What is this ammonium salt?

[Justonium]

It came from an instant cold pack (so it's enthalpy of dissolution is pretty high.) It utterly failed a test for nitrates, melts into a clear liquid under the heat of a blowtorch, and decomposes upon further heating to form a steamy gas and a white compound with a very low solubility in water.

 

(No the cold pack did not say what was inside it besides the water pack.)

[RyanJ]

Sounds like it could be ammonium chloride but I'm not sure that is used in ice packs. As I recall that is ammonium nitrate.

[hermanntrude]

why do you assume it's an ammonium salt?

 

I don't know what it is, just wondering why you narrowed it down to ammonium salts

[aeontide]

Ammonium salts are often used in cold packs, specifically ammonium nitrate. Ammonium chloride also produces similar effects. From what was described, I would go with ammonium chloride as you can use it in ice packs--it is used in newer packs, whereas, you'll more than likely find ammonium nitrate in older products.

Edited October 26, 2009 by aeontide

[UC]

It's urea. The insoluble white stuff is cyanuric acid, melamine, any of various intermediates between the two, or some ungodly polymeric crud. The fumes while heating were probably mostly ammonia, but if you applied a lot of heat, you could be getting isocyanic acid (HNCO, not cyanide) as well.

 

It's being used in cold packs because it works about as well as ammonium nitrate and is less hazardous (and can't be used for pyro).

[Justonium]

Oh, I forgot to mention, I did another test for ammonium ions, and it was positive. I just reacted it with sodium hydroxide and it bubbled ammonia gas which formed a white ammonium chloride mist with hydrogen chloride gas.

 

Thanks for the info, guys.

 

OK, I'm pretty sure it's urea. What does urea dissociate to in water? I've reacted the urea with excess sodium hydroxide and boiled off all the liquids, and have gotten a yellowish white solid. The yellow may, however, be due to the sodium hydroxide reacting with an unfortunate fly that I found had landed in it. Is this new salt possibly sodium cyanate?

Edited October 29, 2009 by Justonium

[UC]

Oh, I forgot to mention, I did another test for ammonium ions, and it was positive. I just reacted it with sodium hydroxide and it bubbled ammonia gas which formed a white ammonium chloride mist with hydrogen chloride gas.

 

Thanks for the info, guys.

 

OK, I'm pretty sure it's urea. What does urea dissociate to in water? I've reacted the urea with excess sodium hydroxide and boiled off all the liquids, and have gotten a yellowish white solid. The yellow may, however, be due to the sodium hydroxide reacting with an unfortunate fly that I found had landed in it. Is this new salt possibly sodium cyanate?

 

Can you smell ammonia coming off of a cold solution of the unknown salt mixed with a little NaOH, or do you have to heat to smell it? It hydrolyzes just like any other amide, but that won't happen with cold, dilute base. The condensation to cyanuric acid only occurs when molten, so you solid is almost surely just carbonate and leftover hydroxide.

[Justonium]

When it was cold, it smelled very faintly of ammonia, but nothing compared to when I heated it. I didn't test it using hydrogen chloride when it was cold though, but it would most certainly have made white mist, because there was enough ammonia for my nose to detect. Note, it did not originally smell of ammonia, and only did after several days of sitting in sodium hydroxide solution. Could urea possibly hydrolize at room temperature very slowly?

 

Anyway, assuming my solid was indeed urea, it would have completely hydrolized upon heating, liberating carbon dioxide that would have then immediately reacted with the sodium hydroxide to make sodium carbonate. That is what I would guess was the solid left behind. Thanks for the info suggestions UC. I also know that there was no sodium hydroxide left in the solid because it was not hydroscopic. This makes sense because I mixed the proper amounts of the (possibly) urea, and sodium hydroxide to stoichiometrically react to make sodium nitrate (I had thought I was handling ammonium nitrate. The same mass of urea would react more sodium hydroxide than would it's equivalent mass of ammonium nitrate.

Edited November 2, 2009 by Justonium

[UC]

Could urea possibly hydrolize at room temperature very slowly?

 

Provided strong base, of course. Heat just makes it go much faster. Even cold ammonium nitrate solution and cold sodium hydroxide solution would have stunk like ammonia as soon as mixed together

[Justonium]

K, its urea then.