Sauron
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Trapping NH3
I am planning a preparation of sym-diphenylurea from aniline and urea. Everything is straightforward but the reaction generates a lot of ammonia and I
want to scrub this rather than condensing it or just letting it escape.
Bubbling into water is out because of the ammonia-fountain effect (suckback.) Will running a tube to just above a stirred level of water absorb it
well enough?
Sic gorgeamus a los subjectatus nunc.
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Ozone
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How about suspending a funnel (just touching the surface of the liquid) over dilute HCl? If there is suck-back, the level rises in the funnel, goes
down in the beaker and breaks the connection. There will be (if careful) no suck-back into your system and very little, if any entrainement of air.
Ammonia is soluble enough to dissolve quickly at the gas-liquid interface and the funnel contains any that would otherwise escape.
This method also works well for scrubbing HCl/SO2 while preparing acyl chlorides from SOCl2.
Cheers,
O3
[Edited on 16-8-2008 by Ozone]
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--Albert Einstein
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evil_lurker
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Or one could pressurize the entire system with N2 or install some inline check valves.
Not all chemicals are bad. Without chemicals such as hydrogen and oxygen, for example, there would be no way to make water, a vital ingredient in
beer.
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Sauron
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Thanks.
The whole thing is a pretty simple setup. An air condenser is used for reflux as the excess aniline is acting as solvent. That's like 180 C so too hot
for water cooling.
When the stirred melt no longer gives off NH3 the reaction is done (5 hours) and the melt is poured into a large amount of water, sym-diphenylurea
ppts and any phenylurea stays in soln. Recrystallize in ethanol. I have used a smilar procedure in the past to prepare phthalimide from urea and
phthalic anhydride. It's a piece of cake.
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Panache
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a loosely packed but longish (~500mm) coiled length of rubber/pvc/silicon hose, id say 1/2' filled with a solid granular acid like NaHSO4 or citric
acid works well provided theres little moisture in your NH3 stream, otherwise it will clog, also if the reaction produces most of the NH3 in a burst
then this is also not practical as you will generate back-pressure.
However if its simply a steady consistent dry stream this works great and you have no worries about suck-back. Afterwards just flush water down the
hose and dispose of appropriately.
I find HCl(aq) traps annoying as besides the suckback issue, HCl gas can be drawn back into your reaction. If you want a liquid trap the two previous
mentioned acids in solution work great although don't have the capacity or adsorption that a 30% HCl solution would.
In respect to the inverted funnel technique, use a straight walled funnel (ie a buchner or a fritted sans frit, it allows much greater gas absorption
capacity (if you match your beaker to your funnel then the gap is quite small and the gaseous path length long) whilst also providing greater
protection from suckback, they also sit better i find. Also clamp the funnel otherwise it will bob up and down and be annoying and less effective
Wow i just wrote an essay for something that's already been answered, oops!!
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Nicodem
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Ammonia dissolves in water without any problem and endothermicaly. So just leading the tube above the water layer in a flask with stirred water is
more than enough. No need to bubble trough the liquid. Once the air is purged out from the flask by the incoming NH3, the atmosphere above the water
has 1 atm partial pressure of ammonia which means the absorption will be very rapid until the concentration remains bellow ~10% (saturated NH3 at RT
is about 25%). So just calculate the amount of water necessary to catch all the theoretically formed NH3 to keep the final concentration in such a
lower range.
Of just lead the tube in the fume hood exhaust, nobody will complain about little ammonia smell.
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Klute
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Isn't there some thing you could do with the stream of (apparently) anhydrous NH3? 2 birds with one stone...
Ever wanted to try a triacetoneamine preparation? Gas the NH3 in cold acetone with some ammonium salt, might consider using a scrubber on the exit for
unabsorbed NH3 (dilute HCl works very well IME), then leave it to stir a bit and distill your TAA.
An oxydation away from 4-oxo-TEMPO catalyst, considering the price it is sold, I consider it as a good aquisition.
Or in the case were you really don't feel ike putting much effort in it, you could use twoscrubbers, on of dH2O or dilute ammonia, and a second one
of dilute HCL, toobtain conc. NH3 effortless, always handy. The second scrubber would insure no escaping gas when the first solution is nearly
saturated.
Think process chemsitry, use eevrything, even the wastes 
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panziandi
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Sauron, could you not liquify the ammonia and bottle it as you are doing with your sulphur dioxide. Surely be useful to have a lecture bottle of
ammonia, get liquid ammonia from it etc?
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tapira1
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ammonia trap
You may use a H2SO4 trap and a safety (empty vessel) trap combined with N2 purge; alternatively you may use the same setup with water and get
concentrated ammonia as a byproduct. For condensing the ammonia you will need at least -33ºC (dry ice-solvent condenser)
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panziandi
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Well Sauron has started a thread on liquifying SO2 and storing the liquid gas, he may well use dry ice for that, If he does he could stretch to
liquifying this as liquid ammonia on tap could be very handy...
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tapira1
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trapping ammonia
Ammonia gas can be trapped as a liquid with a Dry-ice acetone or liquid Nitrogen-EtOAc bath (boiling point is -33ºC). This is what is usually done
when running Birch reactions.
[Edited on 7-9-2008 by tapira1]
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MagicJigPipe
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I've never heard of using a LN2-EtOAc bath for anything. What are the advantages of this? Does it not freeze? Will LN2 even dissolve in EtOAc or
vice versa? What is the BP of the solution/mixture? Melting point?
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question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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gsd
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1)
| Quote: | Originally posted by Sauron
The whole thing is a pretty simple setup. An air condenser is used for reflux as the excess aniline is acting as solvent. That's like 180 C so too hot
for water cooling.
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I routinely use simple water cooled Liebig condensor for fatty acid distillation - still head temperatures upto 250 Deg C.
2)
| Quote: |
Bubbling into water is out because of the ammonia-fountain effect (suckback.)
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Refer to good old Vogel. It has details of an excellent scrubber type water trap to avoid suck-backs.
gsd
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panziandi
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I've heard of acetone-LN supposedly the acetone freezes then dissolves. I was dubious thinking that LOx may build up as it evaporates off causing an
explosion, but I believe the N2 layer blankets from the air. Supposedly it drops the temperature to below LN temperatures but I don't have any info on
the temperatures you can reach.
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