JCE Classroom Activity #92: Testing for Iodide in Table Salt

JCE Classroom Activities are a collection of over 100 chemistry experiments and demonstrations that can be carried out using simple materials found around the house or at grocery store and hardware stores. For example, JCE Classroom Activity #92 describes how to test for the presence of iodide in iodized salt using only water, iodized salt, 3% hydrogen peroxide, vinegar and liquid laundry starch.

This activity makes use of the reaction between iodide ion (I^-) and hydrogen peroxide (H₂O₂) in acid to form water and iodine (I₂):

Iodine reacts with starch to form a bluish-black complex:

In JCE Classroom Activity #92, these two reactions are used to test for the presence of iodide ion in iodized salt. Of course table salt is comprised primarily of sodium chloride (NaCl), but iodized salt has a very small amount (0.006% by mass) of potassium iodide (KI) added. By adding hydrogen peroxide to a mixture of water, hydrogen peroxide and iodized salt, a color change can be observed due to the formation of the blue-black starch-I₂ complex. If you have some Vitamin C (ascorbic acid) on hand, you can extend this activity. Ascorbic acid added to the starch-I₂ complex will reduce any I₂ present in the complex back to iodide ion, causing the black color to disappear:

In the video below, you can check out just how easy it is to pull off this experiment.

If you get a chance, check out some of the other JCE Classroom Activities. Be sure to let me know if you find an activity that is particularly easy to carry out. I’m always looking for interesting, yet easy to perform experiments!

Reference:

Stephen W. Wright, JCE Classroom Activity #92, Testing for Iodide in Table Salt. J. Chem. Educ. 2007, 84, 1616A.