Egyptian night

After mixing two colorless solutions nothing happens; however after a few seconds the colourless mixture suddenly turns dark blue

Scientific name: Iodine clock reaction with potassium iodide, sodium thiosulphate, starch and hydrogen peroxide


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Wear eye protection and lab gloves. Avoid the contact of hydrogen peroxide and acids with skin.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

H2O2 + 2I + 2H+ → I2 + 2H2O

2S2O3 + I2 → S4O62− + 2I

I2 + starch → a blue-black complex

Step-by-step instruction

  1. Mix about 0.2 g of starch and some drops of water. Pour some boiling water onto this.
  2. Dilute resulting solution to around 800 ml.
  3. Add approximately 50 g potassium iodide and 10 g of sodium thiosulfate. The solution A is prepared.
  4. The solution B is 30% hydrogen peroxide mixed with water and a small amount of acid (sulphuric acid, acetic acid). It may be any concentration. Time of the reaction depends on the hydrogen peroxide concentration.
  5. Quickly mix together solutions A and B. A few minutes later solution abruptly becomes dark blue.

Scientific background

There are three principal reactions in this experiment. The first is a slow reaction between hydrogen peroxide and iodide ions in presence of H+ ions. The molecular iodine is produced. The second is a fast reaction between thiosulfate and iodine. This reaction causes the triiodide ion which is consumed faster than it is produced. When all of the thiosulphate reacts, this reaction stops and produced triiodide forms dark blue complex with the starch. Increasing H+ , or iodide or hydrogen peroxide concentration will shorten the time. Increasing the thiosulfate concentration will have the opposite effect.


Published on 10 March 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst