Anybody who has been following this column for a while will know that I'm a bit of a fan of phosphorus,
particularly its white allotrope, but sadly it's really
difficult to get hold of these days. So, when before Christmas I found an article
in the Journal of Chemical Education suggesting an idea for a demonstration
in which you could produce some white phosphorus in situ from the red
allotrope, I thought this has definitely got to be worth a look. So, here we have a
look at what they dubbed the 'white phosphorus flamethrower'.
Now I've just popped some gloves on 'cause I'm gonna be handling some glass wool, but you can actually do
this with mineral wool as well.
You want to start off with a glass Pasteur pipette,
and you're gonna put in a little bit of glass wool or mineral wool
to just block off the end of the nozzle on the inside.
Now, I find that you want
something like a micro-spatula to help you get the wool down inside. But a lot
of the schools that I've worked in, including where I am right now, we don't
actually have micro-spatulas to hand. So I often sort of craft my own mini-micro-spatula
using something I definitely know I always will have to hand which is
a wooden splint. And just running some scissors down along the end of this
gives me a bit of control over the length and the width of it and I can use
that to wedge my glass wool down on the inside.
Okay, nice and easy. Now, the job here is to make sure that really all we're
trying to do is prevent any solids from blowing out from the end but I want to
make sure that actually there's a good throughput of air. So I've actually got
some silicon tubing here, which I can just check with this gas syringe that I
can still easily move the gasses (the air) in and out through there. So that's
absolutely no problem, it's looking good. Next stop, micro-spatula
or the end of my fake, wooden splint here, loaded with some red phosphorus.
And we're going to just put some phosphorus on the inside of the Pasteur pipette leaving
about a centimetre gap if possible to where the end of our glass plug is.
And now I've got a little piece of red phosphorus in there. And the last step I
want to do is just to close off the other end in a similar fashion to that I
did before with some glass wool
So, I've taken my gloves off now because I'm ready to start my demonstration.
And I've got my pipette here: glass wool; red phosphorous; glass wool. And the original
authors of the paper suggested that actually heating this with a Bunsen
burner or a micro-burner would be a good option. And then the objective is to blow
some air through after some heating and to expel any white phosphorus out the end
which is going to auto-ignite, giving a kind of a flame-thrower effect. Now, one
of the first things I did when I saw this was to pass the paper on to
Bob Worley at CLEAPSS who, of course, is the master when it comes to micro-scale chemistry.
And he suggested a couple of perhaps improvements over the original method.
So what we're going to be doing today is actually using an ethanol burner which
is going to give us a flame that's totally hot enough to initiate the
endothermic change from the red form to the white form. And the flame temperature
is going to be a little bit more controlled and that's going to allow us
to actually observe the effect over a longer period of time, and you're less
likely to get the the whole pipette overheating and deforming while you're
trying to do the demonstration. The second thing is the original suggestion
was to use a large pipette bulb or even a small syringe. But actually, a fairly
obvious, perhaps, change was to actually swap over the what we're going
to put on the end here, which is instead of using a large pipette bulb we are
going to actually have this fitted to a gas syringe. And of course I'll have that
set up in the retort stand to help me to blow through it. Okay so stands are
optional but they're kind of handy for freeing up your hands when you're doing
demonstration. As you can see I've got a couple of retorts here. One's holding a
gas syringe and that's propping up, via some silicone
tubing, the end of the Pasteur pipette and on the other end, the Pasteur
pipette is just resting there just to hold it level. And before I go I just want to
do a final check. I can smoothly remove my gas syringe in
and out; the wool that I have inside the pipette is not blocking the air from
getting through. I've got my ethanol burner here and some water with some
universal indicator we're going to come to in a minute you can probably guess
already where I'm going with that.
Okay, so light your burner.
And you can just start heating. One of the first things you'll notice is you might look
for some white smoke around on the inside where phosphorous is reacting
with the air inside the pipette. And then after that you'll start to notice some
cream-yellow deposits building up on the on the wool. Again, that's our white
phosphorus that's beginning to actually form from the red. So, give that a minute
or two just to react. We want to make sure we used up all of the air that's inside
the tube before we actually get going but just gently push through the air and
first of all you might get a little bit of smoke. If you go a bit faster you'll
get a nice, cool flame and some fumes of phosphorous oxide; of course, make sure
you're working in a well ventilated laboratory when you do this.
And if you get bit faster you get a really cool flamethrower kind of effect
Out it comes,
a cool flame.
So, I've given this a few seconds to cool down and you can probably guess where
we're going to finish up with in this demonstration. We now have passed through
and burnt off some of that white phosphorus that's in there.
I just want to give this one more puff, okay, just to make sure that all of the
phosphorus has finished doing its thing ...
Give it one more pass through.
Okay, looks good.
Okay, so I can draw in some some air here
and pass the air through some water,
into which I've added some universal indicator solution.
We can see what we get.
Okay so there we go; some nice, red solution showing the
presence of the phosphoric acid produced from the hydrolysis of the phosphorus oxide.
So, nice simple micro-scale reaction of white phosphorus that we've
produced endothermically from red phosphorus in situ in a Pasteur
pipette. Quick and easy to set up; give it a go.