
Iodine test is a qualitative test for detection
of polysaccharides. By this test we can also
differentiate between starch,dextrin and
glycogen.

This test depends upon the property of
adsorption possessed by the large
polysaccharide molecules. Helically coiled
polysaccharides adsorbs smaller molecules of
iodine on their surfaces to form a complex of ill
defined chemical nature. Atleast eight
monosachharide units should be joined in linear
chain to give the coloured complex. The
starch,iodine and glycogen iodine complexes are
blue,reddish purple and reddish brown
respectively.


The alkalis disrupt the secondary valencies of
polysaccharides,thus iodine is released from
their surface,therefore the colour disappears.
The strong acid restores the secondary
valencies and helps in adsorption of iodine
thus solution again gains its respective
colour.
The property of adsorption decreases on
heating. On heating the iodine and
polysaccharide complex dissociates and
therefore colour disappears. On cooling the
complex is reformed and the colour reappears
except in case of dextrin. It has been suggested
that heating produces a change on the surface
of dextrin molecules as a result of which they
loose the property of adsorption,therefore,
colour does not reappear on cooling.

2%
Lugol’s iodine solution: which consist of 2.5g
iodine and 7.5g of potassium iodide dissolved in
500ml distilled water. Iodine is added slowly untill a
deep yellow colour is produced.
 10% NaOH
 Conc. HCl
 Conc.H2SO4


Take 3ml of carbohydrate solution in a test
tube. Acidify it with one drop of conc. HCl.
Now add one small drop of iodine solution in
this test tube and mix it. Excess of iodine may
give false result so iodine should be added
carefully.




Starch gives a blue colour on addition of
iodine.
Dextrin gives a reddish purple colour on
addition of iodine.
Glycogen gives a reddish brown colour with
iodine.
No change in colour indicates the absence of
polysaccharide in the given solution.




Perform iodine test and add 10% NaOH drop
by drop till the colour disappears.
Now add conc. Sulphuric acid drop by drop till
colour reappears.
INTERPRETATION:
Iodine test is positive only in acidic medium.




Perform iodine test and heat the test tube till
the colour disappears.
Now cool the test tube till colour reappears.
INTERPRETATION:
Iodine test is negative at higher
temperaturesand positive at lower
temperatures. It is better to perform it at or
near room temperature.


This test should not be carried out in an
alkaline medium.
Iodine should not be added in excess.