The Lugol’s test is a widely used method in chemistry and biology laboratories to determine the presence of starch in a substance. This simple technique relies on a color change that is easily observed. It has been a standard procedure due to its reliability. The test provides an indicator for this complex carbohydrate, which is a major energy storage molecule in plants.
Defining the Key Components
The test requires Lugol’s solution and the target molecule, starch. Lugol’s solution, also known as Lugol’s iodine, is an aqueous mixture of elemental iodine (\(I_2\)) and potassium iodide (\(KI\)). Potassium iodide is included because elemental iodine is not very soluble in water, helping to dissolve the iodine for use in water-based samples.
The potassium iodide dissociates in water, and the resulting iodide ions (\(I^-\)) react with the elemental iodine molecules to form the soluble triiodide ion (\(I_3^-\)) and other polyiodide species. Starch is a polysaccharide composed of glucose units, existing as two main polymer types: amylose and amylopectin. Amylose is the linear chain component (about 10–20% of natural starch), and it is specifically responsible for producing the color change in the Lugol’s test.
The Step-by-Step Procedure
The procedure involves preparing a sample of the substance being tested, which can be a liquid solution or a small solid piece. A control sample, typically distilled water, is also prepared to provide a baseline for comparison. This control ensures any color change observed is due to the presence of starch and not the reagent itself.
Two to three drops of the Lugol’s solution are added directly to the material. The reagent should be thoroughly mixed with a liquid sample or allowed to permeate a solid sample. The final step is observing the immediate color change that occurs in the test sample, comparing it to the water control and the reagent.
The Chemistry of the Color Change
The color change that defines a positive Lugol’s test results from the unique structure of the amylose component of starch. Due to the bond angles between its glucose units, the long, linear amylose chain naturally coils into a helical shape. This helical structure creates a hollow interior channel or cavity.
In the solution, linear polyiodide ions, such as the triiodide (\(I_3^-\)) and pentaiodide (\(I_5^-\)) species, are produced. These chains are able to slip and become physically trapped within the cavity of the amylose helix, forming an inclusion complex. This physical trapping of the iodine species changes their chemical properties.
When confined within the amylose helix, the spacing of the polyiodide chains’ energy levels is altered. This means the complex begins to absorb light differently than the free iodine in the solution. The trapped iodine absorbs light from the visible spectrum, causing the mixture to appear deep blue, violet, or black. The intensity of this color is related to the amount of amylose present, as amylose provides the necessary structure for the complex to form.
Interpreting Positive and Negative Results
The result of the Lugol’s test is determined by observing the color that appears after the reagent is added. A positive result is indicated by the immediate formation of an intense blue-black or deep purple color. This confirms the presence of starch in the sample due to the formation of the iodine-amylose inclusion complex.
Conversely, a negative result occurs when the sample remains the original color of the Lugol’s solution (yellow, orange, or light brown). This lack of a dark color change indicates that no significant amount of amylose is present. The test is sensitive to temperature; heating the mixture causes the blue-black complex to dissociate, and the color temporarily disappears. Upon cooling, the helical structure of the amylose reforms, allowing the blue color to return.