pH (Vol. 4) 2.0-3.5 (0.3 in 10 suspension)
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 alginic acid from agar, sodium
carboxymethyl cellulose, carrageenan, de-esterified pectin, gelatin, carob
bean gum, methyl cellulose and starch.
Test for alginate Passes test
Dissolve as completely as possible 0.1 g of 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.