Warning: Hydrobromic acid is corrosive, wear gloves when handling it.
Greetings fellow nerds.
In this video weíll be performing the very useful laboratory technique of fractional distillation.
In previous videos iíve shown how to make various acids like nitric acid, sulfuric acid, hydrobromic acid and so on.
Now with liquids of very different boiling points like sulfuric acid and water, you can do a simple distillation and pretty much get all the separation you need.
But with mixtures of liquids like nitric acid and water with somewhat closer boiling points.
Simple distillation will get you a distillate thatís richer in one liquid, but will still have the other liquid present.
So you have to distill again. Further enriching one component but still not quite pure.
So you distill again, and again, and again until finally you get a constant boiling mixture that doesnít change.
Obviously if you value your time, your money and maybe your sanity if you still have it, youíll want a more straightforward way.
Fractional distillation letís you essentially run a large number of simple distillations in one run and get excellent separation quickly and efficiently.
All you really need in addition to equipment for simple distillation is a fractional distillation column.
There are many types and I have here a Vigreux column.
I got this one from alchemylabsupply and iíve included a link in the video description in case you want to buy your own.
Be sure to match the joints to the size of your existing glassware.
This particular one is a 24/40 type.
On the Vigreux column you can see it has many internal fingers that protrude into the vapor stream.
These are necessary to maximize the surface area and contact time between the vapors and the walls.
Other popular columns include packed columns full of glass beads
or even simply using an extra uncooled condenser column repurposed as a fractionating column with diminished performance.
So here is the fractionating column installed into a distillation setup.
At the bottom we have our flask containing our mixture of liquids.
In this case weíre distilling dilute hydrobromic acid.
The fractionating column is installed right between the flask and the stillhead.
The rest of the apparatus is the usual distillation equipment like condensers, receivers and thermometers.
Okay iíve turned on the heating and the cooling water so we can begin distillation.
Now whatís happening is first the liquid boils and the vapor will have a greater proportion of the more volatile component than the liquid does.
This vapor moves up and condenses on the inside of the fractionating column near the bottom.
The greater surface of the Vigreux column facilitates this.
Now itís a bit cooler than before
but the continued condensation of the hot vapor delivers heat to the bottom of column
and the condensate will start vaporizing again.
But this vapor will have even greater content of the more volatile component.
The liquid containing a greater proportion of the less volatile component will remain liquid and drop downward back into the flask.
Let me give you an example.
Letís say we start with a 50/50 mixture of highly volatile component and a less volatile component.
When it boils the vapor will now consist of the 60/40 mixture in favor of the more volatile component.
This condenses on the column but as the heating continues it will boil again, creating a 70/30 vapor that continues moving up the column.
Meanwhile the condensate which may contain a small amount of say 50/50 mixture, now drips back down.
Now as the 70/30 vapor continues, it will condense and reboil with continued heating
to give an 80/20 vapor that continues going up and a 60/40 liquid that drips back down.
Repeating this process we can get essentially pure vapor of the volatile component at the top of the column if the column is long enough.
Since this vapor also matches the boiling point of whatever purified components it consists of,
we can monitor the progress of the distillation with a thermometer.
Let me zoom into the top half where the action is.
But iíll also show the flask so we can keep an eye on it.
Weíre getting essentially distilled water as it reaches the top of the column and enters the condenser to be condensed off and collected.
The thermometer is reading 99 degrees Celsius rather than 100 for two possible reasons.
One, the barometric pressure today is slightly lower than standard pressure so the water boils at a very slightly lower temperature.
Two, and the more likely reason, my thermometerís calibration is slightly off.
I did after all buy it on ebay for about six dollars.
It doesnít really matter as weíre more interested in the changes in temperature rather than the absolute temperature.
If the temperature is constant, it means weíre distilling off one clean component or an azeotrope, which iíll explain later.
If the temperature is changing then weíre transitioning between two components.
Now whenever i do a video with a distillation in it and mention that itís hard to cleanly remove all the water.
I inevitably get comments from people stating that all you need to do is heat up a mixture to the boiling point of water.
All of the water will boil off clean and leave behind the rest of the components.
This is a blatantly wrong misconception for a number of reasons.
First, mixtures of liquids always boil off as mixtures. Just the proportion is different in the vapor.
For liquids with similar volatility you can have almost the same proportion in the vapor.
You have to use fractional distillation in that case.
Now for some mixtures with very different volatility like water and sulfuric acid,
the proportion of the less volatile component can be nearly zero.
So you can sometimes separate them without fractional distillation.
But you still canít simply boil off water at its boiling point as if the sulfuric acid wasnít there.
This leads me into my second point against the misconception.
Mixtures of liquids have different boiling points than their components depending on how the components interact.
For very strongly attractive components, like water and sulfuric acid, the boiling point will be higher than water alone.
So if i had water with some sulfuric acid added and then heated it to 100 degrees Celsius, it actually wonít boil.
It would have a higher boiling point depending on how much acid was added.
The composition of the vapor depending on the composition of the liquids.
Now what if we had two liquids that were strongly repulsive?
Letís say you mixed together methyl ethyl ketone which had a boiling of 79.6 degrees celsius with about 11% water
then you would actually get a mixture that boiled at 73.5 degrees celsius.
The boiling point would actually be lower than either of them.
The vapor being a mixture of these two liquids.
Bottom line, when youíre boiling a mixture of liquids,
the boiling point is not simply the lowest boiling component.
Nor can you easily remove the lowest boiling component using just that temperature.
Itís much more complicated than that.
Of course you might ask, why are we getting nearly 100 degrees for water in the video?
Didnít i just say the temperature would be different!
This is because the fractionating column, through condensation and reboiling,
is purifying the vapors as they travel up the column.
The top is essentially pure water boiling at 100 degrees.
But the flask however is at a different temperature itís still boiling the raw mixture.
If we had a thermometer there we would be measuring a higher temperature.
Now an interesting thing about mixtures of liquids having different boiling points depending on their properties,
interactions and composition is that you can have a liquid of a particular composition that boils
but does not change its composition in the vapor phase.
This vapor condenses as the exact same mixture and cannot be separated by fractional distillation.
These mixtures are called azeotropes and have constant boiling points and composition.
They behave almost like pure substances.
Not all mixtures produce azeotropes, but a lot do.
In fact hydrobromic acid that we're distilling here is one such azeotrope.
Hydrogen bromide is a gas that boils at -66.8 degrees celsius and water obviously boils at 100 degrees.
But together they form an azeotrope that boils at 125 degrees with a composition of about 47.5% of hydrogen bromide by mass.
So if I keep fractionating my dilute hydrobromic acid and removing the water, the concentration of the hydrobromic acid will rise.
I'll eventually arrive at the azeotrope and be unable to separate them further.
Any attempt to do so and iíll just distill over the azeotrope.
Chemicals that easily form azeotropes during manufacturing are often sold like that without further concentration
as itís generally cheaper to do so.
For example nitric acid tends to be sold at 68%-70% concentration because thatís the azeotrope it forms.
Because such chemicals are so often sold at their azeotrope concentrations those particular concentrations become common names for the chemical.
For example concentrated sulfuric acid often refers to the 98.4% azeotrope it forms with water.
Now there are ways of breaking azeotropes by using chemicals to change the properties of the mixtures
or by distilling under different pressures.
But iíll make a separate video on those processes and demonstrate them directly when i get around to them.
Weíve pretty much distilled off the water and now weíre hitting the azeotrope.
You can see it moving up the column as it clings to the glass differently than water does.
This is useful indicator when it happens but donít rely on it, not all distillations transition this cleanly.
It should be hitting the thermometer about now.
Okay there we go, weíre at a constant temperature now which means weíre just distilling the azeotrope.
We change out our receiver and attempt to collect our acid.
Now this is not strictly necessary as we know weíve removed all the water we can at 100 celsius.
Any liquid remaining has to be the hydrobromic acid.
But iím going to try anyway and hopefully get something cleaner than the orange decomposed acid in the flask.
Now Iím distilling a single batch of hydrobromic acid and i donít need to add more.
But sometimes you might want to add more liquid if you have very large quantities.
Taking apart the apparatus to refill it is cumbersome and time consuming
so if you have some extra funds it might be worthwhile to invest in a claisen adapter.
You install this between the flask and the fractionating column and install a separatory funnel onto that so you can refill the flask with fresh liquid periodically.
Just be sure to turn off the heating and let the apparatus cool during refills so you donít accidentally have hot vapors spitting out at you.
Oh just so you know the arrangement that i have here with the fractionating column on the head and the funnel on the arm is actually not the best arrangement.
Most chemists prefer to have the column on the arm and the funnel on the head.
One, the extra length provides a tiny bit more fractionating turnovers, also known as theoretical plates.
And two a large separatory funnel full of liquid can be much heavier than a secured and properly clamped fractionating column.
The heavy funnel might snap off the arm if you knock it the wrong way during a refill.
Putting it on the head is a little bit stronger and more secure than putting it on the arm.
Anyway, back to the distillation.
Now itís important not to boil too much and go too fast.
The condensate must be able to drip back down and mix with the bulk liquid in the flask.
If you heat too much in attempt to speed up the distillation you risk blasting the the condensate up the column
and never getting the equilibrium necessary for fractional distillation.
This is known as flooding your column and this is especially important in packed columns.
So adjust the temperature and distill slowly.
I prefer these Vigreux columns as opposed to packed columns as the greater open space allows the condensate to easily return to the flask
and so itís much harder to flood this column even at high flow.
That being said, itís still possible so pay attention to your distillation.
Looks like the hydrobromic acid is badly decomposing due to the excessive heating.
And i'm getting bromine vapor up the column and condensing into the receiver.
One of the drawbacks of fractional distillation is that the column necessarily has to lose heat for the continual condensing and reboiling to occur.
So an excessive amount of heat must be applied to the boiling mixture and this can decompose sensitive chemicals.
I have so little liquid left that the dead space in the system is making it difficult to continuing distilling at a reasonable rate.
So iím going to stop now and let it cool.
While Iíve successfully removed all the water thatís not part of the azeotrope,
Iíll likely need to redistill the hydrobromic acid azeotrope to purify it.
Okay, Iíve recombined the hydrobromic acid to be distilled again.
Iím using a much smaller flask to reduce the dead space in the system and thus recover more of the hydrobromic acid.
Now iím not bothering to do a fractional distillation again because i donít have to.
I know that all this fluid has a constant boiling point of 124 celsius so there is no point in attempting to fractionate it.
I can now use a simple distillation and safely assume that all the liquid i collect has that boiling point.
Of course this assumption would be wrong if i had some other, even higher boiling liquid still in the residue,
but i didnít use any such chemicals when i made this crude hydrobromic acid.
Oh yeah, if youíre wondering about the purple color,
itís because i added a small amount of copper metal to the hydrobromic acid.
This is to react with the excess bromine and thus remove it from the hydrobromic acid.
Since copper bromide doesnít distill over, it won't contaminate the acid.
Normally, professional chemists use red phosphorus as an additive to scavenge free bromine.
Unfortunately i donít have any in my amateur lab so I had to make do with copper.
It wonít be as good, but itíll still work to some extent.
And we are done.
There is our concentrated 47.5% hydrobromic acid with a minor trace of bromine giving it that yellow tint.
This is acceptable for my use.
Another way of verifying the concentration is by density.
Iíve poured the hydrobromic acid into this tared graduated cylinder.
The mass of the acid appears to be 84.1g and iíve measured the volume to be 58 mL.
This gives us a density of 1.45g/mL which is very close to the literature value of 1.49g/mL for constant boiling hydrobromic acid.
Anyway, there we have it.
We concentrated 300 mL of dilute hydrobromic acid down to 58 mL of concentrated 47% hydrobromic acid by fractional distillation.
Thanks for watching.