Sciencemadness Discussion Board

Microwaves

killswitch - 2-12-2011 at 12:49

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. :cool:

Sedit - 2-12-2011 at 13:37

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.

quicksilver - 2-12-2011 at 13:43

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]

fledarmus - 2-12-2011 at 14:00

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


Nicodem - 2-12-2011 at 14:29

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.

watson.fawkes - 6-12-2011 at 21:16

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.

bquirky - 6-12-2011 at 21:25

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

Luftwaffe - 7-12-2011 at 05:49

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. :)

AndersHoveland - 8-12-2011 at 00:30

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.

killswitch - 10-12-2011 at 20:36

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.

simply RED - 11-12-2011 at 02:28

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...

quicksilver - 11-12-2011 at 14:22

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.

AndersHoveland - 12-12-2011 at 11:13

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.