K2S2O5 222.33
Disulfurous acid, dipotassium salt.
Dipotassium pyrosulfite [16731-55-8].
Potassium Metabisulfite contains an amount of K2S2O5 equivalent to not less than 51.8 percent and not more than 57.6 percent of SO2.
Identification— A solution (1 in 20) responds to the tests for Potassium and for Sulfite.
Iron— 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: the limit is 0.001%.
Heavy metals— Dissolve 2 g in 20 mL of water, add 5 mL of hydrochloric acid, evaporate on a steam bath to about 1 mL, and dissolve the residue in 25 mL of water: the limit is 0.001%.
K2S2O5 - 222.3 - 16731-55-8
DEFINITION
Potassium metabisulphite (potassium disulphite).
Content:
95.0 per cent to 101.0 per cent.
CHARACTERS
Appearance :
White or almost white powder, or colourless crystals.
Solubility:
Freely soluble in water, slightly soluble in ethanol (96 per cent).
IDENTIFICATION
A. pH (see Tests).
B. To 5 ml of solution S (see Tests), add 0.5 ml of 0.05 M iodine. The mixture is colourless and gives reaction of sulphates.
C. Solution S gives reaction of potassium.
TESTS
Solution S:
Dissolve 5.0 g in carbon dioxide-free water R and dilute to 100 ml with the same solvent.
Appearance of solution:
Solution S is clear and colourless.
pH:
3.0 to 4.5 for solution S.
Thiosulphates:
To 2.00 g add 25 ml of a 42.5 g/l solution of sodium hydroxide and 75 ml of water. Shake until dissolved and add 10 ml of formaldehyde and 10 ml of acetic acid. After 5 min, titrate with 0.05 M iodine using 1 ml of starch solution. Carry out a blank titration. The difference between the volumes consumed in the 2 titrations is maximum 0.15 ml.
Iron:
Maximum 10 ppm.
Selenium: Maximum 10 ppm.
Zinc: Maximum 25 ppm.
Heavy metals:
Maximum 10 ppm.
Potassium Pyrosulfite
K2S2O5 Formula wt 222.31
INS: 224 CAS: [16731-55-8]
DESCRIPTION
Potassium Metabisulfite occurs as white or colorless, free flowing crystals, crystalline powder, or granules. It gradually oxidizes in air to the sulfate. It is soluble in water, but it is insoluble in alcohol. Its solutions are acid to litmus.
REQUIREMENTS
Identification: A 1:10 aqueous solution gives positive tests for Potassium and for Sulfite.
Assay: Not less than 90.0% of K2S2O5.
Lead: Not more than 2 mg/kg.
Iron: Not more than 10 mg/kg.
Selenium: Not more than 5 mg/kg.
