MrHomeScientist
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Sulfur Dioxide and the Barking Dog
I've got an interesting situation that I thought I'd ask about and see if anyone here had any advice for me.
As part of a science stage demonstration show I co-star in, I perform the barking dog reaction as my finale. If you haven't heard of it, it's a very
spectacular reaction between nitrous oxide and carbon disulfide that's very bright and loud, perfect for stage demonstration. The way I do it is to
pre-fill the (stoppered) tube with a measured amount of N<sub>2</sub>O, then on stage mix the CS<sub>2</sub> into it. I then
remove a stopper, ignite the mix, let it run its course, then cap it off again. This is a good video of it, and their setup is very similar to mine:
http://www.youtube.com/watch?v=_8UWlKyBR5M&feature=relat...
My problem is that this reaction tends to produce sulfur dioxide gas, which is extremely irritating and headache-inducing to both the performers and
the first few rows of the audience. I've decided to drop this demo until I can figure out a way to make it safer, despite how much I love it.
There are three pathways that can produce this gas.
In the case of excess nitrous oxide, there is complete combustion:
CS<sub>2</sub> (g) + 6N<sub>2</sub>O (g) → CO<sub>2</sub> (g) + <b>2SO<sub>2</sub>
(g)</b> + 6N<sub>2</sub> (g)
In the case of excess carbon disulfide, we get incomplete combustion:
CS<sub>2</sub> (g) + 2N<sub>2</sub>O (g) → CO<sub>2</sub> (g) + 2S (s) + 2N<sub>2</sub> (g)
This is precisely what we want, <b>but</b> in practice this is near impossible to achieve. What I believe actually happens is that when
there is excess CS2, it consumes all the nitrous and then burns in air to again produce SO2:
CS<sub>2</sub> (g) + 3O<sub>2</sub> (g) → CO<sub>2</sub> (g) + <b>2SO<sub>2</sub>
(g)</b>
In addition to this, I believe the elemental sulfur produced above can then react with air (heated by the reaction's flame front) to directly form the
gas as well:
S (s) + O<sub>2</sub> (g) → <b>SO<sub>2</sub> (g)</b>
Given that those are the three pathways to sulfur dioxide produced by this demonstration, does anyone have any ideas on how to minimize this gas? Or
at least prevent it from escaping into the audience? Much of it is thrown out of the tube as the reaction is going, but I'm reluctant to cap it off
while it's burning for fear of pressure buildup.
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Endimion17
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Try to make a standard. Use the same tube filled with N<sub>2</sub>O and add different amounts of CS<sub>2</sub>.
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Adas
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You can use something different, methanol for example. there is a lot of flammable fluids 
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watson.fawkes
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Quote: Originally posted by MrHomeScientist  | | Much of it is thrown out of the tube as the reaction is going, but I'm reluctant to cap it off while it's burning for fear of pressure buildup.
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Make yourself a cap with a hole in it. To capture the gas, put a short length of tubing in the hole and put
a balloon on the end.
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woelen
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Almost nearly as spectacular is the reaction of methanol in which some boric acid is dissolved with N2O. This also makes a lot of noise and gives a
bright green flame. First try this reaction without audience and at small scale so that you get some feeling with it and know what to expect.
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MrHomeScientist
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Thanks for all the responses.
@watson.fawkes: That's an interesting idea. I wonder if I could string wires from a piezoelectric sparker under the stopper to ignite the mix without
having to uncap it as well. I had also thought of leading the tubing into water instead of a balloon to absorb the gas.
@woelen: That sounds like a really great demo, I had never heard of that! Do you have any specifics on it (reaction equation, amounts to use, etc.)?
I'd like to know what the products of that reaction are. A cursory google search only yields a lot of pages on using nitrous + methanol in cars. I'd
definitely like to try this experiment.
When doing the barking dog, you have to let the CS2 evaporate and mix with the N2O in order to ignite it. In your version, when the methanol
evaporates wouldn't that leave the boric acid behind and take away the green color?
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Endimion17
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Boric acid reacts with methanol, it produces trimethyl borate which is volatile and in fact responsible for the green color.
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woelen
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Endymions explanation is correct. Just try the experiment with CS2 replaced by methanol in similar volume. As a first start that will do. Always first
try at test tube scale before scaling up!
Making the solution of boric acid is easy. Add a spatula full of boric acid to 5 ml of methanol and dissolve. Burn the liquid. If it burns with a deep
grass green flame then it is ok.
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MrHomeScientist
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| Quote: Originally posted by woelen | Endymions explanation is correct. Just try the experiment with CS2 replaced by methanol in similar volume. As a first start that will do. Always first
try at test tube scale before scaling up!
Making the solution of boric acid is easy. Add a spatula full of boric acid to 5 ml of methanol and dissolve. Burn the liquid. If it burns with a deep
grass green flame then it is ok. |
That's excellent. Thanks for the explanation woelen & Endimion17! I've made that green fire mixture before, but I guess I didn't realize that the
trimethyl borate would also be volatile. This is a great idea and I'll try it out as soon as I am able.
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MrHomeScientist
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It works!!!
I tried out the alternate version of the experiment suggested by woelen yesterday, and it totally works! Thanks so much for the idea - I really did
not want to get rid of this demo! It's really striking to see a green fire version of this demo, because it's something I've never seen before. I
unfortunately don't have any pictures to post, but as soon as I do this in the stage show I'll be sure to record it and post it here.
I did exactly what you suggested - replace the CS2 with a similar volume of methanol with dissolved boric acid, and kept all other parameters the
same. I got the same characteristic 'woosh' and visible flame front in the tube. The only difference was it traveled down the tube a bit faster and
was louder (as well as the color change). I think this is just because it's a different reaction going on, so I ended up using too much nitrous.
I was thinking about what the reaction would be for this version, and here's my proposed equation:
18N<sub>2</sub>O + 2B(OCH)<sub>3</sub> == B<sub>2</sub>O<sub>3</sub> + 6CO<sub>2</sub> +
9H<sub>2</sub>O + 18N<sub>2</sub>
The reaction is between nitrous oxide and trimethyl borate, the product of adding boric acid to methanol. I think that boron trioxide is produced
because of the heat of the reaction would decompose most anything else down to the oxide. I also noticed a white smoke left over in the tube after the
reaction, which I believe is small particles of the oxide. There will almost certainly also be some extra unreacted methanol in there too, as well as
regular air, so I imagine side reactions with these will produce similar products (just without the B). Does all this seem reasonable? If this is
correct, it's great that it produces no hazardous byproducts like the original version does, and I will feel much better being able to perform this
demo safely.
[Edited on 10-7-2011 by MrHomeScientist]
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woelen
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Yes, your proposed reaction indeed occurs. Hardly any hazardous products are produced. Most boron oxide remains in the glass tube as a solid deposit
on the glass. Another advantage of this demo is that cleanup of the glass tube is much easier. Just rinse with warm water.
Also try different amounts of boric acid. With very small amounts you can make a cyan flame and adding more makes the flame more green. So you have
the option of playing a little with the color of the flame.
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