SODIUM ALGINATE

Prepared at the 17th JECFA (1973), published in FNP 4 (1978) and in FNP 52 (1992)

SYNONYMS

INS No. 401

DEFINITION

Sodium Alginate, the sodium salt of alginic acid, is a hydrophilic, colloidal substance. The alginic acid is a linear, high polymer consisting mainly of ß-(1→4) linked D-mannuronic acid in the pyranose ring form, with part of the mannuronic acid replaced by L-guluronic acid.

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

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 in filamentous, grainy, granular, and powdered forms. It is colourless or slightly yellow and may have a slight characteristic smell and taste

FUNCTIONAL USES

Stabilizer, thickener

CHARACTERISTICS

IDENTIFICATION

Solubility

Slowly soluble forming a viscous solution in water; insoluble in ethanol, ether and chloroform

Specific rotation

Clarify an 0.5% solution of the sample in sodium hydroxide TS with kieselguhr, and determine the rotation in a 20-cm tube. The specific rotation is not less than -0.8o at 20o

Precipitate formation with calcium chloride

Passes test

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

Passes test

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

Moisten 1-5 mg of the sample with water, 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

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)

pH

6.0 -8.0 (1 in 100 soln)

Phosphate

Not detectable

See description under TESTS

Water-insoluble matter

Not more than 1% on the dried basis

See description under TESTS

Total ash

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

Weigh 3 g of the sample to the nearest 0.1 mg in a tared crucible. Ignite at a low temperature (about 600o), not to exceed a very dull redness, until free from carbon, cool in a desiccator, and weigh. If a carbon-free ash is not obtained, wet the charred mass with hot water, collect the insoluble residue on an ashless filter paper, and ignite, in the crucible, the residue and filter paper until the ash is white or nearly so. Finally, add the filtrate, evaporate it to dryness, and heat the whole to a dull redness. If a carbon-free ash is still not obtained, cool the crucible, add 15 ml of ethanol, break up the ash with a glass rod, then burn off the ethanol, again heat the whole to a dull redness, cool and weigh. Calculate as percentage of the dry weight.

Arsenic

Not more than 3 mg/kg (Method II)

A sample of solution prepared as directed for organic compounds will meet the requirements of the Limit Test for Arsenic

Lead

Not more than 10 mg/kg

A sample solution prepared as directed for organic compounds will meet the requirements of the Limit Test for Lead

Heavy metals

Not more than 40 mg/kg

Test 0.5 g of the sample as directed in Method II under The Limit Test for Heavy metals using 20 ìg of lead ion (Pb) in the control (Solution A)

TESTS

PURITY TESTS

Water-insoluble matter

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. Add 40 ml of hydrogen peroxide solution containing 30% by weight H₂O₂, cover the flask 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. 934 AH, 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 105^o for 1 h, cool and weigh. Calculate as percentage of the dry weight.

METHOD OF ASSAY

Decarboxylation method

Proceed as directed under Carbon Dioxide Determination by Decarboxylation in the General Methods. Each ml of 0.25 N sodium hydroxide consumed is equivalent to 5.5 mg of carbon dioxde (CO2) or 27.75 mg of sodium alginate (equivalent weight 212.00).

Gravimetric method

Dissolve 0.500 g of the sample in 10 ml of sodium hydroxide TS, add 90 ml of water, and filter if necessary. Add 15 ml of 4 N hydrochloric acid and 100 ml of 90% v/v ethanol. Allow this mixture to stand for 2 h, decant the supernatant liquid as far as possible, and centrifuge. Decant the liquid and replace it by 90% v/v ethanol. Mix well, centrifuge and decant again. This washing is repeated until the hydrochloric acid is removed. Then transfer the precipitate by means of 90% v/v ethanol to a fine glass filter, wash with dry acetone, place the filter in a vacuum desiccator, and dry to constant weight at 100o.

Calculate the percent purity of the sample by the formula:

% Purity = (200 x F x W) / % dry substance in sample x 100

in which F is a conversion factor specified as 1.125 and W is the weight (in grams) of the dried precipitate.