In a lab, Lugol's solution is typically used as an indicator for the presence of starch in a solution. Lugol's solution, also called Lugol's iodine, is a solution of elemental iodine and potassium iodide in water that generally causes a solution containing starch to turn deep blue.
In some cases, adding Lugol's iodine to a solution that contains no starch can cause a solution being tested to turn blue, such as when the Lugol's solution or the glassware being used is contaminated. Sometimes a control is used during testing, where Lugol's solution is also added to a solution known to contain starch (the positive control) and to a solution known not to contain starch (the negative control.) If the positive control turns blue, and the negative control does not turn blue, it helps confirm that the test is not defective.
Chemists do not fully understand how Lugol's solution causes a solution containing starch to turn blue. The chemical processes are complex. The mixture of elemental iodine and potassium iodide in Lugol's solution generates free iodine atoms that beta amylose, a starch, seems to force into a linear arrangement with energy level spacings conducive to the absorption of visible light such that the solution appears blue.
Lugol's solution was named after the French physician, J.G.A. Lugol, who invented it in 1829. In addition to being used as a reagent in labs, it is often used as an antiseptic and disinfectant.