Iodine Test Definition
The Iodine test is a chemical process which distinguishes particular polysaccharides, including amylase, dextrin, as well as glycogen, from mono-or disaccharides. The starch-iodine test, a variation of this method, is utilized to identify the existence of glucose generated by plants in leaf tissue.
Objectives of the Iodine Test
- determining the availability of starch and other polysaccharides.
Principle of Iodine Test
- The iodine test is based on the polyiodide ions that generate colourful adsorption compounds with the helical chains of glucose leftover from amylase (blue-black), dextrin (black), or glycogen (reddish-brown).
- Branched polysaccharides like cellulose, monosaccharides, and disaccharides all maintain their colorlessness. An orange-yellow colour is produced by amylopectin.
- The iodine test utilises Lugol’s iodine, an aqueous solution comprising elemental iodine as well as potassium iodide.
- Iodine cannot dissolve in water on its own. A reversible interaction between potassium iodine and iodine produces a triiodide ion, which subsequently combines with an iodine molecule to form a pentaiodide ion.
- The iodide, triiodide, as well as pentaiodide ions are generally colourless, whereas bench iodine solution appears brown.
- It has been noticed that the helix (or coil or spring) shape of the glucose chain is crucial to this assay.
- The distance between the glucose chains also impacts the final colour.
- The resulting linear triiodide and pentaiodide ions slide into the helix framework.
- The complex’s colour is believed to result from variances in the spacing of the energy levels, which may absorb visible light as a consequence of the charge transfer between the helix and the polyiodide ions.
- When the temperature rises and there are organic molecules like ethanol present that are water soluble, the colour intensity drops.
- Because of the disruption of the helical structure during heating, the blue colour and iodine binding ability of the amylase-iodine complex dissociate and then reassemble upon cooling.
- After cooling, the blue hue returns as a result of the helical structure recovering its ability to bind iodine.
- To create Lugol’s iodine, 5 percent elemental iodine is combined with 10 percent potassium iodide.
- Test sample
- Materials Required
- Test tubes
- Test tube stand
- Water bath
Procedure of Iodine Test
- Put 1 ml of the sample that is provided in a dry, clean test tube.
- In another tube, take control of 1 cc of distilled water.
- To each tube, add a few drops of Lugol’s solution, then swirl them together.
- Watch how the test tubes’ appearance changes colour.
- The test tubes should be heated in the water bath until the colour is gone.
- Remove the test tubes so they can cool down.
- Keep a record of the coloration in the test tubes.
Results and Interpretation of the Iodine Test
- A positive test result is when a blue-black or purple hue appears; this shows that starch is present.
- When the colour doesn’t really alter, the result is unfavourable and indicates the absence of starch.
Uses of Iodine Test
- This test is employed to find starch in a variety of materials.
- Similar to that, this test is run to evaluate how well plants’ photosynthesis works.
Limitations of the Iodine Test
- Since starch hydrolyzes in acidic environments, this test cannot be carried out there.
- This test is qualitative in nature and does not reflect starch content.
References and Sources
- Tiwari A. (2015). Practical Biochemistry. LAP Lambert Academic Publishing.
- 2% – https://allmedicalstuff.com/barfoeds-test/
- 1% – https://www.scribd.com/document/429888468/Advances-in-Carbohydrate-pdf
- 1% – https://www.coursehero.com/file/p39m2q7c/IKI-Test-experiments-the-presence-of-starch-in-a-solution-if-positive-color/
- 1% – https://libraryofessays.com/lab-report/lap-report-1895512
- <1% – https://allmedtests.com/iodine-test-starch/