Greetings fellow nerds.
In this video we're going to explore and use a Dean-Stark Trap as well as construct a more
versatile makeshift version.
So first a bit of background.
I've discussed in previous videos the concepts of fractional distillation and in particular
the concept of azeotropes,
mixtures of liquids that boil together and behave as single liquids that have a constant
composition and boiling point.
So far I've been focusing on azeotropes made of homogeneous mixtures of miscible liquids
but another very interesting class is azeotropes consisting of immiscible liquids.
Here i have a mixture of toluene and water.
Toluene boils at 110 Celsius and water obviously boils at 100 Celsius.
They don't mix very much at room temperature
but if you set up a distillation and boil them over an interesting thing happens.
They form an azeotrope that boils together at a much lower temperature of 84 degrees
Celsius.
When they condense and cool they separate back into their separate phases.
This is tremendously useful and we can exploit this for such applications as removing most
of the water from an organic sample.
Just boil it out, pour the organic layer back into the boiling flask while separating the
water and boil it out again.
Now you might ask why we don't simply use a separatory funnel
and while this is certainly the way to go if the mixtures are simple and easy separate,
sometimes you have to deal with complicated sludges, tars, emulsions, viscous gels and
other heterogeneous mixtures.
A separatory funnel isn't going to do much for you if the components just don't separate.
Adding an easy to boil carrier solvent like toluene let's you boil out the water and separate
it that way.
Now a very difficult sample might take hours or even days of refluxing and returning the
organic layer to get a good separation.
Two chemists, E.W.
Dean and D.D.
Stark, ran into the same labor problem when they wanted to analyze the water content of
petroleum samples.
So they invented the Dean-Stark apparatus,
other devices under different names that essentially do the same thing include the Dean-Stark receiver,
the Barrett trap and the Clevenger trap.
What i have in my hand here is technically a Barrett trap because it has a stopcock for
draining the water out as it accumulates.
The original Dean-Stark trap has just a blind tube for measuring the water accumulated.
But the name Dean-Stark trap has become so commonplace for these devices that I'm going
to to call it a Dean-Stark trap as well for this video.
Now how this works is that you attach this to the top of a boiling flask
and as the vapors rise into the trap they condense on a condenser attached to the top.
The liquid falls into the trap here and the denser phase, usually water, accumulates in
the bottom.
The lighter phase fills up the trap and eventually overflows back into the boiling flask
replenishing it allowing more of the azeotrope to be boiled out.
If too much of the heavy phase accumulates then it will also overflow back so it can
be removed using this stopcock here.
Okay, let's see this thing in action.
Here I've set it up over a boiling flask containing a mixture of water and toluene.
And up here is the condenser.
It's all extremely simple to use and setup.
Now all we do is turn on the heating.
And there it goes, the azeotrope of water and toluene is rising up into the trap and
condensing in the condenser where it then falls into the trap.
You can already see the layer of denser water accumulating in the bottom of the trap.
Eventually when the distillate reaches the top it will overflow and the lighter phase
will return to the boiling flask.
Now removing and measuring water from a difficult sample isn't the only use for a Dean-Stark
trap.
It can also be used to remove water from reactions.
For example if you're making esters with very high boiling alcohols and acids,
the water will most likely form an azeotrope with one of the reagents or products and you
can remove it with the Dean-Stark trap.
The trap then returns the reagent to continue the reaction.
Let me flip over this clamp so you can see the overflow.
Anyway as i was saying before you can use the trap to remove water from samples as well
as reactions.
You can see that the process of separating the two phases and returning the lighter phase
requires almost no user interaction
so for extremely slow or difficult samples and reactions,
you can just set it up and walk away to spend your valuable time elsewhere.
The only time you do need to interact with the apparatus during operation
is if there is so much of the heavier phase that it starts overflowing and you need to
empty the trap.
So you usually select a large enough trap that you need to do this as few times as possible
if at all.
Okay we've been running this for some time now and the water layer seems to have stopped
growing,
we've pretty much removed all the water we can by this method.
If you're doing analytical work like Dean and Stark then you can just read off the amount
collected on the graduations.
Alternatively you can remove the water from the trap by using it just like a separatory
funnel.
And there we go.
And here is the toluene after cooling without any visible water present.
And here is the water that was originally added.
This method is a very cheap way of removing water from immiscible solvents
and excellent for pre-drying solvents before using much more expensive chemical drying
techniques,
allowing you to use less chemicals overall.
Now I've seen Dean-Stark traps cost anywhere from twenty dollars to several hundred dollars
depending on size,
quality and extra features like vacuum jackets or the ability to return the heavier phase
rather than the lighter phase.
So you'll have to select the one that suits your needs.
This one here was generously donated to me
but if i had to purchase this exact model it would have cost me around $250 dollars
from the original manufacturer.
So once again to the individual that donated this thank you very much,
i would never have bought one myself.
Now if you use one very often in your work then it's worth getting a professional unit.
But if you only rarely need one then a very crude unit can be constructed from more common
glassware.
And here it is.
I know it's a little bit daunting and far more unwieldy than a professional unit
but it does the job and can be put together in ten minutes.
Let me walk you through it.
Down here is our boiling flask containing our water and toluene test sample.
Above that we have a claisen adapter but you can also a cheaper gas inlet adapter if you
have one.
Now to the claisen adapter we have a condenser column.
This is not actually be using as a condenser as you can see it's unconnected and you can
use just about any column,
what we want here is to increase the height to the still head up here.
So you can use whatever column you want to achieve that.
Now to the still head we have a properly connected condenser column which will actually be used
as a condenser.
At the end we have our distillation adapter although this optional if you tilt the graduated
cylinder down here.
Now key to the whole operation is this tube that siphons out liquids in the graduated
cylinder
and sends them to this hose adapter that connects to the claisen adapter and returns the liquids
to the boiling flask.
Now because this system is dependent on siphon action to work,
it must be properly aligned and pre-filled before you start, unlike a professional trap
that can run empty.
First, make sure the inlet of the siphon tube is below the outlet relative to gravity.
Second the lowest point in between the two ports must be lower than the bottom of the
graduated cylinder.
So going by the shortest distance is not allowed.
Setting it up like this ensures the siphon will always work
and withdraw fluid from the graduated cylinder without over emptying it
and sucking in air that stops the siphon and causing it to overflow later.
Now you need to pre-fill the tube and graduated cylinder with the carrier fluid.
Since we want to collect water our carrier fluid is toluene.
I'm filling through the hose adapter here but you can use whatever method you want.
Fill up graduated cylinder until it's passed the tube inlet.
Now make sure the tube itself is filled without air bubbles in the top.
You may have to move around the tube somewhat to work the bubbles out.
Once the siphon is achieved.
If you move around the hose adapter filled with liquid the level will match the cylinder
as the siphon maintains the balance.
At this point, reinstall the hose adapter onto the claisen adapter.
It's alright if some air now appears in this end of the tube.
The lower midpoint of the tube should prevent it from creeping up to the highest point.
It is vitally important air never gets into this highest point or the siphon will be broken.
As one final test, try to overfill the graduated cylinder with the carrier fluid.
If everything is setup correctly then the excess fluid will get siphoned out and ejected
out the outlet port where it will return to the boiling flask.
You can now adjust the inlet port higher if you wish as long as it always below the top
of the fluid and the outlet port.
Give a centimeter or so margin just in case dropping fluid from the condenser brings down
some air bubbles.
Oh yeah this black clip up here secures the tube so it doesn't change its position while
the distillation is running.
Always have some way to secure your tube.
Now that everything is setup.
Turn on the heating and we can now watch our homemade Dean-Stark trap in action.
And there it goes, first the vapor rises up through the columns and into the condenser.
It condenses back into a liquid that phase separates and drips into the graduated cylinder.
The heavier water phase sinks to the bottom where it collects.
The lighter phase overfills the cylinder and the siphon tube sucks it out and returns it
to the boiling flask through the claisen adapter.
Completely self-running and requiring no user intervention unless it's for emptying the
graduated cylinder.
Now the whole thing costs about a hundred and fifty dollars or about the price of a
moderately expensive dean-stark trap.
But since the components are more commonly used for other experiments,
chances are a well-stocked laboratory already has most if not all of them.
The total effective cost can be considerably reduced.
Now obviously if you're going to use this a lot,
it might be more effective to spend the money on a complete dean-stark trap so you can use
your glassware elsewhere.
For one, this is a hell of a lot nicer looking.
Anyway, looks like all the water has been collected and now it's just cycling carrier
fluid.
Now so far I've just demonstrated light return dean-stark traps that just return the lighter
phase.
But what if we want to do opposite?
What if we want to return the heavier phase and keep the light phase?
This is especially important for herbalists, botanists and other scientists that collect
oils and chemicals from natural sources.
They use steam distillation where boiling water carries the oils out in the steam.
This professional dean stark trap only returns the lighter phase and cannot be easily modified
to return the heavy phase.
We could purchase a trap specifically for returning heavier phases but those are even
rarer and more expensive.
Fortunately our homemade unit can be easily modified for heavy return.
So I've emptied out the cylinder and tube and just like with light return mode we make
sure all the tube positions are where they should be
and we fill this time with water since that is our carrier fluid and our target is the
lighter oil.
Once everything is setup, we just perform one simple change to convert this apparatus
to heavy return mode.
We push the siphon tube in graduated cylinder all the way to the bottom.
And that's it.
Let me turn on the heating.
No when the distillate pours into the graduated cylinder,
the siphon tube sucks out the heavy layer and returns that.
The lighter layer has nowhere to go and thus accumulates in the graduated cylinder.
Heavy distillate will sink through the lighter layer and accumulate in the bottom where it
can be siphoned out again.
So our homemade apparatus, despite being big, slow and ugly,
can easily be converted to both light return and heavy return modes.
A professional unit that can be converted to both modes is extremely rare and somewhat
pricey.
I don't do nearly enough distillation work to justify buying it.
So there you have it, the dean stark apparatus and how to make it.
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