This iodine clock reaction is amazing and one that can truly generate a great deal of interest. Depending on the dilution of the two solutions used the time can be tuned to occur at a point the magician requires it to turn black. Even pouring the mixture into a bottle of coke and telling a story is enough to get that wow factor happening in class.

Here is how its done.

PREPARING AND PERFORMING THE DEMONSTRATION
Safety
• Potassium iodate is mildly toxic by ingestion and is irritating to body tissues. Avoid contact.
• Sodium bisulfite is slightly toxic by ingestion and is a severe body tissue irritant. Avoid contact.
• Sodium thiosulfate is slightly toxic by ingestion and is a body tissue irritant. Avoid contact.
• Iodine is generated in this demonstration. Those who know they are allergic to iodine or do not know whether they are should avoid contact. Wear protective gloves.
• Safety goggles and protective gloves should be worn while doing this demonstration.

Equipment and Materials (for one presentation)
• 0.38 grams of sodium metabisulfite (Na₂S₂O₅
• 1.40 grams of potassium iodate  (KIO₃)
• 0.60 grams of soluble starch
• Distilled (or deionized) water
• Glass stirring rod
• Permanent marker
• 2 X 250-mL beakers
• Hotplate
• Waste container (at least 2-L)
• 10-20 grams of sodium thiosulfate pentahydrate (for waste treatment)

Preparation of solutions

1. Label one 250 mL beaker “A” and place 200 mL of deionised water into the beaker and heat the water on a hotplate.
2. Dissolve 1.40 g of potassium iodate (KIO₃) in the hot water in beaker “A”.
3. Label one 250mL beaker “B” and place 80 mL of deionised water and bring to boil  on a hotplate. Slowly add the 0.60 grams of starch until it is all dissolved. Take the beaker off the hotplate and add 0.38 g of
   sodium metabisulfite (Na₂S₂O₅). Stir until dissolved.

The two videos on the right show the impact of concentration on the speed of the reaction. It is clear that the reaction can be delayed if the solutions are reduced in concentration.

Magicians who are clever chemists can create concentrations such that they will occur at a certain time interval after mixing. Placed around the room the magician, who now is able to view a clock counting the seconds is able to move up to a solution and demand it goes black on their command. Pretty cool and mind blowing for students.

Try it.

Any spills can be washed with excess solution of sodium metabisulfite or sodium thiosulfate pentahydrate.

Concentrated solutions

Dilute solutions