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Topic: Vanillin turning pink  (Read 3087 times)

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Offline lynn71

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Vanillin turning pink
« on: September 12, 2013, 03:32:54 PM »
I was hoping someone could explain the chemistry behind the relationship between vanillin and pH.  The higher the pH the faster my solution goes pink.  My solution will also turn pink faster is I expose it to UV light. A chemist I am not but I could use the help understanding the base reactions. 

Thanks in advance.

Offline spirochete

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Re: Vanillin turning pink
« Reply #1 on: September 12, 2013, 06:30:31 PM »
There is a fairly acidic proton on vanillin. It has a pKa of 7.78, which is a measure of its acidity. As a comparison, water has a pKa of about 16.  A smaller number for pKa means more acidic.

When you lower the pH you make it more basic and there is a deprotonation (acid base reaction) that changes the structure of vanillin.

Molecules with a large number of double bonds in a row tend to absorb visible light. The more double bonds you have in a row, the lower the wave length of absorption. A very over simplified explanation is that the deprotonation basically increases the number of double bonds you have in a row, lowering the wave length of light that is absorbed so that color can be seen.
« Last Edit: September 12, 2013, 07:34:17 PM by spirochete »

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