Botanic characteristics—
Agar—
Usually in bundles consisting of thin, membranous, agglutinated strips or in cut, flaked, or granulated forms. May be weak yellowish orange, yellowish gray to pale yellow, or colorless. Is tough when damp, brittle when dry.
Histology—
In water mounts Agar appears granular and somewhat filamentous; a few fragments of the spicules of sponges and a few frustules of diatoms may be present; in Japanese Agar, the frustules of Arachnoidiscus ehrenbergii Baillon often occur, being disk-shaped and from 100 µm to 300 µm in diameter.
Powdered Agar—
White to yellowish white or pale yellow; in
chloral hydrate TS its fragments are transparent, more or less granular, striated, and angular, and occasionally they contain frustules of diatoms.
Identification—
A:
Iodine TS colors some of the fragments of Agar bluish black, with some areas reddish to violet.
B:
When boiled with 65 times its weight of water for 10 minutes, with constant stirring, and adjusted to a concentration of 1.5%, by weight, with hot water, Agar forms a clear liquid which congeals at 32
to 39
to form a firm resilient gel, which does not melt below 85
.
Water, Method III 
921
—
If necessary, cut it into pieces from 2 mm to 5 mm square, and dry at 105
for 5 hours: it loses not more than 20.0% of its weight.
Limit of foreign insoluble matter—
To 7.5 g add sufficient water to make 500 g, boil for 15 minutes, and readjust to the original 500 g. To 100 g of the uniformly mixed material add hot water to make 200 mL, heat almost to boiling, filter while hot through a tared filtering crucible, rinse the container with several portions of hot water, and pass these rinsings through the crucible. Dry the crucible and its contents at 105
to constant weight: not more than 15 mg (1.0%) remains.
Limit of foreign starch—
A solution made by boiling 0.10 g of it in 100 mL of water does not, upon cooling, produce a blue color upon the addition of
iodine TS.
Gelatin—
Dissolve about 1 g in 100 mL of boiling water, and allow to cool to about 50
. To 5 mL of the solution add 2 to 3 drops of a mixture of 0.2
M potassium dichromate solution and 3 N hydrochloric acid (4:1): no yellow precipitate is formed.
Water absorption—
Place 5.0 g in a 100-mL graduated cylinder, fill to the mark with water, mix, and allow to stand at 25
for 24 hours. Pour the contents of the cylinder through moistened glass wool, allowing the water to drain into a second 100-mL graduated cylinder: not more than 75 mL of water is obtained.
