the blank. Calculate the quantity, in mg, of C
9H
9HgNaO
2S in the portion of Thimerosal taken by the formula:
100C(AU / AS),
in which
C is the concentration, in mg per mL, of
USP Thimerosal RS in the
Standard preparation; and
AU and
AS are the absorbances of the
Assay preparation and the
Standard preparation, respectively.
;)
