What Is Iodine Test?
The iodine test was first described by J.J Colin and H.F Gaultier de Claubry and independently by F. Stromeyer in 1884. The iodine test is used to test for the presence of starch in a given analyte. The test can be qualitative or quantitative.
Objective Of Iodine Test
- To test for the presence of starch in biological molecules.
Principle Of Iodine Test
This test depends upon the property of adsorption possessed by the large polysaccharide molecules. Starch contains alpha-amylose, helical saccharide polymer and amylopectin. Triiodide anion instantly produces an intense blue-black color upon contact with starch. This reaction is as a result of the formation of polyiodide chains from the reaction of starch and iodine. The amylose or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark-blue/black color. The amylopectic or branched portion of starch forms much shorter helices and iodine molecules are unable to assemble, leading the color to be of an orange/yellow hue.
Generally amylopectin, glycogen and cellulose do not form alpha-helices, they do not complex well with iodine, therefore, they do not show the blue-black color; instead they show a purple or brown color. Monosaccharides on the other hand do not interact with the iodine, therefore no color is produced.
The color obtained depends upon the length of the unbranched or linear chain available for complex formation. Also, the intensity of the color produced decreases with increasing temperature and with the presence of water-miscible organic solvents such as ethanol. Iodine test cannot be performed at very low PH due to the hydrolysis of the starch under these conditions.
Reagent And Material Required
- Iodine Reagent: 0.5 ml iodine diluted in 5 ml distilled water and mixed with 10% potassium iodide to form Iodine solution (Lugol’s iodine)
- Test tube
- Dropper or pipette
- Test sample or solution
Iodine Test Procedure
- Take 1 ml of the test sample in a clean, dry test tube.
- Similarly, take another 1 ml of distilled water in another tube.
- Add about 2-3 drops of Iodine solution to both test tubes and mix thoroughly.
- Observe the appearance of color in the test tubes.
- Heat the test tubes in the water bath until the color disappears.
- Take the test tubes out for cooling.
- Note down the appearance of color seen in the test tubes.
Iodine Test Result Interpretation
- Positive Test: A positive test is indicated by presence of a blue-black or purple color in the test tube. This confirms presence of starch.
- Negative Test: A negative test is indicated by no change in color of iodine solution. This confirms absence of starch.
Limitations Of Iodine Test
- This test cannot be done under acidic conditions as the starch hydrolyses under such circumstances.
- This test is a qualitative test and doesn’t signify the concentration of starch.