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Transcript of iodine test
The Iodine Test for Starch is used to determine the presence of starch in biological materials.
why add sodium thiosulfate?
it has no effect on starch
and will yield a clear coloration
iodine dissolved in an aqueous solution of potassium iodide — reacts with the starch producing a purple black color.
prepare a starch soln. by mixing thoroughly 2 grams of starch with 10 ml. of water and then pouring this mixture in 100 ml of boiling water
add 2 drops of a very dilute aqueous soln. of iodine in potassium iodide to 1 ml. of starch soln.
what is the color of the soln.
for the starch; it rendered a purple black color which indicates the presence of starch
for glycogen; it rendered a copper color or somewhat brown and less intense than starch
heat to boiling and observe the effect.
principle behind heating:
The blue color is caused by the trapping of the iodine inside the structure of the starch (note that starch is a polyssacharide that has a helical structure).
This is because the heat causes an unwinding of the helical structure of starch releasing the trapped iodine making the solution colorless. However, if cooled, with no heat present, the starch structure becomes helical once more trapping the iodine returning the blue colored solution.
why is it that the color disappears when heated?
Starch in the form of amylose and amylopectin has less branches than glycogen. This means that the helices of starch are longer than glycogen, therefore binding more iodine atoms.
allow the soln. to cool back down to room temperature. now what do you observed.
add few drops of sodium thiosulfate soln. to the cooled starch soln. and note the results. do the same with glycogen. explain the results.
Na2S2O3 is a reducing agent
that reacts with the iodine, converting
it to colorless iodide
w/ iodin soln.
The use of Lugol's iodine reagent (IKI) is useful to distinguish starch and glycogen from other polysaccharides.