Page 311 - Pharmacognosy 2 PG303

Pharmacognosy-2 (PG303) Level 2 Clinical Pharmacy-Pharm D

Tests for Identity
1- Cooling of hot solution: Boil 1 g of Agar, for 10 mins, with 100 ml of water,

replacing the water lost by evaporation; the solution yields a stiff jelly on cooling.
2- Microscopical examination: Boil 0.5 g of Agar with 10 ml of HCl for 10 mins;

set aside for 10 mins and decant; examine the residue microscopically; sand
particles, sponge spicules and diatoms especially those of disc shaped of
Arachnoidiscus ehrenbergii Baill. are found.
3- Tannic acid test: To a nearly boiling 0.2 % w/v solution of Agar, add tannic acid
T.S.; no precipitate is formed (c.f. Gelatine) and on cooling, the solution becomes
turbid.
4- Barium chloride test: Dissolve 0.1 g of Agar by boiling for 5 mins in 10 ml of
water, add 1 ml of HCl and heat for 30 mins on a water-bath, then add 1 ml of
BaCl2 T.S.; a white precipitate is formed (c.f. Tragacanth).
5- Rhuthenium red test: Mix a little of powdered Agar with a drop of rhuthenium
red T.S.; the particles are stained deep red.
6- Iodine test: Mix a little of powdered Agar with a drop of iodine T.S.; some
particles are coloured reddish but not blue (c.f. Starch).

Constituents
Agar consists chiefly of a calcium salt of a sulphuric ester of a carbohydrate

complex. When hydrolysed by boiling with dilute HCl, it yields a limpid liquid
containing galactose and sulphuric acid (c.f. Gelatine and similar substances). Agar
contains traces of protein insufficient to yield any evidence of ammonia when agar
is heated with dry soda-lime (c.f. gelatine and albumin).

Uses
Agar is classified as a suspending agent. It has been used as a bulk laxative.

It is extensively used as a gel in bacteriological culture media. In industry, it is
employed for making emulsions, for thickening ice cream, for adhesives, for
photographic emulsions, and for sizing textiles especially silks.

Gelatin

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