Iodine clock reaction

How to turn out the “light” in a solution

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves, eye­wear, and a mask. Per­form this ex­per­i­ment in a well-ven­ti­lat­ed area. Ob­serve safe­ty pre­cau­tions when work­ing with acids and hy­dro­gen per­ox­ide.

Reagents and equip­ment

  • 1 g sodi­um thio­sul­fate;
  • 5 g potas­si­um io­dide;
  • 1 L dis­tilled wa­ter;
  • 100 mL 15% hy­dro­gen per­ox­ide so­lu­tion;
  • 5 mL con­cen­trat­ed sul­fu­ric acid;
  • 5 g starch;
  • 400 mL fresh­ly-boiled wa­ter;
  • 3 beakers;
  • a pitch­er.

Step-by-step in­struc­tions

Pre­par­ing a starch so­lu­tion (so­lu­tion A): Com­bine 5 g starch and 100 mL cold dis­tilled wa­ter in the 500 mL beaker. Stir thor­ough­ly. Add 400 mL fresh­ly-boiled wa­ter and stir. Pre­par­ing a so­lu­tion of sodi­um thio­sul­fate and potas­si­um io­dide (so­lu­tion B): com­bine 1 g sodi­um thio­sul­fate and 5 g potas­si­um io­dide in a 500 mL beaker. Add 500 mL dis­tilled wa­ter and stir thor­ough­ly. Pre­par­ing a hy­dro­gen per­ox­ide so­lu­tion (so­lu­tion C): com­bine 400 mL dis­tilled wa­ter, 100 mL 15% hy­dro­gen per­ox­ide so­lu­tion, and 5 mL con­cen­trat­ed sul­fu­ric acid in a 500 mL beaker. Stir thor­ough­ly. The ex­per­i­ment: com­bine 100 mL of so­lu­tion A and 50 mL of so­lu­tion B in a pitch­er. Stir thor­ough­ly. The vol­ume of so­lu­tion C is vari­able – adding more or less of so­lu­tion C will af­fect the speed of the re­ac­tion.

Process de­scrip­tion

The “Io­dine Clock” re­ac­tion (some­times called “Egyp­tian Night”) is an ox­i­da­tion-re­duc­tion re­ac­tion. Col­or­less so­lu­tions are mixed and, fol­low­ing a short pause, the liq­uid rapid­ly turns dark blue, al­most black. Sev­er­al ox­i­da­tion-re­duc­tion re­ac­tions take place in the so­lu­tion, which can be de­scribed as fol­lows

Hy­dro­gen per­ox­ide ox­i­dizes io­dide ions to molec­u­lar io­dine:

H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O

The so­lu­tion re­mains clear be­cause the form­ing io­dine re­acts with sodi­um thio­sul­fate:

2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻

When the form­ing io­dine has al­ready re­act­ed with all of the avail­able sodi­um thio­sul­fate, it be­gins to re­act with starch, form­ing a dark-blue starch-io­dine com­plex. This in­stant dark­en­ing ef­fect, rem­i­nis­cent of the sud­den on­set of night in Egypt, gave this re­ac­tion its po­et­ic name of “Egyp­tian Night.”