Iodine test for starch. Iodine test is used to determine presence of starch. Elemental iodine reacts with starch and produces purple-blue color, whose strength depends on concentration of the starch. Here few drops of Lugol's solution have been added to three beakers with aqueous corn starch solutions with concentrations 1%, 0.1%, and 0.01% (left to right). Lugol's solution (Lugol's iodine) is an aqueous solution of iodine and potassium iodide.