Extracting Iodine

One of the projects coming later uses crystals of the element iodine.

For someone like me, finding iodine crystals is as simple as reaching up to the top shelf in the lab. But for my readers, the pure element is not so easy to come by.

However, there is a tiny bit of iodine in that bottle of tincture of iodine you can find at the pharmacy. There is about half a gram in each fluid ounce of the disinfectant.

Extracting the pure element is simple, and demonstrates how clever chemists can be using simple supplies and materials. The principle is called partitioning, or sometimes liquid-liquid extraction. It takes advantage of the fact that some things dissolve more easily in one liquid than another.

The liquid in tincture of iodine is alcohol. Iodine does not dissolve well in alcohol, and even less well in water. To get the elemental iodine to dissolve in either of these solvents, a little potassium iodide is added. The iodide ion helps more of the pure iodine dissolve.

We will be adding water and toluene to the disinfectant. The water dilutes the alcohol and makes the iodine less soluble in the mixture. The toluene (available in stores either as the pure solvent, or as cigarette lighter fluid) will float to the top, since it does not mix well with either alcohol or water.

Iodine dissolves easily in toluene. In the photo above you can see the bottom layer of 50% water and 50% tincture of iodine, and above it the layer of toluene, which has already started to get pinkish-violet from the iodine it has absorbed.

The next step is to stopper the test tube (or jar, or whatever you have chosen to use) and shake well for 15 to 20 seconds. The two layers will mix, and there will be some froth at the top, but when you stop shaking, the two layers will fairly quickly separate again. After 30 seconds of sitting still, the top layer of toluene can be seen stained a dark purple by the iodine it has extracted from the tincture.

The next step is to use an eyedropper or a pipette to carefully remove as much of the top layer of dark purple toluene as you can into a jar, without getting any of the alcohol and water mixture in with it. There will be a little toluene left in the test tube, but that is OK.

Now we have a jar with dark purple toluene in it. We need to evaporate the toluene while losing as little of the iodine as possible. Iodine slowly vaporizes at room temperature, but if we give the toluene a lot of flowing air, it will evaporate faster than the iodine does. I used a blow dryer that had a setting that allowed the air to flow without any heat. In a few minutes, the toluene had evaporated, and I had pure iodine crystals in the bottom of the jar.

Save the crystals by tightly closing the jar. We will need them for the next project.