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POTASSIUM METABISULFITE

 

Prepared at the 53rd JECFA (1999) and published in FNP 52 Add 7 (1999), superseding tentative specifications prepared at the 51st JECFA (1998), published in FNP 52 Add 6 (1998). Group ADI 0-0.7 mg/kg bw as SO2 for sulfite established at the 51st JECFA in 1998.

SYNONYMS

INS No. 224

DEFINITION

 

Chemical names

Potassium disulfite, potassium pentaoxodisulfate, potassium pyrosulfite

C.A.S. number

16731-55-8

Chemical formula

K2S2O5

Formula weight

222.33

Assay

Not less than 90%

DESCRIPTION

Colourless free-flowing crystals, crystalline powder, or granules, usually having an odour of sulfur dioxide

FUNCTIONAL USES

Antibrowning agent, antioxidant, preservative

CHARACTERISTICS

 

IDENTIFICATION

 

Solubility (FNP 5)

Soluble in water; insoluble in ethanol

Test for potassium (FNP 5)

Passes test

Test for sulfite (FNP 5)

Passes test

PURITY

 

Water insolubles

Dissolve 20 g of the sample in 200 ml of water. The solution should be clear with only a trace of suspended matter.

Thiosulfate

Not more than 0.1%
A 10% solution of the sample should remain clear on acidification with sulfuric or hydrochloric acid

Iron (FNP 5)

Not more than 10 mg/kg
Determine as directed in the Limit Test using 0.5 ml of Iron Standard Solution (5 g Fe) in the control

Lead (FNP 5)

Not more than 2 mg/kg
Determine by atomic absorption spectroscopy (FNP 5)

Selenium

Not more than 5 mg/kg
See description under TESTS

TESTS

 

PURITY TESTS

 

Selenium

Reagents: Hydrochloric acid, hydrazinium sulfate, standard selenium solution (100 g Se/ml)

Procedure

Weigh 2.0 0.1 g of sample and transfer to a 50-ml beaker. Add 10 ml water, 5 ml hydrochloric acid and boil to remove SO2.
Into a second beaker, weigh 1.0 0.1 g of sample, add 0.05 ml standard selenium solution and proceed as above.
To each beaker add 2 g hydrazinium sulfate and warm to dissolve. Let stand for 5 min. Dilute the contents of each beaker to 50 ml in a Nessler tube and compare the colour of the two solutions. The sample should be less pink than the sample with the added standard.

METHOD OF ASSAY

Weigh 250 mg of the sample, add to 50.0 ml of 0.1 N iodine in a glass stoppered flask, and stopper the flask. Allow to stand for 5 min, add 1 ml of dilute hydrochloric acid TS and titrate the excess iodine with 0.1 N sodium thiosulfate, using starch TS as the indicator. Each ml of 0.1 N iodine is equivalent to 5.558 mg of K2S2O5.

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