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hopelessone
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anhydrous ammonia from urea?
Hi all,
I was reading that;
1. heat urea (fertilizer) to dryness 120°C?.
2. heat it further up to about 200°C.
Don´t overheat. Anhydrous ammonia will evolve. Lots of it.
Cool with air or icewater or dry ice in acetone or IPA or similar.
Anyone done this here?
thanks..
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Sauron
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Urea decomposes somewhere if memory serves in the 140-160 C range, before that it will be a liquid melt. In addition to NH3 of course you will get CO
but that will not condense. NH3 will condense but you must remember that it is a refrigerant so you need a LOT of cooling. My advide is to use a LARGE
Dewar type condenser, preferably a series of them, and also cool your receiver flasks in dry ice/acetone.
There are alt methods of generating chlorine that won't come off hot, and drying it before condensing. Ultimately you must store the liq NH3 in
cylinders with regulators. Obviously use a hood and respirator. A scrubber is a good idea is you don't want to stink up house/neighborhood.
HOT ammonia to liq ammonia is a foolish idea! It is hard enough to go from RT ammonia to liq ammonia (refrigerant, remember, look at the delta-phase
change heat of condensation! Try this on a small scale and you will see.
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Organikum
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Iron tubing is suitable. Run it decently through a icewater bath before into something cooled with dry ice and you are fine.
Ammonia does not store so much heat, no real problem.
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Sauron
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Urea melts at 137 C. And decomposes on further heating, yes. But not simply to NH3, it mostly goes to biuret, and some to cyanuric acid.
2 H2NC(O)NH2 -> NH3 + H2NC(O)NHC(O)NH2
120 g urea gets you 17 g ammonia. The other 103 g is biuret, and this is the commercial prep of biuret.
Biuret decomposes at 192 C but, I do not have any details on exactly what it decomposes to. Part of that might be ammonia, or it might be various
other products.
All in all this is an inefficient and IMO unnecessarily hazardous way to prep ammonia. A great way to make biuret though.
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Eclectic
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Urea and ammonium chloride with sufficient heating makes ammonia gas and cyanuric acid according to some posts in the cyanamide or guanidine threads.
NH3 and CaCl2 at room temperature makes CaCl2.(NH3)8 solid (with swelling to double volume). Mild heating releases 6 of the 8 NH3s.
There is a solar ice maker based on this:
Solar Ice Maker
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Sauron
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That's interesting.
As noted, 2 mols urea condense to one mol biuret abd split off one mol ammonia. So 120 g urea will produce 17 g ammonia.
Cyanuric acid is cyclic (-C(O)NH-)3 so one mol of biuret and one mol urea will condense and split off two mols NH3. If we simplify this reaction by
combining the two we get an overall equation
3 Urea -> 1 Cyanuric acid + 3 NH3
which is probably not the actual mechanism.
3 NH2C(O)NH2 -> (C(O)NH-)3 + 3 NH3
So if cyanuric acid is the only product then 180 g urea gives 51 g ammonia
However we know that cyanuric acid is not the only or main solid product - biuret is, and I still do not know details of biuret's pyrolysis, except
that when conducted at higher than 192 the product is melamine.
What are the stoichiometries involved in the reaction between urea and NH4Cl and what else forms, the chloride ions have to end up somewhere.
If the CaCl2.(NH3)8 is built from dry NH3 then it is pretty useless as a source for NH3 (orcept for portability purposes, as in the solar ice maker).
And what is the temperature required for the urea/NH4Cl reaction as compared to urea alone?
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Eclectic
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CaCl2.(NH3)8 is a lot safer to store than liquid ammonia. Also probably easier and safer to work with to initially absorb NH3 from a urea ->
ammonia generator than other options.
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Sauron
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Safer than a lecture bottle and valve with a dip tube?
I'd condense the NH3 directly into the LB (also chilled) and then add the (also chilled) valve and dip tube to close. Usual ptfe tape to seal. KiloLab
steel cylinders are also good.
Same storage for liq SO2 or Cl2. I can buy empty new LBs easily.
I'd like to see more details about absorption of NH3 by CaCl2 esp at normal pressure. These folks with the icemaker are charging the CaCl2 with
compressed ammonia and then running their system at 200 psi on a day/night cycle.
I am reminded of the Harrison Ford film "Mosquito Coast" based on Paul Theroux's book. That character was hung up on NH3 refrigeration too.
Generally all we chemists want is some liquid NH3 on demand and the only ice we need be concerned with in this contect is the dry kind...
[Edited on 14-9-2007 by Sauron]
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franklyn
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If you're going to all this trouble to decompose urea by heating you may as well
obtain 100% of theoretical recovery of ammonia by mixing with strong Alkali in
water. H2N.CO.NH2 + Ca(OH)2 -> CaCO3 + 2NH3 . The lime formed can then be
heated itself to drive off the CO2. Much simpler and neater overall.
There's a proceedure for producing liquified ammonia here.
http://www.erowid.org/archive/rhodium/chemistry/eleusis/ammo...
.
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Sauron
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That is of course same page as on Rhodium, doubtless where erowid got it. I would certainly recommend taking NH3 off of conc ammonium hydroxide, or
basifying NH4Cl depending on what is more available in your locale. Urea, with or without slaked lime as franklyn suggests, is to me a distant third
choice.
The adsorption of NH3 by CaCl2 appears to be rather slow at ordinary pressure, varying between 1 hr and 20 hrs depending on target ammine (CaCl2.NH3,
.2NH3, .4NH3, .8NH3). Since only the octammine is of interest this suggests that preparing CaCl2.8NH3 really needs to be done in a pressurized system
from a cylinder of dry NH3 and not from an ammonia generator at all. My point is that if you have a tank of NH3 what use is the ammine salt? This is
especiually the case since 25% of the ammonia will not be available (only 6 of the 8 mols NH3 are released by mild heating.)
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S.C. Wack
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It can however be handy for assuring a perfectly dry and pure gas, on top of temporary storage without the need for dry ice, ammonia cylinders, or gas
regulators. Not that it is difficult to put together a purification train, but the simpler the better. Of course the same can also be obtained from
combination with ammonium nitrate (Divers' solution), or better, ammonium thiocyanate if you just happen to have some. Or best a mixture of the two.
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garage chemist
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Heating Urea does not make anhydrous ammonia, some water vapor is produced as well. The ammonia has to be dried before use.
Also, it would be more effective to heat urea with NaOH solution, since that releases all the N in urea as ammonia.
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Sauron
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As S/C/ Wack mentions there are a lot of these solid/gas equilibrium mitures with NH3.
Lithium nitrate
Strontium chloride
Calcium nitrate
Barium chloride
Plus the already mentioned CaCl2, NH4NO3 and NH4SCN. The ammonium nitrate/ammonia systems are very corrosive and so are the ammonium
thiocyanate/ammonia systems.
These ammonia-metal salt or ammonia-ammonium salt solid solutions vary widely in vapor pressure, and other parameters as well as cost. Some are
prepared from hydrated salts, and may be liquids. Most are anhydrous solids. Some may be stored in inert solvents (which have to be carefully
selected) and often exhibit faster eqilibrium rates in such suspensions than they do neat.
Calcium nitrate is one of the best such systems being less corrosive to iron and steel than calcium chloride systems and just as cheap. This according
to a JACS article attached below.
Major applications: adsorprion of NH3 in ammonia synthetic process; refrigeration systems (heat pumps).
The preparation of these solid solutions still seems to involve rather long reaction times at ordinary pressure, during which times the finely ground
anhydrous salt (neat or suspended in inert liquid) swells to about double its initial volume.
As far as I can see this makes it hard to charge the salt with ammonia from a typical ammonia gas generator rather than from a cylinder.
@S.C., have you any details on the lab scale prep of these solid solutions?
[Edited on 14-9-2007 by Sauron]
Attachment: ja01362a006.pdf (454kB)
This file has been downloaded 1124 times
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Eclectic
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Also, does anyone have any data on the properties of these ammine salts and ammonia-salt solutions? Ammonia vapor pressure, dissociation
temperature,etc.
SC, anything in detail on Diver's solution, or ANA nitrogen fertilizer?
Google didn't turn up anything of much use.
Is it OK to be interested in ammonia as a refrigerant on this forum, or is any use other than drugs or explosives politically incorrect? 
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Sauron
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I have found a few more articles from ACS journals with vapor pressure/temperature information. I will post them here.
The zip file of these five articles is too large to post here but I will place it on my 4shared.com folder and post that link, so anyone can access
and download it, no PW required,
http://www.4shared.com/dir/2245331/5a78115f/sharing.html
It's the only file not in a subfolder.
One of the articles includes a footnote to a peper by E.Divers which may be the original publication on "Divers' Solution" referenced by S.C.Wack.
Note that while CaCl2-octammine is solid, the solutions of NH4NO3 and NH4SCN are liquid. 100 g NH3 will dissolve 300-400 g of these salts and the
salts rapidly liquify in an atmosphere of NH3. They are however extremely corrosive to ferrous metals, although the papers do not specifically discuss
stainless steels.
[Edited on 14-9-2007 by Sauron]
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Eclectic
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Here is the original article on ammonium nitrate-ammonia:
Divers' Solution
S.C., Any references for ammonia absorption by mixed ammonium thiocyanate-nitrate?
[Edited on 9-14-2007 by Eclectic]
Is anyone up on thiocyanate chemistry? As in "how to make"?
[Edited on 9-14-2007 by Eclectic]
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S.C. Wack
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I'm not so sure that there is an easy convenient amateur-friendly method for making ammonium thiocyanate.
The mention of reduction of nitrate to nitrite in solution with zinc in the Divers Proceedings... is interesting. Perhaps the presence of ammonia
prevents the decomposition to same, making nitrite more favorable. More details on page 869 of the first Watts supplement.
An article in full, JACS 43, 1178 (1921):
A Method for Producing Dry Ammonia.- In the work described previously in this number of THIS JOURNAL, it was desirable to have a source of dry ammonia
gas which could be used at variable pressures up to two atmospheres. For this purpose, we have modified the method for producing ammonia used by Keyes
and Brownlee, [THIS JOURNAL, 40, 25 (1918).] consisting of warming a solution of ammonium nitrate in ammonia, by substituting ammonium thiocyanate for
the nitrate. The thiocyanate has the advantage of taking up more ammonia than the nitrate at a given pressure, but its chief advantage lies in the
extreme rapidity with which it absorbs ammonia.
The apparatus consists merely of a heavy-walled salt-mouth bottle holding about 500 cc., with a stopper provided with an outlet tube, and with an
inlet tube for charging. The bottle is nearly filled with dry ammonium thiocyanate to act as an absorbent. The inlet and outlet tubes may be provided
with glass stopcocks or even with heavy-walled rubber tube and screw pinchcocks.
The apparatus is stored with ammonia by passing in the dry gas, keeping the bottle cooled with ice during the process. When saturated with ammonia at
0° and atmospheric pressure, the solution contains about 45% ammonia. The gas is absorbed with a rapidity comparable to that of ammonia in water, so
that a very rapid current can be run in with almost no loss. By placing the bottle, after being stored, in a waterbath, and keeping at room
temperature or slightly above, the ammonia can be drawn off as desired. The ammonia in this generator is nearly the equivalent of liquid ammonia as
purchased in tanks, except that its freedom from moisture can be assured, and it is delivered at a much lower pressure which can be varied at will by
adjusting the temperature. It is well to recharge the generator before the thiocyanate crystallizes in too great amount. Otherwise, the inlet tube may
become blocked by the salt.
H. W. FOOTE AND S. R. BRINKLEY.
Another by the same authors, this time on vapor pressures, JACS 43, 1018 (1921):
Attachment: jacs_43_1018_1921.pdf (804kB)
This file has been downloaded 1385 times
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Eclectic
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"Industrial Nitrogen Compounds and Explosives" from the Library has some preparations from ammonia and CS2 at 100C under pressure.
So it's sort of a toss up: Ammonium cyanide + ammonium sulfide, or a process that makes lots of H2S.
Maybe CS2 and Urea will work? 
Anyone have a good monograph on CS2 and it's derivatives?
[Edited on 9-14-2007 by Eclectic]
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Sauron
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EERE (at the DOE) discusses CaCl2-ammonia as refrigerant but reminds that ammoni-based systems, while useful for industrial refrigeration, are
unsuitable for residential refrigeration applications.
DUH! Getting away from NH3 as refrigerant was precisely why we went to freons in the first place.
So much for the solar ice maker.
@Eclectic, looks like you will need an autoclave if you want to make your own ammonium thiocyanate.
It does seem to be unique in its rapidity of adsorption of NH3, all the others being rather sluggish.
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12AX7
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...And SO2, and H2S. Now there's a refrigerant that'll discourage tampering!
...But I digress...
Tim
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Eclectic
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Well, I do have a 10 gallon 600 psi rated stainless steel pressure vessel, but somehow, the burst diaphragm on it doesn't give me any sense of safety
when I contemplate a prep scale reaction. 
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Sauron
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Woof! Not when what your burst valve would be venting would lots of hot and angry H2S, I suppose not!
Better look for a different method, or a different salt, or just buy the thiocyanate.
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Eclectic
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I inquired about buying 12.5kg from ESP. Jeeze those guys are nosey and paranoid. Thiocyanate is not even a hazmat material for transport.
[Edited on 9-14-2007 by Eclectic]
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Sauron
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Have you got Ullmann's?
See section 2.1 under Thiocyanates-Inorganic Thiocyanates
Production. Ammonium thiocyanate is obtained from CS2 and NH3 or from the reaction of hydrogen cyanide or cyanides with sulfur and/or polysulfides.
Production from Ammonia and Carbon Disulfide. This continuous, pressure-free process is carried out in two steps [6]. In the first step CS2 and
ammonia gas are reacted in the presence of water over activated charcoal at ca. 50 °C in an interface reactor to give ammonium dithiocarbamate.
The latter is decomposed in the second step at 95 – 100 °C on activated charcoal.
If the NH3 and CS2 used are of high purity, the 35 – 45 % ammonium thiocyanate solution formed can be marketed after concentration without further
purification. The crystals, which are produced by vacuum evaporation and centrifugation, are of higher purity. In this process, apart from H2S and
excess CS2 , no other waste substances are formed. After CS2 and H2S have been separated, the former can be recycled and the latter can be used, for
example, for sulfur production in the Claus process. There is also a one-step variant of this process in which the reaction of NH3 with CS2 and
decomposition take place at ca. 115 °C and 5 bar [7].
-------------------------
That rxn of CS2 and NH3 is at 1 atm. All you need to worry about is scrubbing the H2S byproduct (oversize caustic scrubber ought to do the trick.)
Sounds like a piece of cake as long you don't mind the fire hazard of the CS2.
The alt reactions involve HCN, ugh.
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Eclectic
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Anyone have access to "Inorganic Syntheses" on Wiley Interscience?
Ammonium Dithiocarbamate
[Edited on 9-14-2007 by Eclectic]
OC(NH2)2 + CS2 -> COS + NH4SCN 
This would maybe also work as a two step atmospheric pressure process?
Carbon Disulfide Properties and Uses
[Edited on 9-15-2007 by Eclectic]
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