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Author: Subject: Burning solvents in alcohol lamps
Melgar
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[*] posted on 7-11-2017 at 16:48
Burning solvents in alcohol lamps


Sometimes I'll have an impure solvent for any of a number of reasons, and I have plenty of a much purer grade and have no need to keep small amounts of that solvent. So I've taken to burning the non-halogenated ones in alcohol lamps, made from nail polish bottles, that can be used as a very quick, low-profile source of heat.

Aromatic solvents generate way too much soot to be of much use, at least on their own. Hydrocarbons in general have this problem, and I rarely use any of them.

Isopropanol and any larger alcohol molecules also generate soot, but much less if they're mixed with water. Mixtures with boiling points that are too different will pop a lot when burning though.

Glycerin is too thick to be an effective fuel, but propylene glycol works really well. The only annoying thing is that it's sticky and doesn't evaporate quickly. It also doesn't cool off very fast when you put it out, a property that lower-boiling solvents have that is quite convenient. But propylene glycol generates no soot, which is very convenient.

Mixtures containing hydrogen peroxide generate less soot, and if the wick has gotten dark-colored, H2O2 will clean it. However, I only make such mixtures rarely and cautiously, and do not recommend it because it can potentially be dangerous. You don't want to find out the hard way that there's acetone in your denatured alcohol.

Methanol is my go-to fuel if I don't want to do any experimenting, and just want to be economical. However, it burns with a flame that can be difficult to see. A pinch of boric acid added to the fuel can make that flame bright green, and lithium can make it reddish-orange. However, both substances leave white residue above where they were burning, which is slightly annoying.

Dimethylformamide burns with a really cool-looking purplish-blue flame. It's a fairly hot flame too, and burns with no soot whatsoever.

Nitromethane is the most fun. It is a component of racing fuel, after all. It adds oxygen to any flame, allowing you to burn solvents that would generate too much soot otherwise. By itself, it doesn't burn that strongly, but the flame has a cool whitish-green color. Nitroethane burns similarly, but with more of a white color. (That was strange... it felt like a bunch of unscrupulous chemists all cried out in agony at once. Odd.) Also, acetone and MEK mixed with nitromethane burn perhaps the hottest of any unpressurized fuel. You can even burn aromatic solvents if there's enough nitromethane mixed with it. It will form a very bright yellow-white flame, but produces an unpleasant smell when you put it out.

Once, and only once, I burned a mixture of methanol, nitromethane, and DCM under VERY controlled conditions, to see if this would produce any phosgene, and if so, how much. I had some test paper that could detect phosgene, and wanted to test it. However, I was surprised to find that no phosgene was generated, and the Cl all formed HCl in the combustion gases. I didn't want to push my luck though, and left it at that.




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ninhydric1
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[*] posted on 7-11-2017 at 17:13


Huh, a somewhat effective, entertaining way to dispose of organic solvents compared to mundane evaporation of such wastes.



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The Volatile Chemist
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[*] posted on 7-11-2017 at 18:59


Pretty cool post, thanks for sharing. I'll definitely be doing this with my waste solvents from now on, I've got some mixed acetone-iPrOH right now, but I think it's got too much water/Salicylic acid with it to be an effective fuel.



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Melgar
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[*] posted on 7-11-2017 at 22:09


Oh, I forgot to add that acetone burns up really quickly, with a moderate amount of soot and a lot of heat. Adding water to a solvent like acetone or isopropanol that will mix with it can slow down the combustion, reduce soot, and lower the flame temperature. Up to about 25% water works fine, but more and the flame will go out very easily.

It's also fine if some ammonia is in the fuel. It burns weakly, and adds a purple tint to the flame.

For a wick, I have a length of cotton cord that works really well, but twisting up a piece of paper towel works in a pinch. Cottonballs also work well; if you look at one, you can see that it's actually a rolled up length of cotton. Unroll it, and twist it really tightly along its length, then let it wrap around itself to get a double helix. Or just twist it up loosely. I usually try and size the wick so it fits snugly into the neck of the bottle, but you can do other things too, like put a wire inside the cord to hold it in the mouth of the bottle. It's good to make sure there isn't a gap between the wick and the neck of the bottle large enough for a flame to get through. If one gets through, it tends to use up all the oxygen immediately and burn out, preventing another similar incident, so it's not a big deal.

Acetonitrile burns hot, with little to no soot, and a purple and yellow flame. Nitrogen in the -3 state seems to cause a purple flame color, I've noticed.

Nitromethane added to a fuel will prevent it from being blown out, reduce soot, and also raise the flame temperature for most fuels. 2/3 nitromethane + 1/3 MEK will output more heat than any other mixture I've tried, without soot. Sometimes if an alcohol lamp runs out of fuel, and there's a glowing ember on the wick, refilling it then quickly putting the cap on and carefully adding a drop or two of nitromethane near the ember will reignite it. That only works for my store-bought alcohol lamp though.

Putting a few specks of used Pd/C on the end of the wick will make it catalytic, and much easier to reignite the ember with nitromethane. Also, if it goes out, you'll smell formaldehyde (for methanol) or acetaldehyde (for denatured alcohol) as the ember catalyzes the partial oxidation of the last traces of alcohol. It can also make an alcohol lamp reignite when blown out, like a trick candle.




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