Warning: Reaction must be done outside or in a fumehood due to production of toxic gases including hydrogen sulfide. Concentrated sulfuric acid is extremely corrosive and dangerous and should be handled in glass containers at all times. Always wear gloves when working with it and thoroughly wash any spills with water. Wear eye protection at all times.
The Iodine vapor that is formed throughout most steps of this instructable is toxic and will also stain EVERYTHING it comes in contact with. You can see from the video that the work-area became an absolute mess. So do not do this indoors.
First, get your sodium or potassium iodide and completely dissolve it in a minimum of water. Then place it into an ice bath to keep it cold.
Second, add concentrated sulfuric acid directly to the solution and constantly stir. As you add it will produce iodine. At this point, it will start producing toxic gases including hydrogen iodide, sulfur dioxide, hydrogen sulfide and vaporized sulfuric acid. You have to perform this reaction outside or in a fumehood.
At first the mixture will become pasty and eventually liquefy.
Once it's liquefied, pour it directly into cold water (the ice bath itself is useful for this) and stir it up. Then let it settle for ten minutes or so.
The iodine will settle to the bottom so pour off the water and acid.
The iodine will still be contaminated with acid so you'll need to add water and pour off several times to get out most of the acid.
Filter it and now you have a solid plug of iodine.
It's usable as-is for most purposes but you can purify by placing it in a beaker with a ice-filled round bottom flask on top. Then heat the beaker. The iodine will vaporize and deposit on the flask.
A cool reaction is to take some iodine and pour magnesium powder over it. It will react to form a purple cloud of iodine vapor. If it doesn't react quickly enough, just add a drop of water.
While the iodine vapor is cool, it's also toxic and this reaction should be performed outside.
More cool stuff at our site at http://www.nurdrage.com