Chemical Test for Starch or Iodine:
Amylose in starch is responsible for the formation of a deep
color in the presence of iodine. The iodine molecule slips inside
of the amylose coil.
Iodine - KI Reagent: Iodine is not very soluble in water,
therefore the iodine reagent is made by dissolving iodine in
water in the presence of potassium iodide. This makes a linear
triiodide ion complex with is soluble. The triiodide ion ion
slips into the coil of the starch causing an intense blue-black
Starch Test: Add Iodine-KI reagent to a solution or
directly on a potato or other materials such as bread, crackers,
or flour. A blue-black color results if starch is present. If
starch amylose is not present, then the color will stay orange
or yellow. Starch amylopectin does not give the color, nor does
cellulose, nor do disaccharides such as sucrose in sugar.
Iodine Test: When following the changes in some inorganic
oxidation reduction reactions, iodine may be used as an indicator
to follow the changes of iodide ion and iodine element. Soluble
starch solution is added. Only iodine element in the presence
of iodide ion will give the characteristic blue black color.
Neither iodine element alone nor iodide ions alone will give
the color result.