Starch and iodine reaction

When a few drops of colorless solution are added to another colorless solution it suddenly turns dark-blue.

Scientific name: Qualitative reaction of starch to iodine is demonstrated.


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Reaction formula

I2 + I- → I3-

I3- + (C6H12O6)n → (C6H12O6)nI3-

Step-by-step instruction

  1. To determine the presence of iodine prepare starch indicator.
  2. Take 1g of starch and add it to 10 ml of water.
  3. Shake the mixture thoroughly.
  4. Pour components to 100 ml of boiling water.
  5. Boil it under stirring for a while and leave to cool down.
  6. Put a few drops of starch indicator to the solution under study. If it turns dark blue it contains iodine.
  7. Iodine solution could be used as an indicator of the presence of starch. To prepare it dissolve iodine in an aqueous solution of potassium iodide.
  8. Put a few drops of iodine indicator to the solution under study. If it turns dark blue it contains starch.

9.Iodine solution could be also used to detect starch in fruits and vegetables.

10.Take the vegetable you want to test on starch presence. Peel off the skin.

11.Put a few drops of iodine indicator on the food surface.

Scientific background

The reaction of starch and iodine is very specific and leads to the formation of a dark-blue complex. It could be used as a qualitative test for one of these compounds. The use of iodine as an indicator on starch has one disadvantage. Iodine (I2) is insoluble in water. This difficulty could be easily overcame by addition of I- ions (potassium or sodium iodide as a rule). In presence of iodide iodine forms triiodide I3- ions which are highly soluble:

I2 + I- → I3-

Starch consists of two components – amylose and amylopectin. Amylose is the substance responsible for the appearance of dark blue color in the presence of iodine. It is a helical polysaccharide made of α-D-glucose monomer units. The long polymer chain winds around linear triiodide ions and gives a colored complex. The I3- ions are embedded in amylose helix:

I3- + (C6H12O6)n → (C6H12O6)nI3-

The color of the complex is so intense that even very low concentrations (<0.00002M) could be detected with a naked eye.

Iodine indicator could be used to detect the presence of starch in fruits and vegetables. Potatos contain a lot of starch and when a drop of iodine is placed on a potato peel it turns dark-blue.


Published on 15 April 2015

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