Lugol’s solution, also known as Lugol’s iodine, is a laboratory reagent made of elemental iodine and potassium iodide dissolved in water. This transparent brown liquid is primarily used as a chemical indicator in scientific investigations. It was first developed in 1829 by the French physician Jean Lugol.
Identifying Starch
Lugol’s solution is primarily known for its ability to detect the presence of starch. When added to a sample containing starch, the solution undergoes a distinct color change, transitioning from its original yellowish-brown to a dark blue or black. This reaction forms the basis of the iodine test for starch, a widely used method in biology and chemistry.
The color change occurs due to an interaction between the iodine molecules in Lugol’s solution and the helical structure of amylose, a major component of starch. Starch consists of two types of glucose polymers: amylose, which is a largely unbranched chain, and amylopectin, which is highly branched. The iodine molecules become trapped within the coil of the amylose helix, forming a polyiodide-starch complex.
Molecular iodine (I₂) is insoluble in water, but potassium iodide (KI) in Lugol’s solution allows for the formation of polyiodide ions, such as the triiodide ion (I₃⁻). These polyiodide ions are negatively charged and act as charge donors. They fit into the helical structure of amylose, resulting in the characteristic dark blue or black coloration. This reaction is highly sensitive to the presence of amylose. When starch breaks down into smaller carbohydrate units, such as simple sugars, this helical structure is lost, and the distinctive blue-black color does not form.
Detecting Other Polysaccharides
While starch is the primary target for the Lugol’s solution test, the solution can also react with other polysaccharides, yielding different color outcomes. These reactions occur because these polysaccharides, like starch, possess structures that can interact with iodine, though in a less defined or different manner.
Glycogen, a branched polysaccharide found in animal cells, reacts with Lugol’s solution to produce a reddish-brown color. This distinct coloration differentiates glycogen from starch, which yields a dark blue or black result. The structural differences between glycogen’s more extensively branched form and amylose’s less branched helix account for the varied color responses.
Dextrins, which are smaller carbohydrate polymers formed from the hydrolysis of starch, also interact with Lugol’s solution. Depending on their chain length, dextrins typically produce a red or light brown color.
Practical Considerations and Safety
Performing a Lugol’s solution test typically involves adding a few drops of the solution to a sample. The resulting color change, or lack thereof, indicates the presence or absence of specific polysaccharides. This test is qualitative, meaning it determines whether a substance is present, rather than providing a precise measurement of its quantity.
Handling Lugol’s solution requires careful attention to safety. Avoid direct skin contact, as the solution can cause irritation and stain materials. Prevent ingestion or eye contact due to potential health risks. If accidental contact occurs, rinse the affected area thoroughly with water.
Lugol’s solution contains iodine, and inappropriate ingestion can lead to serious health consequences, including irritation of mucous membranes and systemic effects due to iodine toxicity. Symptoms can include stomach pain, nausea, vomiting, or diarrhea. Therefore, it should never be consumed without explicit medical guidance. The solution should be stored in a secure location, away from children and unauthorized access, in its original container to prevent accidental exposure.