Sciencemadness Discussion Board

Dangers of Isocyanic acid when thermal decompising urea

vanBassum - 17-6-2019 at 12:54


I like to use thermal decomposition of urea to form ammonia. But upon searching the internet for a while I came across some pages claiming it also releases Isocyanic acid. Is this true, or should I be relative safe when doing this?

Urea seems like a nice source for ammonia but dont want to take unessesairy risks.

[Edited on 17-6-2019 by vanBassum]

Vomaturge - 17-6-2019 at 19:28

I asked this exact question a while ago. The consensus was... We don't know if the amount of isocyanic acid produced is hazardous. The 10-minute ocupational exposure limit for isocyanic acid is 70 micrograms per cubic meter, so it wouldn't take a high concentration to be dangerous. What several really helpful posters did say is that regardless of isocyanic formation, heating urea gives only a small amount of ammonia, and a lot of solid byproducts which are hard to get out of glassware.

Maybe garden lime has a high enough amount of calcium hydroxide (as opposed to limestone or dolomite which have more carbonate) that it will react with urea in solution and give off ammonia and calcium carbonate. Lime is pretty cheap, even in fairly small amounts.

So maybe skip the pyrolysis and try reacting urea with lime instead. If that doesn't work, you might need the more powerful and costly NaOH.

walruslover69 - 18-6-2019 at 06:03

I believe if you are decomposing straight urea, you would generate near equal molar amounts of ammonia and isocyanic acid. This can be solved by decomposing urea with a base, I have used KOH and NaOH, but i assume calcium hydroxide and carbonates would work reasonably well. The isocyanic acid gets neutralized to the salt so the only major gas evolved in ammonia. I actually used this method for producing cyanates.

vanBassum - 18-6-2019 at 09:13

hmm, I think Ill refrain from doing this then, cyanide doesn't sound like something I like to be messing around with. Another possibility would be electrolysis. A mixture of urea and some hydroxide can be converted to ammonia. And the rate can be adjusted by regulating the current and temperature. Although this also releases hydrogen at the cathode, since I plan to convert the ammonia to NOx over a platinum catalyst, it seems to be a bit of an explosion hazard.

walruslover69 - 18-6-2019 at 10:32

I think the thermal decomposition of Urea with sodium hydroxide is the way to go. You won't generate any cyanide, only sodium cyanate. Sodium cyanate is not really toxic or dangerous at all.