Iodine test
Encyclopedia
The Iodine test is used to test for the presence of starch
. Iodine solution — iodine
dissolved in an aqueous solution of potassium iodide
— reacts with the starch
producing a purple black color. The colour can be detected visually with concentrations of iodine as low as 0.00002M at 20°C. However the intensity of the colour decreases with increasing temperature and with the presence of water-miscible, organic solvents such as ethanol. Also the test cannot be done at very low pHs due to the hydrolysis of the starch under these conditions.
This reaction is the 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 purple/black color. The amylopectin
, 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. As starch is broken down or hydrolyzed into smaller carbohydrate units, the purple-black color is not produced. Therefore, this test can determine completion of hydrolysis when a color change does not occur.
Iodine solution will also react with glycogen
, although the color produced is browner and much less intense.It is rust colored normally.
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
. Iodine solution — iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
dissolved in an aqueous solution of potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
— reacts with the starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
producing a purple black color. The colour can be detected visually with concentrations of iodine as low as 0.00002M at 20°C. However the intensity of the colour decreases with increasing temperature and with the presence of water-miscible, organic solvents such as ethanol. Also the test cannot be done at very low pHs due to the hydrolysis of the starch under these conditions.
This reaction is the result of the formation of polyiodide chains from the reaction of starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
and iodine. The amylose
Amylose
Amylose is a linear polymer made up of D-glucose units.This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure...
, or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark purple/black color. The amylopectin
Amylopectin
Amylopectin is a soluble polysaccharide and highly branched polymer of glucose found in plants. It is one of the two components of starch, the other being amylose.Glucose units are linked in a linear way with α glycosidic bonds...
, 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. As starch is broken down or hydrolyzed into smaller carbohydrate units, the purple-black color is not produced. Therefore, this test can determine completion of hydrolysis when a color change does not occur.
Iodine solution will also react with glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
, although the color produced is browner and much less intense.It is rust colored normally.