Sciencemadness Discussion Board

Help please

Zelot - 14-7-2008 at 19:49

I was just dehydrating some ammonium nitrate in the oven at about 320*F. I left it for about a half an hour and came back to check on it. When I opened the door to the oven, the ammonium nitrate was smoking! I immediately turned off the oven and took it out. I then noticed that the once perfectly white powder had turned a light-purple color. Can someone give a possible explanation as to what happened? Any speculation would be appreciated.

ShadowWarrior4444 - 14-7-2008 at 20:04

The source of the AN would be helpful here. 160C should be 9.6C below the MP of AN, so I don’t expect any decomposition--it is likely that there was a carbon-containing impurity present that was oxidized.

That, or your oven's heating coils were a bit overzealous.

Zelot - 14-7-2008 at 20:06

I recrystalized it from an instant cold pack.

although the AN had been sitting around for a month or two after that, so some dust or something could have gotten in. Do you think that it is still good, or should I throw it out?

[Edited on 7/14/2008 by Zelot]

ShadowWarrior4444 - 14-7-2008 at 20:15

Originally posted by Zelot
I recrystalized it from an instant cold pack.

Well, that leaves a few possibilites:

1. Your recrystalization wasn't-good-enough. (Ex. There were other soluble compounds that remained, or some of the fine carbon containing [perhaps starch] compounds slipped through the filter.)

2. Localized overheating. (You might want to try the drying at only about 100-110C.)

As for whether it is still good, another recrystalization should remove any carbonized waste. AN should be kept moderately sealed from the environment, as it is hygroscopic for one, and dust can be meddlesome.

Ancillary: A useful filter to remove fine particles is a cotton swab in the neck of a funnel.

[Edited on 7-14-2008 by ShadowWarrior4444]

bfesser - 15-7-2008 at 18:44

What were you drying it in/on? If you were using a coated pan, could it have reacted with the coating?

Zelot - 15-7-2008 at 21:36

It was a pyrex dish.

Picric-A - 27-7-2008 at 02:59

the colours in the AN was probs oxidised material.
what state was the AN? fine powder? crystals?
If it was a powder then trace amounts of oxidisable matrerial would probably be oxidised by the AN.
there is a reason MSDS sheets say about oxidiser 'contact with material may cause fire'

[Edited on 27-7-2008 by Picric-A]