Packaging and storage—
Preserve in cylinders.
NOTE—Sulfur Dioxide is used most in the form of a gas in pharmaceutical applications, and is described herein for such purposes. However, it is usually packaged under pressure, hence the following specifications are designed for testing it in liquid form.
Water, Method I 
921
—
Taking precautions to avoid absorption of moisture, transfer 3 g (about 2.1 mL) to a suitable flask, and add 20 mL of anhydrous pyridine: not more than 2.0% is found.
Limit of nonvolatile residue—
Transfer 300 g (about 209 mL) to a tared, 250-mL conical flask, and allow the liquid to evaporate spontaneously in a well-ventilated hood. When evaporation appears complete, blow a current of dry, filtered air through the flask until the odor of sulfur dioxide is no longer apparent: the weight of the residue does not exceed 7.5 mg (0.0025%).
Sulfuric acid—
To the flask containing the residue obtained in the test for
Nonvolatile residue add 25 mL of water previously neutralized to
methyl red TS. Swirl the flask, and titrate with 0.10 N sodium hydroxide: not more than 1.3 mL is required (about 0.002%).
Assay—
Collect 100.0 mL of gaseous Sulfur Dioxide over mercury, and note the temperature of the sample and the pressure upon it. Slowly introduce 50.0 mL of 0.1 N sodium hydroxide into the air space over the mercury, and absorb the sample in the solution by shaking. When absorption is complete, transfer the solution to a 250-mL conical flask, add 3 mL of starch TS, and titrate with 0.1 N iodine VS until the solution is pale blue in color. Each mL of 0.1 N iodine is equivalent to 1.094 mL of SO
2 at a temperature of 0
and a pressure of 760 mm of mercury.
