![]() ![]() Iodide ions combine to form triiodide ion (I 3 –), which further associate to create polyiodide ions (I n –) in solution. Potassium iodide dissociates, to form iodide ions. Therefore, potassium iodide is used for making the laboratory reagent. Molecular iodine or iodine molecule, i.e., I 2 molecule, is insoluble in water. Now, let us understand the iodine purpose in Lugol’s iodine test. Interestingly, this is also known as the IKI solution. Therefore, for this test, iodine in water, i.e., an aqueous solution of molecular iodine (I) and potassium iodide (KI), which is known as Lugol’s iodine, is used. The helical structure of Amylose forms a charge transfer (CT) complex with iodine, wherein iodine is present inside the spiral or helical structure of the Amylose. The basic principle involved in the iodine test is that Amylose interacts with starch to form a blue-black colored complex with the iodine. It does not give blue color with iodine solution In Amylopectin, glucose units are connected by α-1,6 glycosidic linkagesĪmylose is the water-insoluble fraction of starchĪmylopectin is the water-soluble fraction of starchĭoes not exhibit gelling on the addition of hot waterĮxhibit gelling on the addition of hot water In Amylose, glucose units are linked by α-1,4 glycosidic linkages Straight chain polymer made up of D-glucose subunitsīranched-chain polymer made up of D-glucose subunits Table 1: Difference between Amylose & Amylopectin Source: Modified by Maria Victoria Gonzaga of Biology Online, from Laghi.l’s amylopectin chains arrangement (CC BY-SA 3.0) and Theislikerice’s 3D amylose structure (CC BY-SA 4.0). However, the positioning of the D-glucose unit in Amylose and Amylopectin is different.įigure 1: Structural difference between Amylose and Amylopectin. Chemically both monomers are made up of D-glucose subunits. Natural starch contains two monomeric units: Amylose (10-20%) and Amylopectin (80-90%). Starch is found in abundance in potatoes and cereals (oats, barley, rice, wheat). Glucose synthesized by plants is stored as starch. See also: hydrolysis Principle of Iodine Testīefore understanding the principle behind the iodine test, let us briefly understand starch and its chemistry. Therefore, this test can also indicate the completion of hydrolysis when a color change does not occur. When starch is broken down or hydrolyzed into smaller carbohydrate units, the blue-black color is not produced. The amylose in starch forms helices where iodine molecules assemble, forming a dark blue or black color. The reaction is due to the formation of polyiodide chains from the reaction of starch and iodine. The sample turns blue-black in color when a few drops of potassium iodide solution are placed on the sample. Iodine test is a test for detecting the presence of starch.
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