Chemicals:
| LUGOL SOLUTION (1.3 g of iodine and 2 g of potassium iodide,
dissolved in 100 mL water | |
Foods:
Noodles, waxy rice
Hazards and safety precautions:
  |
Iodine is toxic - may be fatal if swallowed or inhaled. Harmful by inhalation and through skin absorption. Readily absorbed
through skin. Very destructive of mucous membranes and upper respiratory tract, eyes and skin. Severe irritant. |
 |
Lugol Solution is harmful if swallowed or absorbed through the skin. |
Safety glasses, nitrile gloves, effective ventilation. Lugol solution should be prepared in a fume hood!
Experimental procedure
The material that is to be analyzed - noodles and rice, respectively - is cut into small pieces. Water is added to a small amount of
the material. The suspension is heated and afterwards cooled to room temperature. In order to obtain a low concentration the cooled
and filtered samples are diluted with water.
| Test tube 1
| 1.5 mL of diluted, aqueous "noodle sample"
| | Test tube 1
| 11.5 mL of diluted, aqueous "rice sample" |
Results:
|
| Color of solution
| | Test tube 1
| blue
| | Test tube 2
| red-brown |
|
Discussion:
Amylose Helix |
|
Starch consists of two types of molecules, amylose (normally 20-30%) and amylopectin (normally 70-80%). Waxy rice consists of 100%
amylopectin and no amylose.
Amylose and amylopectin are both polymers containing 1000's of glucoses. Amylose molecules consist of single mostly-unbranched
chains of glucose molecules. All of the monomer units are a-D-glucose, and all the alpha acetal links
connect C-1 of one glucose to C-4 of the next glucose. As a result of the bond angles in the a-acetal
linkage the chain of amylose is coiled in the shape of a left-handed helix. Amylopectin differs from amylose in being highly
branched. The glucose momomers within the branches are linked by a-1,4 glycosidic bonds, but where the
branch joins the main branch a-1,6 link forms. Approximately every 12th-20th D-glucose unit of one amylose
chain is "cross-linked" to another amylose chain. Figure below gives a partial structure for amylose and amylopectin.
Structure of amylose and amylopectin
Lugol solution contains polyiodide ions. Iodine is dissolved in water in the presence of iodide producing tri-iodide- and
pentaiodide-ions:
I 2 + I - ®
[ J 3 ] -
I 2 + [ I 3 ] - ® [ I 5 ]
-
When starch is mixed with Lugol solution, a blue colored starch-iodine complex is formed.
|
Amylose - [ I 3 ] - Complex
Tri-iodide ion slips into the
amylose coil
| |
| · Amylose reacts with Lugol solution to produce a deep blue color.
[ I 3 ] - and [ I 5 ] - (primary substructures) are linearly arranged in the central
groove of the amylose helices. The complexes may react repeatedly to bind iodine molecules along the length of the amylose chain
forming structures such as [ I 9 ] - and [ I 15 ] -. The hydrophobic inner surface of
helix amylose holds a spiral of water molecules. The intense blue color of the complex is produced due to donor-acceptor interaction
between water and the electron deficient polyiodides.
When heat is applied, the amylose-iodine complex is destroyed. When the solution has cooled the 'blue' of the amylose/iodine
combination appears.
· Amylopectin reacts with Lugol reagent solution to form a reddish brown to purple solution. It
seems that the amylopectin-iodine complex involves a nearly linear [ I 4 ] - stabilized in the hydrophobic
cavity of a small "amylose-like" helix, which is responsible for the change in color.
· The starch-iodine complex is very useful for indicating redox titrations that involve iodine
because the color change is very sharp.
|
Sources for the pictures:
Microsoft PowerPoint - Carbohydrates_lecture
2006
Starch - Iodine
References:
 Demonstration Experiment on Video
Starch - widely available in many Foods
General experimental instructions and index of experiments
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