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Originally Posted by ffixcth
What happend to a blue litmus when an acid is added to it? Why the blue litmus form a red colour? How the acid work to turn the litmus red? Is there a chemical reaction that turn the litmus red?
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I'm having trouble finding information about the chemistry of litmus. Here's what I have come up with.
This is the litmus molecule:
http://www.chriscooksey.demon.co.uk/lichen/litmus2.htm
This website says that the active ingredient of litmus is called
erythrolitmin.
http://reference.allrefer.com/encycl.../L/litmus.html
Do you know any organic chemistry? I believe those six membered rings in the molecule are aromatic.
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.
Aromatic structures are extended pi-electronic systems, so they are probably where the color comes from. Light of a specific wavelength will excite these pi systems, and the complementary color to that absorbed wavelength will be seen by the viewer.
The complementary color to blue is orange, and the complementary color to red is green. Green is a higher energy wavelength of light then orange. Therefore, when the molecule interacts with acid, the extended pi system of the molecule becomes harder to excite (since higher energy green light is now needed to excite it instead of the lower energy orange).
I don't really know why this happens. The hydrogen ions from the acid will probably bind to the alcoholic oxygens, removing some lone pairs of electrons from participating in the pi-conjugation. The H+ ions may also bind to the other oxygens or the nitrogens which will have the same sort of effect. Somehow this will make the pi-systems harder to excite.