Hey guys. Today we're gonna be making some anhydrous oxalic acid, but the
method that we use today is a little bit special.
Normally, the easiest way to dry a powder is to just put it into the oven, but the problem
with oxalic acid is that it very easily sublimates at elevated temperatures.
When something sublimates, it converts directly from the solid to the vapor
form, with no liquid intermediate, and this means that if you were to crank up
the heat on oxalic acid, you would vaporize a lot of it and cover the
inside of your oven with oxalic acid crystals.
To get around this problem, and to dry my oxalic acid, I used a technique known
as azeotropic distillation, and I'll elaborate on the details of this technique
as the video progresses.
For most applications, the hydrated oxalic acid is perfectly fine, but I want
to make some oxalyl chloride, and to do this I need everything to be perfectly dry.
Once I make the oxalyl chloride, I can make something called TCPO, which is
basically the chemical in glowsticks that allows them to glow.
As usual I'm gonna mention that I used yet another Doug's Lab video as a reference
for this, so if you enjoyed this video, I highly recommend that you check out his
channel, and I'm gonna leave a link in the description.
Besides oxalic acid, the only other thing that we really need in terms of chemicals is some toluene.
Toluene's a good solvent to use because it doesn't dissolve oxalic acid,
but it forms an azeotrope with water.
I don't really want to get into the details of what an azeotrope is, but basically a
mixture of 80% toluene to 20 percent water by weight boils at around 20
degrees lower than pure water, and about 30 degrees lower than pure toluene, so
basically what's gonna happen is as we heat things up, the oxalic acid is gonna
release water into the toluene, and the water and toluene are gonna boil away as a mixture.
We could, in theory, use a distillation apparatus with a large excess of toluene and just
do a continual distillation, but I think it's much smarter to use something called a Dean-Stark apparatus.
So this is what a Dean-Stark apparatus looks like: on the right side, we attach
to the flask, and on the left side above it will have a condenser.
As we continue with the video, you'll see exactly how this nifty piece of glassware works, and why it's so useful.
I started things off by adding about 60 grams of oxalic acid to a 1-liter round bottom flask.
On top of the oxalic acid, I pour in about 400 milliliters of toluene.
Once this is done, above the flask I can put together the Dean-Stark apparatus and the cold water condenser.
Because we're using a stir bar to stir things, we need to use an excess of
toluene, because if we used more oxalic acid, it could easily jam up the stir bar, and things would get stuck.
So just stay on the safe side, we use the large excess of toluene, but if you have something like a
mechanical stirrer, then you could use more oxalic acid.
The way that I heated the flask is a little bit ghetto,
and it's just hovering a little bit above the surface of a hot plate.
The reason I had to do this is because I didn't have a container large enough to
make an oil bath for a one liter flask, and apparently my heating mantle's too thick
for the stir bar to work through it.
To help keep the flask hot, I insulated it with a very small amount of aluminum foil.
As things start to heat up more, we can see our vapor front moving up through the Dean-Stark apparatus.
What's happening here is the hot vapors from the flasks are traveling up to the Dean Stark apparatus
and condensing on the cold walls.
The vapor slowly heat up the walls of the apparatus though, so they slowly travel up.
Eventually, the vapors will make it all the way up to the cold water condenser
where they'll recondense and fall back down to the area on the left.
It's gonna have a slight milky white color here when it recondenses, because remember the
vapors contain both water and toluene.
In the liquid form, water and toluene are immiscible, so when they're mixed, they become slightly cloudy.
As we continue to collect more, the toluene and water will separate into layers,
where the water's on the bottom and the toluene's on top.
If we look closely, every so often we can see a little bit of water sinking from the top to the bottom.
So, ideally, what's happening is the water is sinking to the bottom, but the toluene is falling back into the flask.
At this point, everything is pretty much on autopilot, and we just need to keep
things running until we think that no more water's coming over.
To get an idea of when it's about done, we can do the calculations, and we find
out for 60 grams of oxalic acid dihydrate, we should collect about 17.1 milliliters of water.
As we carry out the distillation, the oxalic acid might go through a few stages
where at some point it's clumping up a lot, and then it becomes much more of a free flowing powder.
Near the end, the oxalic acid started to turn brown, and I think this is because
most of the water is gone and the temperatures start to rise above
100 C, and some of the oxalic acid started to melt.
Anyway when it's done to see how much water we got I drained off into a beaker
and we go ahead and weight it. I get a massive about 16.7 five grams which is about
point 35 grams off of the theoretical. The rest of the water might be trapped
in the cloudy toluene you see in the Deans Stark apparatus, but in the end
point 35 grams is not very much and we have removed the majority of the water.
Once everything is cold, I poured off the toluene into an Erlenmeyer flask and
I actually save this toluene for the specific purpose of drying more oxalic acid.
If you don't wanna throw it out I really wouldn't recycle it in new
reactions because even though the oxalic acid isn't very soluble in it.
It still is going to contain a little bit of oxalic acid.
To get rid of the toluene on the oxalic acid I did a few washings with
dichloromethane because DCM is a much more volatile than toluene.
One thing that's interesting is if you look at the oxalic acid now it's
actually slightly pink. Honestly I don't know why but I don't think the color
really matters too much.
Once they've washed it with a little bit of DCM I transferred everything to a
vacuum filter flask.
When I turn on the vacuum everything is very quickly pulled off and I go ahead
and wash it a few more times with a little bit of DCM
Once the rest of the DCM had been pulled off I leave the vacuum on for several
seconds just to try to pull away as much as possible.
The oxalic acid should be relatively toluene free at this point, but to fully
get rid of most of the DCM, I transfer it to a crystallizing dishes and I let it evaporate.
I leave it out for several minutes which is just enough time for the majority of
the DCM to go away and I don't leave it out for too long otherwise it'll start
absorbing moisture from the air.
I transfered everything to a nalgene bottle for storage and at this point you can see
that it's not really pink but it still is a little bit off white.
In the end I was left with nearly the theoretical amount of anhydrous oxalic acid which
was about forty three grams. Like I said before I do plan to use this oxalic
acid to make something called the oxalyl chloride, which in turn will be
used to make something called TCPO. I'm not really sure when I'm going to get
around to making the oxalyl chloride, but I am gonna promise that it will happen eventually.
So as usual a big thanks goes out to all my supporters on Patreon, but
I have to give a very special thanks to everyone who donated $5 or more.
As I said in a previous video I kind of too many 5 dollars supporters to realistically
readout each of your names, but just know that I still love you all.