[Borek]

I am trying to understand why methyl violet 2B changes color. According to wikipedia (and other sources), this is the methyl violet formula:



In low pH methyl violet gets protonated. This formula shows it already IS protonated. Does it mean at low pH it gets in fact protonated twice?

Color change probably means some isomerisation occurring after protonation. I have no idea what the other isomer is.

I have 11 other indicators that I plan to be able to draw in both forms responsible for different colors, but let's start with this one.

[Borek]

After checking thymol blue transitions (http://en.wikipedia.org/wiki/File:Thymolblau.png) I see no isomerisation is needed, simple protonation can be enough to change the spectra. Still, it doesn't explain what is protonated in the case of methyl violet.