Vomaturge
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Isocyanic Acid Danger?
I've been thinking about trying to make ammonia via the pyrolysis of urea. From some basic research, it looks like apart from ammonia, this type of
reaction will make biuret, triuret, isocyanic acid, cyanuric acid, and maybe ammeline, ammelide, and melamine. The by-product which worries me is
isocyanic acid. I've heard it hydrolyzes to NH3 (great!) and CO2 (just about harmless), but that it needs acidic conditions to do so. Ammonia solution
is not acidic, so I assume that at least *some* HNCO will become airborne.
I couldn't find any detailed toxicity information about it, but I did find a paper contemplating negative health effects at <.01 ppm:
http://www.pnas.org/content/108/22/8966.full
Apparently, the related compound methyl isocyanate is really poisonous, both acutely and long term:
https://en.m.wikipedia.org/wiki/Methyl_isocyanate 
So, my question is, how toxic is isocyanic acid and cyanuric acid?
If it's like ammonia or carbon monoxide, I'll only use a small bit of urea (a gram or two), and I'll open a window.
If it's more like hydrogen sulfide or cyanide, I'll do a tiny amount, outside.
Now, if it's anything like methyl isocyanate or phosgene, then forget it. With only a basic knowledge of chemistry, and only rudimentary home lab
equipment (some of it improvised), I don't think I'm up for dealing with a highly poisonous gas.
I know there are already threads on urea decomposition, but I'm wondering about the specific issue of dangerous byproducts.
Edit: I realize that carbon monoxide is a lot more dangerous than ammonia, both because it is more toxic and because it's odorless and thus can cause
illness or death without warning.
Similarly, hydrogen sulfide is not as toxic as hydrogen cyanide, and AFAIK, H2S smells worse too.
Isocyanic acid in an ammonia solution would likely make ammonium isocyanate, although a lot of ammonia and isocyanic acid would still escape in their
free state.
[Edited on 21-2-2018 by Vomaturge]
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j_sum1
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Do you have to use this route for ammonia? I have found ammonium sulfate fertiliser and NaOH to be a sensible route. (Ok, almost the only route
where I live -- ammonia in Aus is really hard to get for a home chemist.)
I do not know much about urea decomposition except that quite a few on this site use that as their source of ammonia and manage the risks ok.
Cyanuric acid is an OTC pool chemical and should not pose to many concerns. I don't know about isocyanic acid.
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Vomaturge
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Thanks for the reply! As a matter of fact, I've got a bottle of 3% (I think) cleaning ammonia solution, and could obtain ammonium sulfate based
fertilizer (I have NaOH drain cleaner). If I made NH3 from urea, it would be more for the sake of doing it, observing a new process. Urea could also
be very cheap, although ammonium sulfate can be too.
Good to know that the cyanuric acid is probably relatively safe.
Sorry to hear that NH3 is hard to get in Austrailia
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j_sum1
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You can buy 3% ammonia solution from the supermarket as a floor cleaner but it is full of other crap.
Buying from a chem supplier involves sky-high shipping charges.
The most readily available ammonia salt is crude (NH4)2SO4 which is dirt cheap. It really is not too difficult to synthesise NH3 from that. Urea is
also easy to get at decent purity but, you know, once you have a simple procedure that works, why change?
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Chemetix
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Thermal hydrolysis of urea, that is heat with water, is a better option. It's slow but steady. Depends on how much you need at a time. Pyrolysis is a
mess chemically and practically. Urea decomposes with co evolution of CO2, so that needs to be taken in to account. The sulphate method is a good lab
scale preparation though.
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Magpie
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I can easily buy 30% aqueous NH3. To make dry ammonia I have boiled this, leading the wet gas through a column packed with KOH flakes to obtain the
dry gas.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Vomaturge
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Okay, I couldn't find an MSDS for isocyanic acid itself, but I found one for urea which mentioned it as a possible decomposition product:
http://lntsolutions.com/downloads/IceMelt-MSDS.pdf
Imagine that! The permissible exposure over 8 hrs is .02 mg per cubic meter, and for 10 minute exposure, .07 mg per cubic meter. Makes NO2 and HCN
look like a refreshing breeze!
I don't know how much isocyanic acid is produced by thermal decomposition of urea, but I don't plan on doing this experiment any time soon.
If I want ammonia as a reactant for some other experiment, I'll get it some other way. Thank you everyone for sharing some other useful processes to
make ammonia. Not only do they sound safer, they will also likely make a purer product
I bet a distillation-like process would work more or less on the 3% solution. I don't expect anhydrous gas from something like that, but I don't
expect to need it now, either. By the way, it seems that 3% cleaning ammonia is around 1$/Liter, but the 10% is around 5-10$/L. I've also seen several
other threads on sourcing or generating ammonia, so I have plenty of ideas now. If I were dead set on using urea as the precursor, I could use hot
hydrolysis, or perhaps calcium hydroxide, and hopefully avoid isocyanic acid production.
Again, thanks.
Edit: I think some of the chemical methods of making ammonia are actually cheaper than buying it at an amateur scale. Urea and ammonium sulfate are
made from ammonia, but they are available over the counter at something closer to the bulk price, as opposed to cleaning ammonia solutions and ammonia
from chemistry supply stores. Those are inflated in price from packaging costs, supply/demand, purification, retail markups, hazmat fees, and a bunch
of other reasons.
One more thing: Hopefully the people who write mostly bogus DIY weapon 'black books' don't hear about HNCO. I can imagine them giving a recipe for a
'super deadly gas' beginning with "fill a glass bottle half full of urine, then add gasoline and... don't do this in an a town. It is superlethal for
at least 100 m" 
[Edited on 22-2-2018 by Vomaturge]
[Edited on 22-2-2018 by Vomaturge]
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Rhodanide
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Quote: Originally posted by Vomaturge  |
One more thing: Hopefully the people who write mostly bogus DIY weapon 'black books' don't hear about HNCO. I can imagine them giving a recipe for a
'super deadly gas' beginning with "fill a glass bottle half full of urine, then add gasoline and... don't do this in an a town. It is superlethal for
at least 100 m"
[Edited on 22-2-2018 by Vomaturge]
[Edited on 22-2-2018 by Vomaturge] |
What?
What the hell would that make?
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Texium
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Rather than pyrolyzing your urea and making a huge mess, you can simply add it to a solution of NaOH to generate ammonia quickly and cleanly (not as
fast as an ammonium salt, but still a good steady rate), with sodium carbonate as your only byproduct.
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Vomaturge
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| Quote: Originally posted by Rhodanide | | Quote: Originally posted by Vomaturge |
One more thing: Hopefully the people who write mostly bogus DIY weapon 'black books' don't hear about HNCO. I can imagine them giving a recipe for a
'super deadly gas' beginning with "fill a glass bottle half full of urine, then add gasoline and... don't do this in an a town. It is superlethal for
at least 100 m"
[Edited on 22-2-2018 by Vomaturge]
[Edited on 22-2-2018 by Vomaturge] |
What?
What the hell would that make? |
Probably nothing. It was more of a joke. Sometimes people post to sciencemadness asking why the "recipes" the tried out from The Anarchists' Cookbook
or similar don't work. The reason is that the people who make these books don't really understand the science, so they give recipes that involve
nitrations without sulfuric acid, or even putting a fuse into a jar of normal Vaseline to make a "fragmentation grenade", or scraping (dry) nitrogen
triiodide out of a glass bottle. Some of it's really dangerous, and the rest is just about harmless since it doesn't work. The joke was that if these
people read that heating urea could make a toxic gas, they would immediately write that burning piss (1% urea?) works great as a poison. Not anything
meaningful, just me goofing around.
And thanks, Texium, for the sodium hydroxide suggestion. It probably gives better yields than pyrolysis, too, since you don't waste urea producing all
the other crap. I definitely won't be trying to get ammonia by pyrolysis of urea, unless I want some of those other products in their own right, and
can figure out how to separate them safely and healthily.
Thanks everybody, I feel like you have just handed (spoonfed?) me more ways to get ammonia than I could have ever asked for!
Edit: Come to think of it, a discussion on safety, and avoiding toxic materials, is probably not the most appropriate place for a joke.
[Edited on 26-2-2018 by Vomaturge]
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