Sulfur Dioxide
SO2 64.06

Sulfur dioxide.
Sulfur dioxide [7446-09-5].
»Sulfur Dioxide contains not less than 97.0percent,by volume,of SO2.
[Caution—Sulfur Dioxide is poisonous. ]
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 Iá921ñ— Taking precautions to avoid absorption of moisture,transfer 3g (about 2.1mL)to a suitable flask,and add 20mLof anhydrous pyridine:not more than 2.0%is found.
Limit of nonvolatile residue— Transfer 300g (about 209mL)to a tared,250-mLconical 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 7.5mg (0.0025%).
Sulfuric acid— To the flask containing the residue obtained in the test for Nonvolatile residueadd 25mLof water previously neutralized to methyl red TS.Swirl the flask,and titrate with 0.10Nsodium hydroxide:not more than 1.3mLis required (about 0.002%).
Assay— Collect 100.0mLof gaseous Sulfur Dioxide over mercury,and note the temperature of the sample and the pressure upon it.Slowly introduce 50.0mLof 0.1Nsodium 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-mLconical flask,add 3mLof starch TS,and titrate with 0.1Niodine VSuntil the solution is pale blue in color.Each mLof 0.1Niodine is equivalent to 1.094mLof SO2at a temperature of 0and a pressure of 760mm of mercury.
Auxiliary Information— Staff Liaison:Catherine Sheehan,B.Sc.,Scientist
Expert Committee:(EMC)Excipients:Monograph Content
USP28–NF23Page 3096
Phone Number:1-301-816-8262