Answer to Question #119915 in General Chemistry for Brittney

Question

During the preparation of Lugol's solution, if you are mixing Iodine with water first then adding potassium iodide, what type of observation you will see and for compounding what process you will recommend and why? (6mks)

Answer 1

The iodine test is used to test for the presence of starch. Starch turns into an intense "blue-black" colour upon addition of aqueous solutions of the triiodide anion. Not much is known of this reaction but a theory is that the iodine in the form of I5-ions gets embedded in the helix shape of the amylose and is pushed straight by the starch. A transfer of electrons between the starch and iodine changes the arrangement of electrons and absorbs much of the visible light which gives it its dark blue colour. In the absence of starch however, the brown color of the aqueous solution remains. This interaction between starch and triiodide is also the basis for idometry.