SODIUM ALGINATE

Prepared at the 39th JECFA (1992), published in FNP 52 Add 1 (1992) superseding specifications published in the Compendium of Food Additive Specifications (1992)

SYNONYMS

INS No. 401

DEFINITION

Sodium alginate is the sodium salt of alginic acid, is a hydrophilic, colloidal substance.

C.A.S. number

9005-38-3

Chemical formula

(C₆ H₇ NaO₆)_n

Structural formula

Formula weight

Structural unit : 198.11 (theoretical), 222 (actual average)

Macromolecule : 32,000 - 250,000 (typical average)

Assay

Yields, on the dried basis, not less than 18.0% and not more than 21.0% of carbon dioxide (CO₂), equivalent to not less than 90.8% and not more than 106.0% of sodium alginate (C₆H₇ NaO₆)_n.

DESCRIPTION

Occurs as white to yellowish brown filamentous, grainy, granular or powdered forms

FUNCTIONAL USES

Stabilizer, thickener

CHARACTERISTICS

IDENTIFICATION

Solubility

Dissolves slowly in water, forming a viscous solution; insoluble in ethanol, ether and chloroform

Precipitate formation with calcium chloride

To a 0.5% solution of the sample in sodium hydroxide TS add one-fifth of its volume of a 2.5% solution of calcium chloride. A voluminous, gelatinous precipitate is formed. This test distinguishes ammonium alginate from gum arabic, sodium carboxymethyl cellulose, carrageenan, gelatin, gum ghatti, karaya gum, carob bean gum, methyl cellulose and tragacanth gum.

Precipitate formation with ammonium sulfate

To a 0.5% solution of the sample in sodium hydroxide TS add one-half of its volume of a saturated solution of ammonium sulfate. No precipitate is formed. This test distinguishes ammonium alginate from agar, sodium carboxymethyl cellulose, carrageenan, de-esterified pectin, gelatin, carob bean gum, methyl cellulose and starch.

Colour reaction

Passes test

Dissolve as completely as possible 0.01 g of the sample by shaking with 0.15 ml of 0.1 N sodium hydroxide and add 1 ml of acid ferric sulfate TS. Within 5 min, a cherry-red colour develops that finally becomes deep purple.

Test for sodium

Passes test

Dissolve the sulfated ash of the sample in dilute acetic acid TS and filter. Add to the filtrate uranyl zinc acetate TS. A yellow, crystalline precipitate is formed within a few minutes.

PURITY

Loss on drying

Not more than 15% (105^o, 4 h)

Phosphate

Not detectable

To an 0.5% aqueous solution of the sample add one-fifth of its volume of 4 N nitric acid and 1 volume of ammonium molybdate TS and warm. No yellow precipitate should be formed.

Water-insoluble matter

Not more than 1% on the dried basis

Disperse 2 g of the sample, weighed to the nearest 0.1 mg, in 800 ml of water in a 2,000-ml flask. Neutralize to pH 7 with sodium hydroxide TS and then add 3 ml in excess. Cover the flask, heat to boiling, and boil for 1 h with frequent stirring. Filter while hot through a tared Gooch crucible provided with a glass fibre filter (2.4-cm, No. 934AH, Reeve Angel & Co., Clifton, N.Y., or equivalent filter). If slow filtration is caused by high viscosity of the sample solution, boil until the viscosity is reduced enough to permit filtration. Wash the crucible thoroughly with hot water, dry the crucible and its contents at 105o for 1 h, cool and weigh. Calculate as percentage of the dry weight.

Total ash

Not less than 18% and not more than 27% on the dried basis

Arsenic

Not more than 3 mg/kg (Method II)

Lead

Not more than 10 mg/kg

Heavy metals

Not more than 40 mg/kg

Test 0.5 g of the sample as directed in the Limit Test (Method II)

METHOD OF ASSAY

Proceed as directed under Carbon Dioxide Determination by Decarboxylation. Each ml of 0.25 N sodium hydroxide consumed is equivalent to 5.5 mg of carbon dioxide (CO₂) or 27.75 mg of sodium alginate (equivalent weight 222).