[kims]

I don't understand how the acid would react with this compound. I know when you add -OH, it turns into a tetrahedral structure and completely loses color, but when acid is added, it just turns a lighter shade of green but I dont know why.

[cyberajnabee]

I have the same exact question and there is no solution for this available on the internet. Can anyone please address this question. Thanks alot

[agrobert]

http://en.wikipedia.org/wiki/Malachite_green

This dye has two transitions which are pH dependent.  Adding acid or base to change within the transition ranges accounts for observed color change.  Light green may indicate partial transition.  Structures are shown on wikipedia page.  The structures are intuitive with an understanding of acid-base catalyzed mechanisms.

[cyberajnabee]

I understand how adding HCl causes the molecule to turn into a salt, which is basically the molecule malachite green. However, if you keep adding HCl to the solution the solution will then slowly start to lose its color. it seems that excess protons actually take away the molecules color. My question is why is that? Where exactly do the protons attack "http://en.wikipedia.org/wiki/Image:Malachite_green.svg" on this molecule, so that it loses it's colors.

[agrobert]

What is the pH of water? Where is the pH transition?  Protons do not attack, but they will be picked up by basic sites.

[Rico]

Hey dakims and cyberajnabee

I have attached the acid-base dependent equilibria of malachite green in a jpeg file. Upon treating malachite green with basic water the carbinol is formed and this is colorless, since in this form, none of the benzene rings are conjugated.
The reason why the color is diminishing upon treatment with acid is, that one of the amines is protonated, giving the acid salt. The acid salt has less conjugation than malachite green since the lone-pair on the nitrogen is now used in a bond to hydrogen and it is therefore also less colored.

Rico 

[AWK]

http://www.chemie.uni-regensburg.de/Organische_Chemie/Didaktik/Keusch/cassy_KV_prot-e.htm

[weiguxp]

the pH of water is 7.

pH increases (more basic), the concentration of the dye cation decreases. This means as the solution gets more basic, me more chromatic malachite green is converted into the carbinol base (which passes through membrane easier)

I put up a diagram showing this :
  http://wikichem.net/index.php?title=Malachite_Green#PH_sensitivity_of_Malachite_green

More information could be found on the old website i made when i was doing a school project:

http://malachitegreen.tidalswan.com