16MillionEyes
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Dyes
Some of you might be already familiar with this annoyance if you buy OTC reagents. In reality it's more than an annoyance, it's utter pain!
Yesterday I bought some store grade potassium hydroxide whose MSDS suggested the presence of relatively pure potassium hydroxide and water (45% to 55%
water to be exact). Obviously this sounded like a great deal so I went ahead and bought it and went home with high expectations. To my misfortune,
when I opened the bottle I found it to be a green liquid. Obviously a little test confirmed the presence of potassium hydroxide but the green dye
ruins it making it almost useless for any qualitative analysis.
Does any of you have any idea what the dye might be made of (I found no information on the types of dyes used) and how it might be removed out of the
chemical?
Same thing happened to me with some sulfuric acid last time and I'm getting tired of throwing away my money. Being able to get rid of the dyes will be
of great help.
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woelen
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Unfortunately, in this case of KOH, it is really difficult to get rid of the dyes, without adding other new impurities. KOH does not crystallize
easily, so concentrating the liquid by boiling off the water does not give you crystals of KOH. You might have some success if you add a lot of
ethanol to the liquid, causing part of the KOH to precipitate, but how much dye coprecipitates is hard to tell. Also, is it worth the cost of the
ethanol to get KOH from such an impure source?
Besides the dyes I bet that there also will be all kinds of detergents in the liquid, making it stick more to the walls of the tubes for which it is
intended. This kind of reagents indeed is close to useless for home chemistry.
I had a similar thing with HCl, to which a lot of yellow/green stuff was added. This HCl could be used, however, simply by distilling it over, leaving
the yellow/green stuff behind. Sadly, this kind of things is done more and more. Most likely they are doing this to make the chemical better for the
purpose for which it is intended, but I also sometimes have the impression that impurities are added to assure that the chemical only is useful for
its intended purpose.
In the future, be even more careful with buying this kind of OTC material. There are drain cleaners, which consist of almost 100% solid NaOH. I'm not
in the USA, but I'm quite sure that this chemical also is easily available in the USA.
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not_important
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You might try extracting the dye with an inert-to-KOH solvent, ethers and aromatic hydrocarbons for example.
Evaporating the solution to dryness, which means fusing the KOH, and then heating to 500 C or so, would likely cause destruction of the dye. After
that the KOH could be brought back into solution and the carbon and whatnot from the dye filtered off.
The alternative suggested by woelen has the problem that KOH is somewhat soluble in alcohols, isopropyl would be a better choice that ethanol as it
would dissolve less of the KOH.
If you had a metal distillation 'flask' you could try EtOH or iPrOH plus 10 to 20 percent toluene, and the axeotrope off the water, returning the
toluene+alcohol layer to the still pot. As the water seemed to be fully removed, add more toluene. This would give you a solid mix of the hydroxide
and alkoxide, likely with all the dye remaining in solution. Filtering off the solid, washinh with toluene, then adding it to a several-fold excess
of water in the metal flask, followed by distilling off alcohol-water would give you back KOH. But that is a lot of work.
In some cases the dyes are added, at least in part, to make it obvious that the liquid is not water, although the range of colours used in "sports
drinks" would seem to allow for confusion. Making it difficult to use the product as a reagent may well be another reason.
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16MillionEyes
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"There are drain cleaners, which consist of almost 100% solid NaOH. I'm not in the USA, but I'm quite sure that this chemical also is easily available
in the USA."
No, this was banned from the market long ago. It used to be sold as "Red Devil" but ever since junkies decided to give a mediocre try at chemistry
(for obvious purposes) it got removed.
I had already thought of crystallizing the potassium hydroxide and then dissolve in a sparingly polar substance (as suggested and also because since
the dye is soluble in water it must also be polar) thinking perhaps the dye (which I consider to be organic in nature) might dissolve off more than
KOH. Main problem with this approach is that I've noticed that when the solution is heated, water vapor comes out with some of the KOH dissolved in it
creating a very choking and dangerous environment. I'm thinking that the best to avoid this problem is carry out the vaporization in a closed flask
and distill. Problem with that is that I lack the fancy equipment to do it.
It's also possible that heating to high temperatures the dye might combust off but this would have to be tried out after I resolve a way to
crystallize the KOH in a safe manner (if possible). See if I figure out something with plastic bottles. 
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chromium
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KOH will not come over with water vapour but those nasty fumes are probably due of small liquid particles that go airborne because of intense
bubbling.
I do not think that combustion of that dye is anything dangerous because of low concentration.
If i were you i did not think of cleaning this colored KOH as its not worth the work. For example you can not use common glassware when heating KOH
solution and you need to exclude any CO2 - or part of your KOH will turn to carbonate.
If you need pure hydroxide then why not make it using lime and sodium or potassium carbonate? You can use this colored KOH for making hydrogen
(electrolytically or by reacting with Al), for precipitaing insoluble hydroxides, for driving ammonia off from its salts, absorbing CO2 etc. There are
actually quite lot of uses were this kind of impurities are not a problem. You could even staturate this solution with CO2 to make potash and after
crystallisation and strong heating use it for making rather pure KOH by reaction with lime.
[Edited on 21-8-2007 by chromium]
When all think alike, then no one is thinking. - Walter Lippmann
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chemist1066
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Are you sure it is a dye? IN HCl it is more likely Iron impurities and not a dye.
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16MillionEyes
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Actually, after neutralizing this solution and doing some reactions I've seen that the dye seems to "disappear". I'm starting to believe that this so
called dye is nothing more than a pH indicator of some sort as the reactions that I'm talking about consumed completely the available potassium
hydroxide and either disappear or turned clear-yellow. I think the strongest evidence for this is when I neutralized it the color almost disappeared
leaving just a faint yellow.
This is just an idea I really have no idea but also I don't see why the dye would dissappear proportionally as the KOH is consumed.
"KOH will not come over with water vapour but those nasty fumes are probably due of small liquid particles that go airborne because of intense
bubbling"
You right. I was basing what I said from the highly exothermic reaction with aluminum which gave intense bubbling and water vapor with KOH dissolved
in it. I thought this was due to the heat and not bubbling.
"If you need pure hydroxide then why not make it using lime and sodium or potassium carbonate?"
Well, I have some sodium carbonate laying around but the problem is that I have nothing that will take the reaction to reach 1000C. The most I've ever
gotten is ~350C but that's not enough to decompose those carbonates.
I really need some strong base because I have none. Even if this requires some work I guess it's worth it more than having nothing unless you know
some OTC product that you recommend. That would be helpful as well.
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not_important
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The K/Na2CO3 + Ca(OH)2 reaction is done in water, not by fusion. metathesis not decomposition.
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16MillionEyes
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Ah I see. I replaced the "and" for "or" apparently.
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