Over
the past several tutorials we’ve performed a few separation techniques, those being filtration
and extraction.
Now let’s look at another one, distillation.
This is a technique that is very common in chemistry labs as well as industry, as it
is an effective and inexpensive way to separate liquid components of a mixture by virtue of
their boiling points.
So this is different from extraction, where the two solvents were immisicible and we could
easily separate them with a separatory funnel.
Here we have a mixture of miscible liquids that have sufficiently different boiling points,
such that we can boil one and not the other.
This is achieved by heating the mixture to a temperature above the boiling point of one
component but below the boiling point of the other, so that the liquid with the lower boiling
point turns into vapor, leaving the other liquid behind.
We can then collect, cool, and condense the vapor into a pure liquid on the other side
of the condenser in a receiving flask.
For this to be achieved, a significant difference in boiling point is required, otherwise too
much of the other component will vaporize as well.
For a simple distillation setup, we’ll need a heat source such as a hot plate or Bunsen
burner.
We’ll also need a round bottom flask, a three-way adaptor like this one, a thermometer
adaptor, a thermometer, a condenser, a vacuum adaptor, and something with which to collect
the condensing vapor, also called the distillate, such as a beaker or Erlenmeyer flask.
We’ll also need a support adaptor for our round bottom flask, and some connector clips.
Lastly, we’ll need ring stands and some clamps.
To build the setup, let’s start with our round bottom flask.
We’ll place it on a round bottom support and put that on a hot plate.
On top of the flask, we can add the three-way adaptor with a side arm like this.
We’ll then connect the thermometer on top using a thermometer adaptor.
We need to make sure that everything is sealed so that our vapor does not escape.
You can add some grease to the joints and turn them so everything fits well.
Your thermometer should be placed at a level that is just a bit lower than your sidearm
to get an accurate measurement of the temperature of the vapor.
Let’s then secure our flask by clamping it at the top like this.
Now on this arm of the three-way adaptor, we can add the condenser.
Since the condenser tends to fall off, as gravity will cause it to slide down, we also
need to secure it.
We can do that by clamping our vacuum adaptor like this and connecting it to the condenser.
We now have support on both sides.
To keep the condenser cool, we will run cold water through this outer chamber.
The vapor will move through the inner section of the condenser, and as its being cooled
by contact with the glass that has been cooled by the cold water in the outer section, it
will begin to return to the liquid phase.
To enable the flow of cold water, on the bottom part of the condenser, let’s connect a hose
that brings in cold water from the tap.
Then connect another hose where the water will exit on the top, and just let that empty
into the sink.
Let’s then finish the setup by adding our collector underneath the end of the condenser
to collect the distillate as it drips out.
In this particular example we are going to be using a graduated cylinder here because
we want to measure the volume of our distillate as time goes by.
So this is our complete setup.
Make sure you examine every joint to ensure that you have a tight seal.
Also check your clamps and adaptors to ensure that nothing is loose.
We can further secure our setup by adding clips that look like this.
These can be used to clamp all the joints together so that the glassware does not separate.
You can now turn on the cold water and make sure that it flows smoothly through the condenser.
We are now ready to start the distillation.
Today we will be distilling a 50:50 mixture of isopropyl alcohol and water with some dye
added to it.
That will give our solution some nice color.
The boiling point of isopropyl alcohol is 82.5 degrees Celsius, while water’s is 100,
so the alcohol is the substance that will vaporize here.
Additionally, because the dye is much more soluble in water than in the alcohol, our
distillate will be clear, leaving all of the dye behind in the water, acting as a visual
confirmation that separation is occurring.
When adding your solution to the round bottom flask, the liquid level should be around halfway
up the flask or lower, as we do not want any liquid to splatter up into the condenser.
Next, let’s add a stirring bar so that we get even heating in our mixture.
We can also add a boiling chip to make the mixture boil more calmly.
Let’s turn on the stirring function and start out by stirring the solution at a slow
rate like this.
You don’t want it to go too slow or too fast, so let’s put this one at around 5.
Be sure to experiment with your stirring function, as different instruments will give you different
stirring speeds.
We can then double-check that everything is connected and that our water is running through
the condenser.
Now let’s turn on the heater.
We’ll turn this up to a medium level to get even heating.
Do not use a high temperature because the solution will start bubbling violently when
it boils.
If that happens, lower the temperature, or remove the heat source completely and wait
until everything cools down before you start again.
Also the discrepancy in the two boiling points is not enormous, so we want to make sure we
don’t heat to a temperature so high that both substances end up boiling.
If you set up the heating and stirring correctly, the mixture should soon come to a nice even
boil.
Vapor will rise up into the three-way adaptor, where it will start condensing.
Eventually, vapor will start collecting in the top arm of the condenser.
The vapor will then travel through the condenser where it will begin to condense, and this
liquid will start falling into the collection vessel.
The isopropyl alcohol being collected is a highly flammable substance that can cause
serious eye irritation and damage to your skin, so if doing distillation of this type,
make sure you wear protective equipment and stay away from fire or sparks.
For this particular experiment, to add a component of data analysis, we can also collect some
measurements.
For every milliliter of distillate collected, write down the total volume collected and
the temperature of the vapor as shown by the thermometer.
We should keep collecting the distillate until we get the amount of alcohol we originally
measured and added to the mixture.
The temperature should go up as the volume of distillate goes up.
This is because the composition of the mixture changes as more of one component is vaporized
and removed.
When all the alcohol has been collected, the condensation should stop, and the temperature
should stop rising.
A graph of your data for a simple distillation should look like this.
The temperature goes up as more distillate is collected, and it will level off after
we’ve removed all the alcohol.
If you know the identity of your target compound as we do in this case, you should collect
your distillate until you get to a range that is slightly above its boiling point.
You can then change the flask when the temperature goes much higher than that.
This is to avoid impurities that have a boiling point higher than your desired product, such
as water in this case.
And with that we have completed a distillation, in which we isolated isopropyl alcohol from
the rest of the mixture.
And as expected, we can see how clear this is compared to our colored mixture.
When the boiling points of the components in the mixture are very close, we can also
use another method called fractional distillation.
This technique is similar to a simple distillation except that we use a fractional column for
the vapors to travel through before they hit the condenser.
This column contains layers of glass beads that improve the separation between the liquids
that are being separated.
But for today, that’s all we need to know about distillation.
Let’s move forward and examine some other techniques.