Iodine testing is a chemical test that distinguishes mono- or diaccharides from polysaccharides such as amylase, glycogen, and dextrin. Starch-iodine is a variant of this test. Iodine test is used to confirm the presence of glucose in the leaves.
To detect the existence of polysaccharides, primarily starch.
Objectives of Iodine Test
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.
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