The iodine test is a chemical method used in biology and chemistry to identify specific organic compounds. The reagent employed is typically an aqueous solution of elemental iodine and potassium iodide, often referred to as Lugol’s solution. The solution relies on a visual change to signal a positive result. The test is a standard procedure in laboratories and educational settings due to its quick results and ease of use.
The Target Substance
The iodine test is designed to detect the presence of starch, a large carbohydrate molecule known as a polysaccharide. Starch is the primary way plants store glucose, and it is composed of two main types of glucose polymers: amylopectin and amylose. The test’s specificity comes from its interaction with amylose, which makes up about 10–30% of natural starch.
Amylose molecules are linear chains of glucose units that naturally coil into a helical or spring-like structure. This helical conformation is required for the reaction to occur. The iodine solution does not produce a color change when mixed with simple sugars, such as glucose or sucrose, because these smaller molecules lack the coiled structure necessary to trap the reagent.
How to Interpret the Color Change
A positive result is an immediate shift from the reagent’s original amber or yellow-brown color to deep blue, purple, or black. This coloration indicates starch is present in the sample. If the sample contains no starch, the solution remains the original color of the iodine reagent, which constitutes a negative result.
The underlying chemistry involves the iodine molecules forming linear polyiodide ions, such as the triiodide ion ($\text{I}_3^-$), when dissolved with potassium iodide. These polyiodide ions become physically trapped within the inner channel of the amylose helix. This encapsulation is a charge-transfer interaction that alters the way the complex absorbs light from the visible spectrum. The resulting absorption shift is what the human eye perceives as the characteristic deep blue-black color.
Common Uses of the Iodine Test
The test is useful for various applications, particularly in food science and education. In classrooms, it demonstrates photosynthesis by testing leaves for starch. Consumers and quality control technicians use the test to identify starch in food items like potatoes, bread, or flour. Additionally, the principle of the iodine-starch reaction is applied in certain specialized medical and forensic procedures, such as the Minor test for assessing human perspiration.