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Chemistry / Can the colour change of litmus indicator be explained by conjugation?
« on: 30/10/2016 21:44:20 »
I have been trying to understand the colour change in litmus paper from a quantiative point of view. I have knowledge of the particle in a box model of quantum mechanics, and i would like to apply this to the conjugated system in the active indicator-component of litmus dye.
In other indicators I have looked at, you can clearly see that when the dye molecule dissociates or is protonated, there is change to the length of the conjugated system (number of alternating double bonds in a row), which explains the colour change. An example is given below for BTB (bromo thymol blue)
[Image 1]
However, i can't find any explanation like this as to why the indicator dye in litmus paper (7-hydroxyphenoxazone) changes colour when protonated/deprotonated. From what I can see, the length of the conjugated system is the same in both versions of the molecule.
[Image 2]
[Image 1]: https://i.stack.imgur.com/Wtsb7.png
[Image 22]: https://i.stack.imgur.com/gpZ8t.png
In other indicators I have looked at, you can clearly see that when the dye molecule dissociates or is protonated, there is change to the length of the conjugated system (number of alternating double bonds in a row), which explains the colour change. An example is given below for BTB (bromo thymol blue)
[Image 1]
However, i can't find any explanation like this as to why the indicator dye in litmus paper (7-hydroxyphenoxazone) changes colour when protonated/deprotonated. From what I can see, the length of the conjugated system is the same in both versions of the molecule.
[Image 2]
[Image 1]: https://i.stack.imgur.com/Wtsb7.png
[Image 22]: https://i.stack.imgur.com/gpZ8t.png