Sciencemadness Discussion Board

anhydrous ammonia from urea?

hopelessone - 13-9-2007 at 05:12

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?


Sauron - 13-9-2007 at 07:22

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.

Organikum - 13-9-2007 at 09:14

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.

Sauron - 13-9-2007 at 10:10

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.

Eclectic - 13-9-2007 at 10:58

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

Sauron - 13-9-2007 at 17:01

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?

Eclectic - 13-9-2007 at 17:42

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.

Sauron - 13-9-2007 at 18:01

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]

franklyn - 13-9-2007 at 19:28

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.


Sauron - 13-9-2007 at 21:04

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.)

S.C. Wack - 14-9-2007 at 03:41

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.

garage chemist - 14-9-2007 at 04:28

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.

Sauron - 14-9-2007 at 04:29

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 757 times

Eclectic - 14-9-2007 at 06:46

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? :P

Sauron - 14-9-2007 at 07:17

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 folder and post that link, so anyone can access and download it, no PW required,

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]

Eclectic - 14-9-2007 at 09:25

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]

S.C. Wack - 14-9-2007 at 13:05

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.

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 970 times

Eclectic - 14-9-2007 at 18:33

"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]

Sauron - 14-9-2007 at 19:21

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.

12AX7 - 14-9-2007 at 19:30

...And SO2, and H2S. Now there's a refrigerant that'll discourage tampering!

...But I digress...


Eclectic - 14-9-2007 at 19:40

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. :o

Sauron - 14-9-2007 at 19:49

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.

Eclectic - 14-9-2007 at 20:10

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]

Sauron - 14-9-2007 at 20:30

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.

Eclectic - 14-9-2007 at 20:48

Anyone have access to "Inorganic Syntheses" on Wiley Interscience?

Ammonium Dithiocarbamate

[Edited on 9-14-2007 by Eclectic]

OC(NH2)2 + CS2 -> COS + NH4SCN :D

This would maybe also work as a two step atmospheric pressure process?

Carbon Disulfide Properties and Uses

[Edited on 9-15-2007 by Eclectic]

Sauron - 14-9-2007 at 21:25

COS = carbonyl sulfide, how to deal with that?

S.C.Wack used to provide a compendium of selected monographs from Inorg.Syn, I think on rapidshare, and perhaps on ftp #1 (Axehandle) but thus far I he has resisted all blandishments to re-up this item, to FTP #2, or 4shared or something.

I dunno if this item is even among his handpicks.

Eclectic - 14-9-2007 at 21:32

You feed the COS with oxygen to your heated vanadium catalyst tube. :D

Mr. Wizard - 14-9-2007 at 21:42

You can liquefy ammonia with simple steel and iron fittings. There used to be a lot of links when you search for Crosley IcyBall. There was even one made with regular steel plumbing pipe and it worked. (Larry Hall Icyball) also see IcyBall Patent #1,740,737 and IcyBall Patent #1,811,523
I downloaded a copy of the sites main information. Try searching for it. Liquid ammonia is dangerous and attacks brass and copper. Liquid propane tankers are sometimes used to haul liquid ammonia in the summer season, if they have the right fittings, that is stainless steel or steel valves.

Strong ammonia fumes will attack every moist, damp, or wet area on your body. It can render your eyes to trash in seconds and leave your crotch and armpits a scalding wound. Emergency workers are known to spread generous amounts of petroleum jelly (Vaseline) on sensitive areas, before using a self contained breathing apparatus to go in and stop a leak at a plant using ammonia. On the other hand a spray of water can absorb enormous amounts of the gas. An ammonia leak is a real nasty item, for you and all your neighbors, and it goes on for hours and hours and hours. Liquid ammonia refrigerates itself so well that it will take forever to evaporate. I've been told a glass of liquid ammonia, without any insulation will last over 24 hours.

I don't know how this will work using Urea, but to say it can't be done isn't true.

Sauron - 14-9-2007 at 21:55

The German patent (looks like Akso's chemists) for the two step process:

Enka Glanzstoff, DE 1 297 088, 1967 (H. Maegerlein, H. Rupp, G. Meyer).

One step process at 115 C and 5 bar, this one definitely Akso

Akzo, DE 3 508 404, 1986 (G. Meyer, G. Sudheimer, H. Zengel, H.

BTW ammonium thiocyanate and thiourea are isomeri just as urea and ammonium cyanate are. Heating ammonium thiocyanate gets you thiourea. But slowly and incompletely. I wonder if this goes both ways? The answer is yes, it is an equilibrium but the two (thiourea and thiocyanate) are very hard to seperate so this method is not employed preparatively and is only of historical interest.


Org.Syn. to the rescue

Ammonium dithiocarbamate. Gaseous ammonia is passed into 250 ml. of 95% ethanol (Note 1) contained in a 1-l. Erlenmeyer flask immersed in an ice bath until the gain in weight is 39 g. (2.3 moles). To this solution, still cooled by the ice bath, is added a well-cooled mixture of 76 g. (60 ml., 1 mole) of carbon disulfide and 200 ml. of ether. The flask is stoppered loosely (Note 2) and allowed to remain in the ice bath for 2–3 hours and then at room temperature overnight (Note 3). The mixture is again cooled in an ice bath or refrigerator, and the crystals are collected by filtration (hood), sucked dry, and washed on the filter with two 50-ml. portions of ether. Air is drawn through the crystals for 5 minutes (Note 4) to effect removal of most of the ether. The product is used promptly without further treatment; the weight of the lemon-yellow solid varies between 80 and 90 g., depending principally on the completeness of the removal of the solvent.

Collective Volume 3, p.763

I will attach the entire prep, which is of Rhodanine, so you will have the references and the notes for Part A which is the ammonium dithiocarbamate. No interface reactor, no activated charcoal, just ethanolic NH3 and CS2/Et2O and you have what you want to pyrolize at 95-100 C to NH4SCN.

So there IS a simple easy way to make ammonium thiocyanate after all.

[Edited on 15-9-2007 by Sauron]

Attachment: CV3P0763.pdf (120kB)
This file has been downloaded 1073 times

Sauron - 15-9-2007 at 01:13


The attached is the JACS article on which the Org.Syn. prep of rhodanine was based, for comparison of their procedure for ammonium dithiocarbamate. I suspect the Org.Syn. variation is a little more thoroughly described.

Sorry about the double post!

[Edited on 15-9-2007 by Sauron]

Attachment: ja01309a053.pdf (336kB)
This file has been downloaded 686 times

S.C. Wack - 15-9-2007 at 04:32

Some corrections if I may digress from the point of this thread entirely.

I specified "easy convenient amateur-friendly method", which CS2 is not exactly, as much fun as it is to make or spend outrageously large sums of cash on.

Axehandle has FTP#2. There is a thread with this title.
No one has asked me for the Inorg Syn collection. [edit- I obviously do not recall such a request] The relevant preparation isn't in that djvu anyways.
I uploaded everything I had of my own at the time to the newer FTP, and it was all erased.
I put all of my book scans on 4shared along with other useful pdfs for a total of 975 MB for many months, in fact I started the thread on 4shared if you look, and the WWW button which is no longer at the bottom of my posts, but was for some time, took you directly to them. One can find amongst my scans the preparation of ammonium thiocyanate from CS2 and ammonia.

The 4shared collection is no longer there because my last 3 uploads and the rest of the collection got a single download between them in the last 3 months, and that one only because Mephisto wanted to see it, from its mention in the book scanning thread.

On topic- "from urea?"
No. Stick with what has been tried and true for many years for ammonia generation or just about anything else and you'll get more done. OK that may be construed as discouraging experimentation... Experiments have shown that it is often a more efficient use of time and money to let other people do the major experimentation for you. One often has their hands full with just the little tweaks.

Originally posted by 12AX7
...And SO2, and H2S. Now there's a refrigerant that'll discourage tampering!

I ran into a working SO2 fridge a couple years ago from what, the 40's? Still around. Wonder if they're worth anything.

[Edited on 15-9-2007 by S.C. Wack]

Sauron - 15-9-2007 at 04:51

Actually I asked you for the Inorg.Syn. files back in Dec or Jan and you gave me an unresponse. Axhandle isn't around any longer to answer requests for admission to his FTP site. To the best of my knowledge your stuff is not on MadHatter, I have been all over that one and have not seen it. I still would like to have it, but, whatever.

I think we have got ammonium dithiocarbamate pretty well covered, details of the conversion of that to the thiocyanate and purification of same would be nice (for Eclectic). As for myself I will just buy the thiocyanate if I decide to try this. Fortunately this is still a free country and I can pretty much buy what I like. That includes cylinders of NH3 should I so desire.

Eclectic - 15-9-2007 at 06:25

Ammonia based refrigeration is good in situations where you need cooling powered by a low grade heat source, or electricity is not available. Such a RV camping, or remote locations off the power grid, or situations where you might need refrigeration even if the power goes off for a week.

S.C., I think the old G.E. SO2 refrigerators may be collector's items if they are in good shape and still working. Please post a link to the Inorganic Synth. prep of Ammonium Thiocyanate in volume 3 if you have it? I don't understant kmno4's link: http://55158856/wasny ???

[Edited on 9-15-2007 by Eclectic]

OH! I see, it's an IQ test. I guess I passed. :D

[Edited on 9-15-2007 by Eclectic]

DeAdFX - 15-9-2007 at 07:19

Would the catalytic decomposition of urea via Urease be worth a try? A large amount of Ammonia/CO2 gas could be formed with little use of other chemicals (just a little bit of phosphates for pH buffer). Sperating the two gases and maintaing optimal conditions for the enzyme are the only things I can think of.

Or perhaps for the engineering type one could desgin a vessel in which urea + water react at high pressures/temperature?

Ammonia on demand

Eclectic - 15-9-2007 at 08:12

V2O5 (with or without tungstates) will catalyze urea hydrolysis at 100 C or so. You'd need a good condenser design to avoid blockage by solid ammonium carbonate, then you still have to get rid of the CO2 and water, unless you want to just use ammonium carbonate as your ammonia source in your reactions.

Check your 4share root folder.

[Edited on 9-15-2007 by Eclectic]

DeAdFX - 11-8-2008 at 15:20

Originally posted by Eclectic
V2O5 (with or without tungstates) will catalyze urea hydrolysis at 100 C or so. You'd need a good condenser design to avoid blockage by solid ammonium carbonate, then you still have to get rid of the CO2 and water, unless you want to just use ammonium carbonate as your ammonia source in your reactions.

Check your 4share root folder.

[Edited on 9-15-2007 by Eclectic]

Sorry for a thread revival but...I was wondering if this could some how work out as a crude way to get mostly CO2 free ammonia...

The ammonium carbonate and water is mixed with 2 mols of an organic amine (high boiling point). The mixture is lightly heated and hopefully mostly pure ammonia is collected in a container full of water.

The organic amine could then be regenerated by heat and or vacuum?

(NH4)2CO3 (aq) + NHxRx (aq) + heat ------> NH3 (g) + (NHxRx)2CO3
NH3 (g) + H2O ----> NH4OH(aq)
(NHxRx)2CO3 + heat/vacuum? --------> 2NHxRxOH + CO2(g)

or perhaps just using a diver's solution(ammonia/ammonium thiocyanate would be a better idea to seperate the two gases?

[Edited on 11-8-2008 by DeAdFX]