Greetings fellow nerds.
I got a new piece of lab equipment.
This is the short path distillation apparatus.
If you look at closely, it seems to be just a really compact integration of a full sized distillation apparatus.
Here you have the still head that connects to a boiling flask.
There is a joint up here for a thermometer assembly.
The rest of the head is integrated with a really short condenser column.
And we even have vacuum adapter for vacuum distillations.
The overall compact size means there is less dead space and surface area so there is less product loss and better recovery for small quantities.
Being fully integrated this is a very convenient way to do distillations without all the hassle of assembling the full-sized apparatus.
And for the most part, that's exactly how it's used by most amateurs and even some professionals.
In fact that's why i bought it, just because it's convenient.
I'm probably very rarely going to use its advanced function.
Interestingly, i find most professionals don't realize the short path distillation apparatus even has an advanced function.
Many assume it's called short path because it has a short condenser column.
Okay so what is so special about this.
First, i want to do a very simple demonstration you already know but probably haven't thought about in this particular context.
I have here a beaker of warm water.
The temperature is about 45 celsius or so, well below the boiling point of water.
To distill something you usually need to heat it up past the boiling point.
So clearly we can't do it here, or can we?
Now i have here a beaker of room temperature water.
It's not cold enough to condense water from the air, but if i place it above the beaker of warm water and wait, it'll start condensing.
The reason of course is well known.
The warm water is evaporating and the cooler beaker of water condenses it.
The vapor pressure of the warm water is higher than the vapor pressure of room temperature water.
So the water condenses out when it hits the room temperature beaker.
You've seen this effect before, this is not new.
But think about this for a second, we are distilling water, below the boiling point of water.
And this isn't some quirk or tiny edge effect, we are over fifty degrees below the boiling point of water.
To get this kind of temperature improvement we'd normally need vacuum distillation.
But here we're doing it at atmospheric pressure, no vacuum pump, we're not even using any complicated apparatus.
But there is one very important consideration when using this effect.
The molecules have to be able to travel from the surface of the evaporating liquid to the condenser.
In boiling distillation, the vapor pressure of the boiling liquid itself pushes the molecules over.
So the distillation apparatus can be large and unwieldy, as long as it's boiling it'll work.
It'll push against atmospheric pressure and reach the condenser.
But when you're using evaporation alone, you have to rely just on the diffusion of molecules.
This can be very slow for large apparatus.
What you need is for the molecules to travel as little as possible.
You need a short path.
And this is why this is called short path distillation.
It's not because the apparatus uses a short condenser column,
but because you're taking advantage of the short distance between the evaporating liquid and the condenser surface.
So when is this used.
It's used primarily for extremely high boiling compounds that can't be distilled in traditional boiling distillation at common temperatures.
By distilling below the boiling point a lot more compounds can be purified.
It's also used when common lab temperatures are simply too destructive for the mixture you're purifying.
If the other compounds in your mixture start decomposing and reacting with your target compound before it can boil then distilling at a lower temperature is preferable.
Short path distillation is almost always augmented with vacuum distillation.
Afterall if vacuum distillation can directly lower the boiling point, then short path distillation can let you distill at even lower temperatures than that.
But more importantly, short path distillation is works terribly without a vacuum.
The vapor has to push through the ambient gas in the apparatus.
It can't rely on the pressure of the boiling liquid to push the molecules through.
If the vapor is very dense this can be extremely slow or even stop completely as the vapor just sits around never reaching the condenser.
Under vacuum the vapor is forced to distribute evenly and reach the condenser much faster.
This is why almost all short path distillation apparatuses have a vacuum adapter.
All of that being said, you're probably wondering why this isn't more popular.
Why isn't this a staple of every lab.
Well there are some serious drawbacks and considerations when using this.
As said before this is apparatus is intended for extremely difficult high boiling substances like essential oils.
Such substances distill very slowly so there isn't a need to have a long distillation column.
As such the distillation column is very short, it's not meant for high flow.
If you try and distill a very easy substance like ether or methanol then it's very easy to exceed the capacity of the column
and flood the system with vapor, resulting in lost product as the vapors don't condense in time and are lost.
Another consideration is that with a short path you need a very well-behaved substance that doesn't pop, bump, splash, or foam.
If it does, it would overflow the apparatus.
At least with a full sized apparatus, any upsets are contained in the boiling flask or using splash guards.
You actually shouldn't need a short path distillation apparatus if your mixture does indeed behave badly.
The fact that it can pop, or foam means its vapor pressure is high enough to do so.
If that was the case, then simple distillation would be adequate.
Overall, short path distillation should only be done when you specifically need it.
If your substance is already accessible with more conventional techniques then a short path distillation apparatus is not only unnecessary,
it can be much slower and/or perform much worse, losing valuable product and causing more trouble than it saves.
If you don't try and reduce the pathway or take advantage of diffusion then this is no better than simple distillation.
So when using this you want to keep that in mind and set it up accordingly.
Use small flasks to reduce the distance, otherwise this will be no better than full sized apparatus.
It also helps to tilt the flasks so the liquid is as close as possible to the apparatus without spilling over.
Vapors travel sideways better than they travel straight up.
Also use a vacuum whenever possible.
If we can reduce the boiling point with a vacuum, and distill below the boiling point, we can stack two advantages to distill extremely difficult substances.
It's also necessary to get reasonable flow rate.
So there you have it, the short path distillation apparatus.
Thanks for watching.
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