Potassium Metabisulfite
(poe tas' ee um met'' a bye sul' fite).

K2S2O5 222.33
Disulfurous acid, dipotassium salt;    
Dipotassium pyrosulfite     [16731-55-8].
Potassium Metabisulfite contains an amount of K2S2O5 equivalent to NLT 51.8% and NMT 57.6% of SO2.
•  A. Identification Tests—General, Potassium 191 and Sulfite 191: A solution (1 in 20) meets the requirements.
•  Procedure
Sample:  250 mg of Potassium Metabisulfite
Titrimetric system 
Mode:  Residual titration
Titrant:  0.1 N iodine VS
Back titrant:  0.1 N sodium thiosulfate VS
Endpoint detection:  Visual
Analysis:  Transfer the Sample to a glass-stoppered conical flask containing 50.0 mL of 0.1 N iodine VS, and swirl to dissolve. Allow to stand for 5 min, protected from light, and then add 1 mL of hydrochloric acid. Titrate the excess iodine with 0.1 N sodium thiosulfate VS, adding 3 mL of starch TS as the endpoint is approached.
Calculate the percentage of SO2 in the portion of Sample taken:
Result = {[(VB VS) × N × F ]/W} × 100
VB== Back titrant volume consumed by the Blank (mL)
VS== Back titrant volume consumed by the Sample (mL)
N== Back titrant actual normality (mEq/mL)
F== equivalency factor, 32.03 mg/mEq
W== weight of the Sample (mg)
Acceptance criteria:  51.8%–57.6% of SO2
•  Iron 241
Test preparation:  Dissolve 1.00 g in 14 mL of dilute hydrochloric acid (2 in 7), and evaporate on a steam bath to dryness. Dissolve the residue in 7 mL of dilute hydrochloric acid (2 in 7), and again evaporate to dryness. Dissolve the resulting residue in a mixture of 2 mL of hydrochloric acid and 20 mL of water. Add 3 drops of bromine TS, and boil to expel the bromine. Cool, and dilute with water to 47 mL.
Acceptance criteria:  NMT 10 ppm
•  Heavy Metals, Method I 231
Test preparation:  Dilute 2 g in 20 mL of water, add 5 mL of hydrochloric acid, and evaporate on a steam bath to 1 mL. Dissolve the residue in 25 mL of water.
Acceptance criteria:  NMT 10 ppm
•  Packaging and Storage: Preserve in well-fitted, tight containers, and avoid exposure to excessive heat.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, M.S.
Scientific Liaison
(EXC2010) Monographs - Excipients
USP35–NF30 Page 1928