Potassium Metabisulfite
K2S2O5 222.33
Disulfurous acid, dipotassium salt;
Dipotassium pyrosulfite
[16731-55-8].
(poe tas' ee um met'' a bye sul' fite).
K2S2O5 222.33
Disulfurous acid, dipotassium salt;
Dipotassium pyrosulfite
[16731-55-8].DEFINITION
Potassium Metabisulfite contains an amount of K2S2O5 equivalent to NLT 51.8% and NMT 57.6% of SO2.
IDENTIFICATION
• A. Identification Tests—General, Potassium 
191
and Sulfite 191:
A solution (1 in 20) meets the requirements.
191 and Sulfite 191:
A solution (1 in 20) meets the requirements.
ASSAY
• Procedure
Sample:
250 mg of Potassium Metabisulfite
Titrimetric system
(See Titrimetry 541.)
541.)
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
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
IMPURITIES
• Iron 241
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
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
ADDITIONAL REQUIREMENTS
• 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 1-301-816-8335 |
(EXC2010) Monographs - Excipients |
USP35–NF30 Page 1928