Use of Sulphur in Pyrotechnics

[aj47]

I understand the reason why sulphur is used in so many fireworks, flash powders etc is because it increases sensitivity and lowers activation energy to make essentially easier to light mixtures, but why is this?

 

Does the sulphur act as a reductant or is it the sulphur compounds formed that give rise to this effect?

[YT2095]

IIRC it`s something to do with the fact that sulpher melts at a very low temp (compared to the other "ingredients") so it`s the 1`st player in many of these sorts of reactions.

[RyanJ]

IIRC it`s something to do with the fact that sulpher melts at a very low temp (compared to the other "ingredients") so it`s the 1`st player in many of these sorts of reactions.

 

Can't sulphur be replaced by phosphorus in most reactions? Phosphorus gunpowder seems to react the same as sulphur gunpowder except it ignites a hell of a lot easier...

 

Cheers,

 

Ryan Jones

[YT2095]

for Some things yes you can, but Phos is many times more reactive than sulpher, often making mixtures far too sensitive and unpredictable as a result.

 

as a general rule if you have a chem that sulpher is "incompatible" with, you can bet the phos will be much more dangerous!

[aj47]

IIRC it`s something to do with the fact that sulpher melts at a very low temp (compared to the other "ingredients") so it`s the 1`st player in many of these sorts of reactions.

 

aha thanks, so for example in black powder, the sulphur will be oxidised first generating the heat for the KNO3 + C reaction to take place?

[YT2095]

yup, simplified sure, but yup

[budullewraagh]

wouldnt it be that some of the S is oxidized and then the rest is reduced to K2S?

[YT2095]

there is K2S left as a product sure, along with other K compounds

the breakdown of K2S is largely responsible for the slight "Rotten egg" smell a short while after the BP has burned.

[budullewraagh]

well, the white stuff that is sprayed everywhere when a cannon is fired is the K2S, correct?

[YT2095]

no idea, but on the decomp of BP under pressure the reaction will make all sorts of interesting compounds, it really isn`t as Simple a reaction as it looks at face value when you consider only 3 main ingredients

 

you`ll have carbonates and nitrites as well.

[woelen]

Indeed, when BP is "burnt", then a lot of interesting compounds are formed. I once have done a crude analysis of the products, and I found at least the following:

 

K2S (smell of rotten eggs)

K2CO3 (bubbling, when acid is added). This appears to be the main product.

K2SO4 (precipitate with BaCl2, which does not dissolve in excess acid, such as HCl or HNO3)

KNO2 (this sometimes is formed, especially, when the mix is low in sulphur, this can be detected, because of formation of brown gas on addition of an acid).

 

Indeed, the reaction is very complex and there are MANY competing reactions.

 

Besides these solid products, I'm quite sure that the gaseous stuff contains SO2 (typical sulphurous smell), CO2, and N2.

[aj47]

yup, simplified sure, but yup

 

Thanks great help. One more thing though, does this also account for the increased sensitivity of impact sensitive compunds i.e. flash powders with the addition of sulphur?

[YT2095]

no, not really, that`s nothing to do with Heat exactly, but more along the lines of decompositions that occur naturaly and having these decomp products combine in a phase likely to cause reaction, Solids won`t react (although it May seem that way very much with certain mixtures), but gasses are given off, and THESE CAN react and set the whole lot off.

interestingly enough Woelen and I have been experimenting along similar lines recently also

evidence would indicate that Phos and Bromates are FAR TOO incompatible to even get a good mix with which to test! for instance.

 

I`m sure he will tell you more about our findings when he reads this, it`s Very Interesting stuff

[aj47]

That is really interesting stuff. Do you know of any examples of this as I would have expected the gases to be released from the decomposition of the reactants to be vanishingly small, even with impact. So primarily what kind of gases would be responsible for this reaction in something like a potassium chlorate, sulphur and aluminium mix?

[YT2095]

for a start KClO3 will indeed Detonate Propper, I don`t mean Burn fast or cause a mechanical explosion, I mean KClO3 will Detonate!

so for a kick-off you`ve got some high powered material that`s unstable on it`s own as is Ammonium Nitrate.

however, the activation energy to trigger such a thing is VERY HIGH!

in Flash powder KClO3/Al it doesn`t detonate exactly, but it IS self confining that could give that impression, I`ll not go into any hydro-dynamics or explainations of condensation phase transitions, bur#t this is to just give you an idea of the duality of such a compound.

 

now, Sulpher or Phos will give of H2S Phosphine respectively, these are VERY STRONG reducers, and they will occur natualy.

open a jar of S or P after it`s been on your shelf for some time and take a sniff (well don`t, but a waft wont hurt).

in a mixture the exact same thing will occur (it doesn`t care if it`s in a jar or next to an Oxidiser).

 

there`s the instabilty factor

[aj47]

so essentaily is it again a redox reaction that takes place except the regents are in the gas phase or do these decomposition products in turn react with the orginal reagents?