killswitch
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Microwaves
When I tried to dry ammonium nitrate powder (made from a pharmacy's instant cold packs) in the microwave, it melted. I knew that AN solubility in
water goes up by nearly an order of magnitude from 0 ºC to 100 ºC, so if the powder was wet it would appear to melt as the microwave heated the
water. That's happened to me before, but what I am about to describe is a different behavior.
The AN was quite dry, as it had just recently been ground (between one hour for the start of the batch to 15 minutes for the last few prills in the
grinder) and the ambient humidity was not very high (cans of soda would sweat, but not enough to bead). Yet when I poured the powder into a mason jar
and microwaved it for 1 minute on high (700 watt, 2450 MHz, 8.5 amp, rotating model), a gap had formed in the pile of powder, the underside of this
gap being solid like styrene rather than caked powder, and a liquid had puddled in the bottom of the jar (which promptly cracked, as I bought it at
the grocery store). This liquid emitted the smoky fumes i have come to associate with the thermal decomposition of AN. The liquid eventually
solidified as a brownish-white mass, the color of which I assumed was due to trapped breakdown products. The only conclusion I could draw was that the
AN was directly heated by the microwave radiation, in contradiction of my prior assumptions.
So, is there a way to adjust the microwave settings (changing the power, cycling it on and off, etc) to avoid this problem, or would I have to use a
different EM wavelength entirely?
And incidentally, do all ammonium salts display this behavior of absorbing microwave radiation?
P.S. if there are any cool tricks for performing reactions with microwaves/blacklights/IR lamps, this would be a cool place to share them. 
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Sedit
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Ammonia will directly absorb microwaves really quickly, I am unsure if the salts exhibit this behavior the same way but in a solution they should be
as there ions anyway possibly causing it to react the same.
Take a solution of H2O and one of dilute Ammonia solution, after a few seconds in the microwave measure the temperature variations in the two glasses.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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quicksilver
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Use a standard oven.
This issue is not unique depending on where you live or the conditions on where the microwave is located. There is (often) a fan which draws outside
air across the "food' when using a microwave and unless you live in a particularly arid area you will be condensing whatever water is in the humid
air. A standard oven is actually a better desiccator in many instances for several other reasons.
edit:
I think there are safety considerations as well in using microwaves with water saturated ammonium nitrate.My inclination would be to investigate
further.
[Edited on 2-12-2011 by quicksilver]
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fledarmus
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At least ethylammonium nitrate does -
http://jcp.aip.org/resource/1/jcpsa6/v132/i10/p101101_s1?vie...
I haven't been able to find a microwave absorption spectra for ammonium nitrate around 2.4 GHz yet
If there is any water in the ammonium nitrate, you can actually use the microwave as an ignition source -
Trends in Sample Preparation
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Nicodem
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All materials absorb microwaves, some more, some less. Polar liquids generally absorb much better than solids. Ionic liquids, such as molten salts,
absorb microwaves very well. That's why you get violent decompositions of some salts when heated with microwaves - at their melting point they start absorbing several magnitudes better than
their corresponding solid form and just heat up above their decomposition temperature in matter of seconds, with the corresponding consequences. There
is no way to control or limit the temperature of the heated sample in a domestic microwave oven!
The melting point of ammonium nitrate is about 170 °C and in my opinion heating it in a microwave oven is just plain stupid. I would expect at least
a minimum of thinking abilities from somone who enjoys playing with potentially energetic compounds, but I guess I'm just overly optimistic.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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watson.fawkes
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Quote: Originally posted by Nicodem  | | All materials absorb microwaves, some more, some less. Polar liquids generally absorb much better than solids. Ionic liquids, such as molten salts,
absorb microwaves very well. |
Let me elaborate on this. What you're seeing here is simple thermal runaway. As
soon as there's the tiniest amount of liquid, the partition of microwave energy dumping into the solid vs. liquid changes drastically. The first hot
spot gets very hot indeed, because it's all of a sudden absorbing at least an order of magnitude heat more. If it can't dump that heat to the other
phase (solid) quickly enough, it will just keep getting hotter, eventually getting hot enough to vaporize.
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bquirky
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this applys to odinary solids too ceramics and metals above a certain temperature will start absorbing energy sometimes quite fast
I have sintered compressed copper powder in a microwave oven before
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Luftwaffe
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Even with decomposition of AN it should leave a white power behind, you can check the decomp products to confirm this. I did alot of experimenting
with AN from cold packs and found that the single ingredient cold packs contain substantial impurities in them, I dissolved the white prills in water
and filtered to get a clear solution that turns almost opaque brown when boiled down, I first tried to dry the raw prills in the oven which in turn
they dissolved themselves and turned brown as well.
Now about the thermal decomposition I have experienced some interesting properties on AN from experimenting. I managed to get a relatively pure sample
from a different brand of cold packs which I outlined in one of my threads where I boiled down the clear solution on a hotplate to the point where it
emmitted white vapour that had a somewhat sweet smell, this is undoubtably the decomposition of AN but there was never a change of state I would have
predicted. The solution was liquid all the way until decomposition which I find strange that there would be water in the sample at temperatures far
above 110C. I didnt want to risk explosive decomposition so I took it off a hotplate and poured it on a pan where it solidified instantly. it has
appreciable moisture still so I put it in an oven at 125C to get the water out.
Another observation was that it remained white after decomposition, there was a slight and I mean very slight tint of yellow/brown in the
product but to the untrained eye it appears white.
To sorta sum it up, get new cold packs and dry your AN in an oven. 
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AndersHoveland
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I think the problem with microwaving AN is, ironically, uneven heating. The surface tends to absorb all the microwaves, causing decomposition, while
the inside remains unheated. It might be a better idea to wrap the AN with some other microwave compatible solid material that will more evenly
absorb, then transfer the heat by conduction to the AN shielded within.
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killswitch
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| Quote: Originally posted by AndersHoveland | | I think the problem with microwaving AN is, ironically, uneven heating. The surface tends to absorb all the microwaves, causing decomposition, while
the inside remains unheated. It might be a better idea to wrap the AN with some other microwave compatible solid material that will more evenly
absorb, then transfer the heat by conduction to the AN shielded within. |
That, actually, gibes perfectly with my observations. The melting seemed to originate at and radiate from a point source near the largest fracture.
| Quote: Originally posted by Nicodem |
The melting point of ammonium nitrate is about 170 °C and in my opinion heating it in a microwave oven is just plain stupid. I would expect at least
a minimum of thinking abilities from somone who enjoys playing with potentially energetic compounds, but I guess I'm just overly optimistic.
|
You're right, I should have just mailed an ACE-brand Instant Cold Compress and request form to Is It a Good Idea to Microwave This?
But what's done is done, and you have my sincere thanks for the out-of-left-field passive-aggressive insults. I understand they do naught but improve
the atmosphere of level-headed discussion nurtured on these boards.
Sarcasm aside, I understand your concerns (first post on forum, all OTC reagents, k3wlish username, engaged in behavior previously associated with
less-than-enlightened forum posters, ended the post with a smiley). But really, labeling my actions (and more insultingly, myself) 'stupid'
seems rather precipitate given the facts you had to hand. I didn't mention whether I was prepared to lose the microwave (thrift store clearance), how
much AN was involved (less than 15 grams, of which only ~2.8 melted), my position relative to the microwave (viewing it through my back porch window
at the limit of two extension cords), or about any number of other mitigating factors beyond those three, which I consciously implemented in order to
err on the side of caution in case of the unexpected.
Yes, I was unaware of this tendency of salts to 'flashover' in a microwave, seeing as microwaves don't come up very often in the chemistry literature.
In which case, I'm glad I took the precautions I did, and would describe my endeavor as being prudently carried out, if uninformed in key nuances.
But stupid? With respect, sir, your first impression leaves something to be desired.
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simply RED
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There is always some water in the AN that starts the absorption, the temperature rises, the AN melts and the absorption rises as ionic liquids tend to
absorb microwaves well. If you continue microwaving it, the AN may ignite and then the ionized gasses will absorb microwaves too, so do not do it. No
cool experiment could be done this way, just fire ar home.
Juts use normal oven to dry the AN...
When logic and proportion have fallen sloppy dead...
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quicksilver
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As it turns out there may be some public dissemination (via the internet or crap-books) that deal with drying ammonium nitrate in a microwave. Frankly
when I had heard about that, it appeared to be VERY dangerous. The use of microwaves as a desiccator mechanism may be extremely dangerous from several
factors. I would never attempt such a thing regardless of who had attempted it previously as (1.) not all microwave appliances are built the same,
(2.) Ionic liquid lens effects pointed out by simply RED may be VERY valid, (3.) uneven distribution of heat w/ ammonium nitrate has been
substantiated as a material handling issue via USDOT.
The Port Chicago (CA navel shipyard) ammonium nitrate exposition may have been initiated by localized over-heating.
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AndersHoveland
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Molten ionic compounds absorb microwaves much more than solid ionic compounds. As soon as one part of the ammonium nitrate is melted, that part
absorbs and is heated much more than the remaining unmelted part. The result is decomposition of part of the AN, before it has all melted. When melted
in a liquid state, the ions are much more mobile, so can more easily absorb. In a solid, the ions have much less freedom of movement.
To properly melt AN, it needs to be protected from direct exposure to microwaves.
Another potential hazard is that, while AN is usually very insensitive, it becomes about as sensitive as TNT in its molten form, just as it begins
decomposing.
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