Iodine

[ffixcth]

When starch is added to iodine, a blue black colouration is observe. can anyone tell me what cause the blue black colouration?

[biochem]

Iodine on its own is brown in colour. Adding iodine to starch results in a blue-black colour which is due to the starch-polyiodide complex. This blue-black colour indicates the presence of starch and is used as a test for it.

[ffixcth]

how the starch-polyiodide complex cause the blue black colouration???

[biochem]

I'm not sure myself but it's possible that it is because the complex absorbs a certain frequency of light corresponding to a cetain colour. The observed blue-black colour is the colour complementary to the one that is absorbed by the complex.

What do you think ffixcth?


Also, anyone who has a better explanation would be most welcome. Thanks.

[RobJim]

Questions like this aren't easy to answer with as much detail as I'd like generally. Here's the basic theory behind color in organic molecules (which starch is).

All compounds can be excited electronically by electromagnetic radiation...when an electromagnetic transition is in the visible range...the compound will appear to us to be colored...Light of a given wavelength is perceived as [a particular] color. However, if that wavelength is absorbed, we perceive the complementary color...Intensely colored materials have absorptions in the visible region [of the electromagnetic spectrum]. For organic compounds, such electronic absorptions are generally [pi => pi*] or [n => pi*] transitions and involve extended [pi]-electronic systems. That is, color in organic compounds is generally a property of [pi]-structure. (1224-1225)

Streitwieser, Andrew, Clayton H. Heathcock and Edward M. Kosower. Introduction to Organic Chemistry Fourth Edition. Upper Saddle River: Prentice Hall, 1998.

Basically, the idea is that molecules, just like atoms, can get excited by photons. When this happens with visible light photons, the color that corresponds to the wavelength absorbed does not get reflected, and the combination of all the colors that do get reflected are seen by a viewer as the complementary color of that which is absorbed.

Starch molecules generally form a coiled shape. When iodine molecules and iodide ions are put in water, they form a triiodide ion complex which can slip into the inside of the coil. This apparently adjusts the pi system(s) in starch such that they begin to absorb photons in the visible light range; yellow photons maybe?

http://www.elmhurst.edu/~chm/vchembo...rchiodine.html

[sanda786]

hello, could you tell me why iodine is coloured please? i cnt find information anywhere! i would be obliged if you could msg me back regarding this enquiry. thnx

[adrian]

Originally Posted by RobJim

Questions like this aren't easy to answer with as much detail as I'd like generally. Here's the basic theory behind color in organic molecules (which starch is).

All compounds can be excited electronically by electromagnetic radiation...when an electromagnetic transition is in the visible range...the compound will appear to us to be colored...Light of a given wavelength is perceived as [a particular] color. However, if that wavelength is absorbed, we perceive the complementary color...Intensely colored materials have absorptions in the visible region [of the electromagnetic spectrum]. For organic compounds, such electronic absorptions are generally [pi => pi*] or [n => pi*] transitions and involve extended [pi]-electronic systems. That is, color in organic compounds is generally a property of [pi]-structure. (1224-1225)

Streitwieser, Andrew, Clayton H. Heathcock and Edward M. Kosower. Introduction to Organic Chemistry Fourth Edition. Upper Saddle River: Prentice Hall, 1998.

Basically, the idea is that molecules, just like atoms, can get excited by photons. When this happens with visible light photons, the color that corresponds to the wavelength absorbed does not get reflected, and the combination of all the colors that do get reflected are seen by a viewer as the complementary color of that which is absorbed.

Starch molecules generally form a coiled shape. When iodine molecules and iodide ions are put in water, they form a triiodide ion complex which can slip into the inside of the coil. This apparently adjusts the pi system(s) in starch such that they begin to absorb photons in the visible light range; yellow photons maybe?

http://www.elmhurst.edu/~chm/vchembo...rchiodine.html

The theory is OK, but: if I dissolve I2 in any oxygen-containing solvent (ethanol, acetone, diethyl ether, etc.), the colour will be brown. When I use a solvent without oxygene (hydrocarbons, CCl4, etc), the colour is violet.

[countygrl77]

Can anyone tell me if it is still indicative of starch if a reaction is light blue when dropped with iodine??? Thanks.

[RjByrne]

Adrian, it's brown in those liquids due to their polarity. They don't dissolve in polar solvents such as water. Solvents like hexane are non-polar and do not interfere with the electronegativity of the elemental iodine or ionic iodide. That's all I can remember anyway. It is definately related to polarity anyway.

[mzubair]

Iodine is insoluble in water, to make it soluble potassium iodide (KI) is added, Can any one let me know to prepare x mol/l or x N solution of Iodine, how much iodine and potasium iodide is required?