fermium
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Methyl violet and Acid Fuchsin solubility (help!)
My problem is to remove stains caused by two aniline dyes. For my purposes I want to remove the stains as completely as possible by solvation, not by
bleaching or destroying the dyes because that might also be destructive towards the substrate.
So from the point of view of solubility theory, can anyone tell me or guess just by looking at the structures and
functional groups what these will be MOST soluble in?
I already know that they are soluble in water and in alcohol, those are the textbook answers that I am not looking for.  The red one likes to dissolve in basic aqueous solution more than neutral. The
purple one likes to dissolve in higher alcohols more than in methanol.
If you can choose ANYTHING which can be prepared as liquids at room temperature what would you choose?
Would aniline itself be a good choice ?
Rather than just hit these with a semi-random battery of organic solvents I want to know first from the point of view of theory. 
Here is the suspected structure of the red dye:
And here is the suspected structure of the purple dye
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chemoleo
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Well I don't think, Fermium, that there is much more to be said!
The fuchsin is obviously more soluble in bases as it quite acidic due to the sulphonic acid groups.
Methylviolett, conversely, should dissolve better in acid due to the three NR3's.
Using aniline as a solvent ... are you serious?
I DO think a battery of select solvents would be best.
You can, however, attempt to diazotise the fuchsin, and add groups that make it even more soluble. While of course, a number of the NH2s are lost as a
result of reacting with the substrate in the first place (which is why they are called 'dyes'.), so diazotising might not really help.
I don't think there are many other easy ways to break those triphenyl dyes down, not for no reason are they still used today and very popular.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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fermium
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aniline dyes solubility
The fuchsin is obviously more soluble in bases as it quite acidic due to the sulphonic acid groups. Methylviolet, conversely, should dissolve
better in acid due to the three NR3's.
Yes this is true and the methyl violet is in fact more soluble in acid than in neutral solution. This is helpful to me because you are getting at the
reason WHY it is more soluble.
Using aniline as a solvent ... are you serious?
They are known to be soluble in aniline and there is a preparation called "aniline water" which is used for some biological stains based on
aniline dyes. Aniline is about 2% soluble in water and this somehow increases the solubility of the dye as well. They add 2% aniline by weight to the
water, shake it up, and then add the dye. Can you speculate on why aniline is used for this purpose? Why does aniline itself seem implausible as the
solvent ?
I DO think a battery of select solvents would be best.
Do you have any suggestions other than alcohol and water? That's basically what I would like to know. Even if it's only an educated guess I
can try it.
You can, however, attempt to diazotise the fuchsin, and add groups that make it even more soluble.
This is maybe an elegant theoretical method but the nitrous acid would be too destructive towards the cellulose fibers of the substrate, I think. Does
not seem feasible.
While of course, a number of the NH2s are lost as a result of reacting with the substrate in the first place (which is why they are called
'dyes'.), so diazotising might not really help.
This is also an interesting and important statement - can you clarify this in the context of the dyes binding to cellulose? Thanks
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BromicAcid
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You're methyl violet, although a salt, has more organic character. Your nitrogen are surrounded for the most part by alkyl groups and as such
this molecule has a more non-polar feel to it, it might not be happy in something completely non-polar like carbon tetrachloride but it seems like it
would dissolve well in something like methyl ethyl ketone or pyridine seem like something this molecule would like to dissolve in, because although it
has some polar character (if it is the salt) it also has significant non-polar character, being for the most part alkyl groups. If however you stick
this in an acid solution you will protonate the nitrogens which will make them more ionic and therefore make the salt dissolve better.
Your acid fuchsin is a different matter, it has many polar substituents, sulfonate groups, easily protonated nitrogen molecules, if you try to put
this in a basic enviroment it has three sulfonate groups that will be 'ionic anchors' due to their interaction with the hydroxide making
them into salts. Although the amine groups will be in their non-protonated state they are still more water soluble then a simple alkyl, this molecule
would do well in base, there are probably other polar solvents that would also solvate it, but basified water seems to be a good choice.
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chemoleo
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On the note of aniline, I can speculate, yes 
I believe those triphenyl dyes undergo pi-coordinated complexes which are very stable (but not covalent). I believe cresols and such can also be
complexed in such a manner. I remember reading this somewhere but can't recall where. For instance, picric acid also complexes lots of
coordinated compounds, such as anthracene and such.
| Quote: | | Why does aniline itself seem implausible as the solvent ? |
Well first I thought in terms of reactivity. I
can't see any easy conjugation to solubilise those dyes. However, odd things happen with highly conjugated systems, who knows.
I'd search the internet, there's heaps of info on triphenyl dyes. Maybe you'll stumble across the info I once read.
| Quote: | | Do you have any suggestions other than alcohol and water? That's basically what I would like to know. Even if it's only an educated guess I
can try it. |
Lol - I didnt quite think along the lines of ethanol and water. I thought more along the lines of NMP, DMF, THF, acetonitrile, acetone, etc. Probably
even in that order in terms of effectiveness. Don't know how cellulose will like those solvents however. It may not dissolve it but it may wreck
the microcrystalline structure of the cellulose, thus rendering it brittle etc.
| Quote: | | This is also an interesting and important statement - can you clarify this in the context of the dyes binding to cellulose? Thanks
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Good question, I was wondering this myself. With hair, obviously the chemical reactivity of various amino acids in the keratin are exploited. Indigo
also binds cellulose, but I dont know the mechanism of action. Some dyes have a reactive chloride, that in the presence of base, reacts with the H of
the cellulose OH, form a permanent covalent linkage. Bromodyes (such as eosin, which is also a triphenyl) could react the same way I suppose
(that's speculation).
Do you know for a fact that fuchsin/methylviolet is used for cellulose dying btw?
Otherwise, did you search the 'books' thread? I seem to remember there was a book on dye chemistry.......
Further, I was reminded that there are four (!) books on dyes in the SM library!
[Edited on 29-11-2005 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
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fermium
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aniline dyes solubility
Thank you for the many helpful suggestions and information!
The idea that the fuchsine will be most soluble in aqueous base may be correct because so far the best method for the red dye is 10% ammonium
hydroxide, it gets most of the dye out. I guess the next step would be to try concentrated KOH or NaOH to see if it works any better. You are right
that caustic solutions are not great for the structure of cellulose. But they are more acceptable than concentrated acid. I like the ammonia because
it evaporates as a gas and therefore will not leave a residue. But the flip
side of that is that my vacuum pump is blowing ammonia out of the exhaust and filling up the room with the gas. 
I will look into trying some of the organic solvents mentioned. Organics which have already been tried and don't really work are: acetone,
toluene, and DMSO
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BromicAcid
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| Quote: | | Organics which have already been tried and don't really work are: acetone, toluene, and DMSO |
Did you
try these on both dyes of just one? You have a nice little range here, if a dye were insoluble in toluene I would have thought it would be at least
somewhat souble in acetone, or vice versa, DMSO being more toward the polar end. If they really are not soluble in any of these solvents you might
want to take solvents from the more polar side of things, like water (which I assume you've tried) or acetonitrile, or DMF or, although I'm
not realistically suggesting this, hydrazine.
When you start working on these things with bases, specifically methyl violet I would consider using something like KOH dissolved in water/ethanol,
when the molecule comes into contact with the KOH it will revert to the non-protonated form, the result of which will be to make it less polar, which
will make it more soluble with the help of the ethanol then in water alone, hopefully. Again though, these KOH solutions and such are not the best
things for cotton. I don't know weather or not the acid fuchsin would have any advantage by treating it with ethanol/water/KOH vs. just treating
it with ammonium hydroxide solution.
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ordenblitz
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I use aniline dyes in my business and they are horrible, staining everything in sight.
While soluble in acetone and isopropyl and many other solvents, using them on anything short of glass and ceramics, the dye will be further driven
into the substrate.
I did some experimentation trying to tear off the amine group and thus destroy the dyes. This proved to be more difficult that I had first thought.
The few things I found that worked were:
1) KMNO4, but that stained badly, porous surfaces.
2) Caro's acid works great but you can imagine why that is impractical for removing stains.
3) Mix of H2SO4+HNO3, but is also impractical.
4) Conc. H2SO4
I threw a battery of solvents at the dyes and these were the best for removing the stains.
1) N-Methyl pyrollidone, either straight up or diluted with water.
2) D-Limonene
Chemoleo is right on track with his solvent list.
[Edited on 29-11-2005 by ordenblitz]
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fermium
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Aniline dyes
I will go through those three solvents on both dyes again and report back - I believe but don't know for sure that they were already tested on
both.
Also thanks for the tips on limonene and pyrollidone. The pyrollidone was actually suggested to me before by a different person but not yet tested
(because I didn't have it).
FERMIUM is essentially a 'ghetto' chemist who is strong on practice but fairly weak on theory, hence these questions. I heard that he also
has no real analytical equipment so the results are just qualitative observations.
I heard that back in the good old days, FERMIUM, or someone like him, was known for making certain soaps with exotic scents but he has since,
according to rumor, given up that activity.
Your comments are very valuable because even when I ask a professional chemist about this stuff they usually don't know and don't want to
take the time to think about it. This type of forum is more democratic and more generous regarding the sharing of information.
To be continued...
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ordenblitz
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There a couple of brands of paint striper that contain NMP that can be found in the home building center.
There is one, if I am not mistaken that contains both limonene, NMP and some gelling goop. It's an easy distillation away from obtaining both.
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chemoleo
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I guess you realise that the NMP I mentioned is N-methylpyrrolidone, which ordenblitz mentioned independently.
Do note, it's a devilish solvent, if you drop it on your PVC floor you are going to dissolve it.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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fermium
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NMP
Yes, I know. And I will just buy it if that seems like the way to go, it's not worthwhile to distill hardware-store mixtures.
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ordenblitz
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Chemoleo....
That is why I mentioned about your list of solvents.
NMP will do a number on a lot of plastics but it is miscible in all proportions in water. Mixing it in this way will tend to slow it's attack.
The trick for me, was finding the concentration that got the dye but left the floor.
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fermium
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Updated solvent results
Hello, all.
Here are the results for a range of solvents (on a semi-quantitative scale of 0 to 5) which I have just replicated today.
So, given this information what does it predict about the best solvents for these two dyes? The red and the purple clearly have different
characteristics.
The purple was noted to change color at very low pH which is why some of the dyes in this family are used as pH indicators.
Most of the predictions you all made before are supported by the evidence so I feel we are getting closer to the truth !
Fermium.
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