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Starch is a polysaccharide formed as a way for plants to store glucose. It can be separated into two fractions--amylose (10-20%) and amylopectin (80-90%).
The structure of amylose consists of long polymer chains of glucose units connected by an alpha acetal linkage. As a result of the bond angles in the α acetal linkage, amylose actually forms a spiral much like a coiled spring. Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The iodine molecule slips inside of the amylose coil.
I-KI Reagent: Iodine is not very soluble in water, therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble that slips into the coil of the starch causing an intense blue-black color.
Procedure:
Pour 5 mL of your test substance into a test tube.
Add 5 drops of iodine (I-KI or Lugol’s solution) to your test tube. (Caution: use care when handling iodine as it is poisonous & can stain hands or clothing!) Iodine will change color from yellow-brown to blue-black in the presence of starch.
Gently shake/swirl the contents of the test tube. Look for any color change in the test substance.
Clean your test tubes thoroughly.
Results:
A blue-black color results if starch is present. If starch amylose is not present, then the color will stay amber or orange or yellow. Starch amylopectin does not give the color, nor does cellulose, nor do disaccharides such as sucrose in sugar.
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