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Old 02-08-2007, 07:07 PM
DesertDog DesertDog is online now
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Black powder: What's the sulfur do

The potassium nitrate is the oxidizer, the charcoal is the fuel, but what does the sulfur do? A catalyst? Wiki has a nifty article including the classic proportions of 15:3:2 and where to get 'saltpeter' but on the why of the third ingredient, it is mum.

Any pyrotechnic chemists out there?
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  #2  
Old 02-08-2007, 07:24 PM
Shagnasty Shagnasty is offline
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It adds a lot to the smell.
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Old 02-08-2007, 07:28 PM
Stranger On A Train Stranger On A Train is offline
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I'm not a pyrotechnic chemist, but I have your answer anyway. Sulfur (the correct IUPAC-approved spelling) is a desiccant used to prevent the marginally less hygroscopic (water attracting) powdered charcoal from becoming saturated with humidity and incombustible. It is possible to make black powder without sulfur--although not in the manner displayed in Star Trek--but it'll have a very short shelf life even in nominally arid environments. It is important to use sulfur of high purity, as small impurities can cause it to form unstable side products and spontaneously combust.

Making black powder at home is definitely ill advised, as many amputees and burn victims can attest. Once the mixture dries you are left with a layer of finely concreted dust which has to be ground up; too much pressure or a tiny spark is sufficient to ignite an entire batch. Even the professionals like DuPont or Hercules occasionally blow up a mixer or lose an entire plant.

Stranger
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Old 02-08-2007, 07:31 PM
Caught@Work Caught@Work is offline
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Supposedly to reduce the ignition temperature.
http://www.musketeer.ch/blackpowder/history.html
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  #5  
Old 02-08-2007, 07:35 PM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by Stranger On A Train
I'm not a pyrotechnic chemist, but I have your answer anyway. Sulfur (the correct IUPAC-approved spelling) is a desiccant used to prevent the marginally less hygroscopic (water attracting) powdered charcoal from becoming saturated with humidity and incombustible.
I hate to disagree with you, but I've found several credible cites which say the main purpose of the sulfur is to lower the ignition temperature of the mixture, making it more easily and reliably touched off. It may also act as you describe, but it's apparently not significant enough to bear mention, as far as I can tell.
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Old 02-08-2007, 07:46 PM
Caught@Work Caught@Work is offline
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W00T! I beat Q.E.D. by 4 minutes and the same cite.
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  #7  
Old 02-08-2007, 07:48 PM
Stranger On A Train Stranger On A Train is offline
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Quote:
Originally Posted by Q.E.D.
I hate to disagree with you, but I've found several credible cites which say the main purpose of the sulfur is to lower the ignition temperature of the mixture, making it more easily and reliably touched off. It may also act as you describe, but it's apparently not significant enough to bear mention, as far as I can tell.
Hmm...well, I'll have to retract my claim, or at least partially. From your first cite:
A recipe without sulfur yields only little less energy (Joule/g). But its advantage lies in its little smoke produced. Its fog then consists of potassium carbonate (potash). Unfortunately its ignition temperature is 100 C higher than usual. For cap lock arms no problem, but a nuisance to flint and match lock sportsmen.
I had always understood the reaction resulting in the K2S product to be incidential and inconsequental in terms of energetic yield, but I hadn't considered its effect on reactivity. But then, like I said, I'm not a pyrotechnic chemist, or indeed, any kind of chemist other than the kind behind the bar. Color my ignorance fought for today.

And you can disagree with me anytime I'm wrong and you're right, or at least have good reason to think so. I'd like to think that I still have a few things to learn.

Stranger
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  #8  
Old 02-08-2007, 07:50 PM
DesertDog DesertDog is online now
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Quote:
Originally Posted by Stranger On A Train
Making black powder at home is definitely ill advised, as many amputees and burn victims can attest. Once the mixture dries you are left with a layer of finely concreted dust which has to be ground up; too much pressure or a tiny spark is sufficient to ignite an entire batch. Even the professionals like DuPont or Hercules occasionally blow up a mixer or lose an entire plant.
Oh, this was strictly intellectual curiosity. I like my fingers right where they are, and unburnt.

Interesting that the sulfur also seems to be a major contributor to the smoke of "non-smokeless" powder.
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  #9  
Old 02-08-2007, 07:50 PM
Crescend Crescend is offline
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This cite states that the reaction is:

KNO3(s) + C(s) + S(s) -----> N2(g) + CO2(g) + K2S(s)

I haven't had to balance a redox reaction in years, but this looks to me like the potassium nitrate (oxidizer) splits into K+ and NO3-. NO3- will then react with the carbon (in a very energetic reaction) to produce N2 and CO2. The K+, on the other hand, will be taken up up by the sulfur. According to Le Chatelier's principle, by removing product, you will drive the reaction to the right. By sequestering K+, you'd skew the reaction towards the NO3- + C, rather than NO3- + K+.

Last edited by Crescend; 02-08-2007 at 07:51 PM..
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  #10  
Old 02-08-2007, 08:00 PM
Squink Squink is offline
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The reaction is roughly 2 KNO3 + 3 C + S ---> K2S + 3 CO2 + N2
Without the sulfur you form KO rather than K2 and the reaction is less energetic.

Black powder at Numericana. I've never heard of this particular site before, but it looks not unreasonable.
Sure, you could probably use selenium rather than sulfur (ie one row down on the periodic table), and get toxic potassium selenide smoke instead of stinky potassium sulfide, but sulfur does have a nice low melting point.
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Old 02-08-2007, 08:14 PM
WarmNPrickly WarmNPrickly is offline
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I've never thought about this but I know a few things that may help. Assuming the above reaction is correct, the sulfur is also being oxidized. Sulfur is one of the more easily oxidized elements. This strongly suggests that the first step in the mechanism is oxidizing sulfur from S(0) to S(+2). Therefore, the initial source of the electrons that end up on nitrogen is probably sulfur. That would explain why sulfur lowers the activation barrier (ignition temperature). Once the reaction gets going, there is enough energy to oxidize the carbon directly. This is why a stoichiometric amount of sulfur is not needed. (I did not calculate that a stoichiometric amount of sulfur isn't used just guessed.)
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  #12  
Old 02-08-2007, 09:42 PM
Paul in Qatar Paul in Qatar is offline
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Yep, it lower the ignition temperature.
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  #13  
Old 02-08-2007, 10:50 PM
WarmNPrickly WarmNPrickly is offline
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Quote:
Yep, it lower the ignition temperature.
Right to the point. But how does it lower the ignition temperature? I'm curious too.
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  #14  
Old 02-08-2007, 11:30 PM
Squink Squink is offline
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Quote:
Originally Posted by Christopher
Right to the point.
Not at all. It's a different reaction if you don't put the sulfur in there. The end products are NOT THE SAME. Folks should learn some chemistry before trying to answer questions like this.

That said, yeah, sulfur's got a nice low melting point, and it ignites easily. Once melted it probably starts to dissolve the carbon and KNO3 particles, which vastly speeds up the reaction. If you make gunpowder with calcium nitrate instead of potassium nitrate, you'll actually see little liquid globs of molten sulfer/nitrate/charcoal spitting fire. I've never made the mixture where selenium replaces sulfur, but sulfur melts at 239F, while selenium requires 430F.
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  #15  
Old 02-09-2007, 06:55 AM
WarmNPrickly WarmNPrickly is offline
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Quote:
Not at all. It's a different reaction if you don't put the sulfur in there. The end products are NOT THE SAME. Folks should learn some chemistry before trying to answer questions like this.
Then please educate me as to what the products are if they aren't essentially CO2 and N2. I suppose the potassium would prevent complete reduction of the N2. (gotta have an anion for a cation like that.) But since you apparently know, please tell me. I'm here to learn.

So you are suggesting that sulfur does not lower the activation barrier? I know some chemistry, so please help me understand what you are saying.
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  #16  
Old 02-09-2007, 07:16 AM
WarmNPrickly WarmNPrickly is offline
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I suppose the potassium would prevent complete reduction of the N2.
Actually, that really helps answer the question. Without the sulfur, the nitrogen can't be completely reduced. The potassium has to go somewhere as a cation (Otherwise you end up with potassium metal as a byproduct and that doesn't seem likely.). The sulfur provides a nice anion for potassium. This suggests a stoichiometric requirement for sulfur. (Fine I'll do that calculation some time.)
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  #17  
Old 02-09-2007, 08:00 AM
WarmNPrickly WarmNPrickly is offline
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I suppose that potassium oxide could be a product without sulfur, but that still leaves a lot of oxygen unaccounted for. I still say incomplete reduction of the nitrogen would be a result of not having sulfur around. Squink Let me know what you know. This is an interesting problem. I'd like to work it out with another chemist.
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  #18  
Old 02-09-2007, 08:02 AM
Malacandra Malacandra is offline
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Could you not just get atomic oxygen from the nitrate, and be left with potassium nitrite?
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  #19  
Old 02-09-2007, 08:34 AM
dropzone dropzone is offline
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Quote:
Originally Posted by Squink
Sure, you could probably use selenium rather than sulfur (ie one row down on the periodic table), and get toxic potassium selenide smoke instead of stinky potassium sulfide, but sulfur does have a nice low melting point.
Yeah, that and the whole part where you're trying to kill the other guy and not yourself!
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  #20  
Old 02-09-2007, 09:50 AM
Fear Itself Fear Itself is online now
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Quote:
Originally Posted by Christopher
Then please educate me as to what the products are if they aren't essentially CO2 and N2. I suppose the potassium would prevent complete reduction of the N2. (gotta have an anion for a cation like that.) But since you apparently know, please tell me. I'm here to learn.
The byproducts of combustion of black powder are actually quite complex:
Quote:
One study's results showed it produced (in order of descending quantities): 55.91% solid products: Potassium carbonate, Potassium sulfate, Potassium sulfide, Sulfur, Potassium nitrate, Potassium thiocyanate, Carbon, Ammonium carbonate. 42.98% gaseous products: Carbon dioxide, Nitrogen, Carbon monoxide, Hydrogen sulfide, Hydrogen, Methane. 1.11% water
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  #21  
Old 02-09-2007, 01:37 PM
WarmNPrickly WarmNPrickly is offline
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Thank you Fear Itself. mostly I was questioning squinks assertion that the byproducts are completely different in the absence of Sulfur.

I think I'm going to stick with my assertion that the Sulfur acts as the initial source of electrons for the reduction of the nitrate. This would, as people have stated, have the effect of lowering the ignition temperature.

This also suggests that any reasonably stable reductant may also suffice. As Squink mentioned selenium may be effective, but perhaps a borohydride would be interesting. (Of course I wouldn't want to be the poor sap with the job of grinding that mixture together.)

It's also possible to make gun powder with things other than nitrates. For example potassium permanganate can be used.
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