Lugol’s solution, also known as the iodine-potassium iodide (IKI) solution, is a mixture of elemental iodine and potassium iodide dissolved in water. This reagent is a simple indicator used in chemistry and biology laboratories. Its primary function is to act as a highly specific indicator for the presence of starch, a complex carbohydrate. The original amber or yellowish-brown color of the solution changes dramatically upon contact with starch.
Identifying Polysaccharides
The Lugol’s test is not a general carbohydrate test but is specific to certain large polysaccharide molecules, chiefly starch. Starch is the main energy storage carbohydrate produced by plants, found abundantly in potatoes, grains, and corn. Simple sugars like glucose and fructose, or disaccharides, do not cause the characteristic reaction, confirming the test’s selectivity for complex structures.
Starch is composed of two different types of glucose polymers: amylose and amylopectin. Amylose is the linear component, typically making up 20% to 30% of natural starch, and its coiled structure is responsible for the intense color change. Amylopectin is the highly branched component, forming the remaining 70% to 80% of the molecule, and reacts much less strongly with the iodine.
A positive test for starch results in the solution turning a deep blue, dark purple, or black color. This visual change indicates that amylose is present in the sample. Conversely, a negative result means the solution retains its amber or yellowish-brown color, indicating the absence of starch.
The branched structure of amylopectin, if present alone, typically yields a less vibrant, reddish-brown or purple-red color. This difference in color intensity allows researchers to infer the relative amounts of linear and branched components within a starch sample. The resulting color is linked to the length of the glucose chains in the polysaccharide.
How the Iodine-Starch Complex Forms
The chemical mechanism behind the color change is a physical trapping process involving iodine and the coiled structure of amylose. Elemental iodine (\(\text{I}_2\)) is not very soluble in water, so potassium iodide (\(\text{KI}\)) is added. The iodide ion (\(\text{I}^-\)) reacts with molecular iodine to form polyiodide ions, predominantly the triiodide ion (\(\text{I}_3^-\)).
The linear amylose molecule coils into a helical shape, similar to a spring. The polyiodide ions, such as the triiodide ion, become physically trapped within the hydrophobic inner channel of this amylose helix. This entrapment forces the iodine atoms into a linear chain arrangement inside the coil.
The confinement of the polyiodide chains within the helix causes charge transfer. This interaction alters the spacing of the electron energy levels within the iodine chain. The new electronic configuration causes the complex to absorb light at a different wavelength than the original solution.
The complex absorbs light from the yellow and green parts of the visible spectrum. The human eye then perceives the complementary color, which is the characteristic deep blue or black. The intensity of the color is proportional to the amount of amylose and the concentration of the polyiodide ions trapped inside the helix.
The iodine-starch complex is sensitive to temperature changes, demonstrating the nature of the physical entrapment. Heating the solution causes the amylose helix to unwind, releasing the trapped iodine molecules. This disruption causes the deep blue color to disappear, returning the solution to its amber hue. When the solution cools, the amylose coil reforms, the iodine is re-trapped, and the blue color returns. This reversible reaction demonstrates physical complexation rather than a permanent chemical bond.
Common Uses Beyond the Classroom
While often used in educational settings to demonstrate basic biochemistry, the Lugol’s test has several applications in food science and medical diagnostics. In food analysis, it serves as a simple method to detect starch adulterants or confirm the presence of starch in processed foods like sauces and certain fillers. It is a quick field test for determining the starch content in grains or vegetables.
In biological research, the test is routinely used to demonstrate the process of photosynthesis in plants. Leaves are tested for starch to confirm that they have converted light energy into stored chemical energy. The test is also used in microscopy as a biological stain to make certain cell structures, including starch granules, more visible under the lens.
In medical contexts, a variation of the test is used for cervical cancer screening, known as Schiller’s test. The iodine solution is applied to the cervix, where normal, healthy cells contain glycogen, a polysaccharide similar to starch. The reaction stains the glycogen-rich tissue a dark brown color.
Cancerous or abnormal cells are often glycogen-deficient and therefore do not stain, appearing pale against the healthy tissue. The solution can also be used in histology to stain glycogen in other tissue samples, producing a reddish-brown color that distinguishes it from the blue-black reaction with starch. Historically, Lugol’s solution was used as an antiseptic, a disinfectant, and a pre-treatment for thyroid conditions.