SODIUM THIOSULFATE

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 sulfites established at the 51st JECFA in 1998.

SYNONYMS

Sodium hyposulfite; INS No. 539

DEFINITION

Chemical names

Sodium thiosulfate

C.A.S. number

7772-98-7

Chemical formula

Na₂S₂O₃ � 5H₂O

Formula weight

248.17

Assay

Not less than 99.0% on the dried basis

DESCRIPTION

Colourless crystals or coarse crystalline powder; deliquesces in moist air and effloresces in dry air above 33�

FUNCTIONAL USES

Antibrowning agent, antioxidant, sequestrant

CHARACTERISTICS

IDENTIFICATION

Solubility (FNP 5)

Very soluble in water; insoluble in ethanol

Reducing activity

To a 1 in 10 solution of the sample add a few drops of iodine TS; the colour is discharged

Test for sodium (FNP 5)

Passes test

Test for thiosulfate (FNP 5)

Passes test

PURITY

Loss on drying (FNP 5)

32-37% (40-45�, 16 h, under vacuum)

Lead (FNP 5)

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

Iron (FNP 5)

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

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

Dissolve about 0.5 g of the dried sample, accurately weighed, in 30 ml of water and titrate with 0.1 N iodine solution using starch TS as the indicator. Each ml of 0.1 N iodine is equivalent to 15.81 mg of Na₂S₂O₃.

	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 sulfites established at the 51st JECFA in 1998.
SYNONYMS	Sodium hyposulfite; INS No. 539
DEFINITION
Chemical names	Sodium thiosulfate
C.A.S. number	7772-98-7
Chemical formula	Na₂S₂O₃ � 5H₂O
Formula weight	248.17
Assay	Not less than 99.0% on the dried basis
DESCRIPTION	Colourless crystals or coarse crystalline powder; deliquesces in moist air and effloresces in dry air above 33�
FUNCTIONAL USES	Antibrowning agent, antioxidant, sequestrant
CHARACTERISTICS
IDENTIFICATION
Solubility (FNP 5)	Very soluble in water; insoluble in ethanol
Reducing activity	To a 1 in 10 solution of the sample add a few drops of iodine TS; the colour is discharged
Test for sodium (FNP 5)	Passes test
Test for thiosulfate (FNP 5)	Passes test
PURITY
Loss on drying (FNP 5)	32-37% (40-45�, 16 h, under vacuum)
Lead (FNP 5)	Not more than 2 mg/kg Determine by atomic absorption spectroscopy (FNP 5)
Iron (FNP 5)	Not more than 10 mg/kg Proceed as directed in the Limit Test using 0.5 ml of Iron Standard Solution (5 �g Fe) in the control
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	Dissolve about 0.5 g of the dried sample, accurately weighed, in 30 ml of water and titrate with 0.1 N iodine solution using starch TS as the indicator. Each ml of 0.1 N iodine is equivalent to 15.81 mg of Na₂S₂O₃.