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Date: ……/..../….
PRACTICAL NO:- 3
PERFORM IODINE TEST
THEORY
Iodine test is a chemical test used to distinguish mono- or disaccharides from certain
polysaccharides like amylase, dextrin, and glycogen. This test has a variation termed starch-
iodine test that is performed to indicate the presence of glucose made by plants in the leaves.
OBJECTIVE
To detect the existence of polysaccharides, primarily starch.
PRINCIPLE
Polyiodide ions form colourful adsorption complexes with helical chains of glucose residue
in amylase (blue-black), dextrin (black), or glycogen (red) (reddish-brown).
Cellulose and other branching polysaccharides, like monosaccharides and disaccharides, are
still colourless. The pigment amylopectin produces is an orange-yellow.
Lugol’s iodine is an aqueous solution of elemental iodine and potassium iodide that is used
as the reagent in the iodine test.
By itself, iodine does not dissolve in water. Iodine ions react with each other to produce
triiodide ions upon addition of potassium iodine; these ions then react with an iodine
molecule to produce pentaiodide ions.
The iodide, triiodide, and pentaiodide ions are colourless, whereas the bench iodine solution
is brown.
It has been found that the helix (coil or spring) shape of the glucose chain is crucial to the
success of the experiment.
Even more, the length of the glucose chains determines the final hue.
The resulting triiodide and pentaiodide ions are linear and can diffuse freely within the helix.
It is suggested that the complex’s hue arises from a shift in the gap between energy levels
brought about by charge transfer between the helix and the polyiodide ions.
As the temperature rises and organic molecules like ethanol, which are miscible in water, are
present, the colour intensity fades.
When heated, the helical structure of amylose is disturbed, causing it to no longer bind
iodine and so lose its blue hue. However, the iodine-binding capacity of amylose is restored
when it cools.
After being cooled, the blue hue returns because the helical structure is restored, allowing it
to once again bind iodine.
Prepared By: - Ashok C. Keshari
HOD, Food & Dairy Tech.
NLPI, Bardibash
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REQUIREMENTS
1. Reagent
Lugol’s iodine: 5% elemental iodine is mixed with 10% potassium iodide to form the
Lugol’s iodine.
Test sample
2. Materials Required
Test tubes
Test tube stand
3. Equipment
Water bath
PROCEDURE
1. Take two test tubes and label your test tubes as- test sample and control sample
2. Take a small sample (solid sample:500 mg -1000mg; liquid: 1ml) in a clean and dried test
tube labeled as a test sample.
3. Take 1ml of the purified water in the clean and dried test tube labeled as the control
sample.
4. Add 2-3 drops of Lugol’s iodine solution to both the test tubes and mix it thoroughly on a
vortex mixer.
5. Observe the color that develops in both the test tubes.
6. The test tubes should then be heated on a water bath until the color disappears.
7. Allow the test tubes to cool down completely and observe the color in both the test tubes
Prepared By: - Ashok C. Keshari
HOD, Food & Dairy Tech.
NLPI, Bardibash
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Result and Interpretation of Iodine
A positive test is when a purple or blue-black color appears. This indicates the presence of
starch.
If the color does not change, it is negative. This indicates that there is no starch.
OBSERVATIONS
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RESULT AND DISCUSSION
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Important Points to Remember
Lugol’s Iodine Solution is sensitive to light. It should be kept in dark bottles in a dark area.
The iodine testing is only for starch. Lugol’s solution of iodine will not cause cellulose to
change in color.
Temperature changes are not a problem for the test.
Prepared By: - Ashok C. Keshari
HOD, Food & Dairy Tech.
NLPI, Bardibash
