the iodine test is a biochemical test to detect and distinguish certain polysaccharides such as starch dextrin and glycogen from monosaccharides disaccharides and polysaccharides such as cellulose a positive iodine test is reflected by the development of color which depends on the type of polysaccharide amylose gives blue black color amylopectin gives orange yellow color dextrin gives a red color while glycogen gives a reddish brown color the iodine starts test was first described by j.j cullen an hf gottierdo clubri in 1814 and independently by f stromer in 1815 the reagent used in the iodine test is a very diluted form of lugol's iodine also known as aqueous iodine lugol's iodine was first made in 1829 by the french physician jean lugol it's an aqueous solution of elemental iodine and potassium iodide iodine on its own is insoluble in water addition of potassium iodide results in a reversible reaction of an iodide ion with iodine to form a tri iodide ion which further reacts with an iodine molecule to form a penta iodide ion and so on make sure to check out my short video on how to prepare iodine solution by clicking on the link given in the screen right now or the link in the description below polysaccharide molecules such as starch dextrin and glycogen are comprised of a large number of alpha d glucose units that are bound together by alpha 1 4 glycosidic bonds or alpha acetyl linkages with occasional alpha 1 6 glycosidic linkages that result in branched chains except in a milos as a result of the bond angles in the alpha acetyl linkages these molecules form branched or unbranched 3d spiral structures much like a coiled spring starts can be separated into two fractions the linear chain amylose and the branched chain amylopectin natural starches are mixtures of amylose and amylopectin amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble iodine test is based on the principle that polyiodide ions mainly the triiodide ions present in iodine solution forms colored absorption complexes with the helical chains of glucose residues of certain polysaccharides the tri iodide and penta iodide ions formed are linear and slip inside the helix structures which then produces visual colors upon contact with these polysaccharide molecules the amylose component in starch is responsible for the formation of a deep blue or blue black color in the presence of iodine while amylopectin produces an orange yellow hue dextrins form a red chromogen while glycogen produces reddish brown color upon contact with iodine the amilos component of starch forms a very dark blue black complex with iodine the color is intense enough to be able to effectively mask the orange yellow color formed by amylopectin this is why starch in general appears to produce only blue black color with iodine the color of the starch complex is so deep that it can be detected visually even when the concentration of the iodine is as low as 20 micromolar at 20 degrees celsius further the resulting color depends on certain factors such as the length of the glucose chains temperature presence of water miscible organic compounds like ethanol and ph the color changes that occur are believed to be caused by so-called charge transfer complexes or ct complexes after the polyiodide such as triiodide is inside of the polysaccharide helix such as amylose a charged transfer complex is formed between the two in this chart's transfer complex the helical amylose acts as a charged donor and the polyiodide as an acceptor this transfer of charge between the two entities excites the electrons of the acceptor molecules in this case the polyiodide when the electrons from the acceptor molecule return to their ground state they give off electromagnetic radiation that is in the uv visible spectrum in the case of the amylose component in start this radiation corresponds to a deep blue color it takes 40 glucose molecules to form a complex helix around the polyiodide molecule the iodine test is also popularly used in plant physiology experiments as an indirect test to check if a plant is photosynthesizing starch is formed as reserved food material primarily in the leaves of plants during photosynthesis the formation and presence of starch in such plants is confirmed or ruled out using the iodine test do check out my photosynthesis related experiment videos to know more about iodine test and photosynthesis click on the links given in the description below in this video we'll be demonstrating positive and negative iodine tests using a variety of test samples to perform this experiment we'll need the following test samples consisting of one percent glucose one percent sucrose one percent starch cellulose sample in the form of a small ball of cotton and distilled water as control sample you may also consider any starchy food items such as a cut piece of potato white bread boiled rice etc point one normal aqueous iodine solution or leukocyadine diluted about five times with distilled water four clean dry identical test tubes five graduated droppers or pipettes of one or 2 ml capacities test tube stand test tube holder to begin the test take the 4 labeled test tubes in the test tube stand using different droppers or pipettes add one ml each of the test samples in the tubes also take one ml of distilled water in the fourth tube this will be the negative control for our experiment also take the cellulose cotton sample now add a few drops each of the dilute iodine solution to each of the four tubes and also to the cellulose sample observe the change in color in the samples you'll notice that the test tube containing the start solution shows a rapid development of a deep blue black color all the other samples including the control test tube retain the original iodine color similarly starchy food stuffs like potato bread and boiled rice all give a blue black coloration with iodine this shows that iodine gives a color reaction only with certain polysaccharides such as starch both starch and cellulose are polysaccharides the main reason why starch shows a positive iodine test while cellulose does not is because as explained previously at the start of this video the alpha d-glucose monomer units of starch are linked together by alpha-1 for glycosidic bonds which results in a 3d helical arrangement of the glucose units the polyiodide ions can slip inside the helix and eventually produce visible color through the formation of charge transfer complexes on the other hand polysaccharides such as cellulose have their constituent beta d-glucose units linked together by beta-1 for glycosidic linkages which gives rise to long linear straight chains of glucose units instead of a helix so there is no room for polyiodide and ions to slip into and form colored complexes the result is that there is no significant chemical interaction between the iodine solution and the cellulose structures hence a negative iodine test so this is all about the iodine test for polysaccharides such as starch click on the end screen card shown on the screen right now along with the various links given in the description below to watch my other videos on iodine solution preparation protocol and also my other biochemical test videos thanks for watching