PAUL ROBERTS: Hi, my name is Paul Roberts. I'm a chemistry teacher here at Andover College.
I've been teaching chemistry for eight years. And today, I'm going to talk you through how
to do the core practical of distillation. So there are some important safety considerations
in this practical.
First of all, we're using a Bunsen burner. So it's very important that students know
how to safely light a Bunsen burner and how to control it. Also, we're going to be heating
the water in an elevated position. So it's important to clamp the conical flask.
The first part of the practical set up, you're going to need to prepare the bung thermometer
delivery tube apparatus. An important safety consideration in this delivery tube setup
is that there are no blockages in the delivery tube. Any blockages could result in the overpressure
of the heating system and could result in injury.
So first of all, just check that there are no blockages. This can be done by carefully
blowing through. And you can feel air coming through out of the other end. So now we have
our delivery tube apparatus set up.
Now, I'm going to add some ink to the conical flask here. And I've got some black ink. I've
prepared this by putting a few drops of just fountain pen ink in 200 mils of water. I need
to add approximately 50 centimeters cubed.
The volume isn't critical. We're going to have some leftover at the end anyway. Now,
to ensure a nice smooth boil, we need to add some anti-bumping granules. The amount of
these anti-bumping granules isn't critical, just a spatula full in the flask is fine.
So now we have our ink and anti-bumping granules in our conical flask. We can now put in the
thermometer bung delivery tube apparatus and push it in nice and snuggly. Notice at the
other end here, where we have the delivery tube going into a test tube. And the test
tube is in an ice bath. That way when the water boils, it condenses in the tube and
will be continued to be condensed in the test tube, OK.
So I'm going to light the Bunsen now. Always light a Bunsen with the hole either closed
or partially closed. And I'm going to slowly increase the temperature of the Bunsen. And
the Bunsen is going to be heating fairly vigorously. And I wanted to bring the water, or the ink,
which is mostly water, up to a boil.
Now, I can keep an eye on the thermometer there. But the thermometer should start rising
as I heat it up. And it's going to take a couple of minutes for the ink to come to a
boil. So watch you're heating the ink. It's very important to keep one hand on the Bunsen
burner. That way if the ink starts to boil quite violently, you can quickly move the
Bunsen out of position.
I can just see the ink just starting to boil now, bringing it to boil. And I'm being mindful
not to leave the Bunsen there. The temperature is still rising. And at this point, I can
just start to see condensation appearing in the delivery tube. So now this is a kind of
boil we're looking for, a nice steady rolling boil.
I don't want to heat it too violently. If I heat it too much, then the ink will actually
boil all the way to the top of the bung. And we'll get carry over into our receiver flask.
I can now see liquid slowly coming down the tube. It's condensing here in this tube. Now,
bear in mind this tube is going to be very hot. This tube is going to be 100 degrees.
So don't touch it.
And I'm continuing this in and out type process where I'm heating the ink, keeping it on a
nice rolling boil. Now, after a couple of minutes of doing this, you should be able
to see through the receiver, to the ice bath and the receiver that we have collected some
water. And at this point, I can turn the Bunsen burner off.
Now, I'm not going to do anything to the apparatus until the boiling has stopped and until there's
no more steam coming out of the delivery tube. Bear in mind, you can see the thermometer
now. The thermometer is at 100 degrees. And so is the whole of this apparatus.
However, my ice bath and my receiver is actually quite cool now. This is actually cool to touch.
Now, I can carefully remove the ice bath. There is a matter about any water dripping
here. And I can see now when I take out my test tube, my receiver test tube, that I've
got pure water remaining. And there's no ink in the original sample.
So it's very important now to keep an eye on the temperature and not to start dissembling
the apparatus until the temperature drops to 50 degrees. Everything here will still
be very hot. The rest of this can go down the sink as there's no hazards in pure water.
So there are a few common problems that can occur in this practical. First of all, you
get black ink in your test tube, in your collection vessel. This can be caused essentially by
the ink boiling too vigorously and the ink carrying over into the cheek.
A simple remedy to this is not to heat the conical flask as strongly, and constantly
be moving the Bunsen burner in and out. Another problem can be, if you're using a small conical
flask, then it boils to quickly. And there isn't the volume for the boiling to calm down.
Again, I suggest using a 250 mil conical flask.
Now, the other problem might be you get no water at all in your receiver. Now, this can
be due to there being a hole or not a snug fit in the thermometer and delivery tube,
or there not being a snug fit in the bung into the conical flask.
So in summary for the distillation practical, first of all, you prepared the delivery tube
apparatus and made sure there are no blockages. We've also looked at how to safely heat the
apparatus and make sure there's no carryover. And last of all, we've looked into some troubleshooting
of how to get over some problems that might occur.
Now, the practical I'm going to talk you through today is the core practical of chromatography
in ink. This is a very straightforward practical. It's something the students might have come
across before even at primary school. But I'm going to give you some guidelines today
of how we can get some very clean and accurate results.
As with all chemistry practicals, we have to consider safety. The solvent we're going
to use in this chromatography is water, so there's no hazards there. But as ever, if
you're working in a laboratory, it's standard precautions to wear a lab coat and safety
glasses.
So we're going to start the practical now. The first thing we need to look at is the
beaker. Now, the beaker is going to dictate the dimensions of the chromatography paper.
It's very important that the chromatography paper goes into the beaker, and it doesn't
touch the sides.
So what I've used here is a piece of filter paper. And I've drawn my chromatography paper
outline on there and cut it out. Now, once we've got the chromatography paper in the
beaker, checking the heights so it's just at the top of the beaker there.
What I'm going to do on the chromatography paper is mark a line about two centimeters
up from the bottom. Now, it's very important that I mark this line in pencil. As with all
lines here drawn on my template, everything needs to be marked in pencil. Otherwise, it
will run and interfere with the results. I've marked my line now in pencil. And this is
what is known as the baseline.
Now, the next part is to dot the inks on the chromatography paper. Now, I've got four marker
pens here. Well, two are marker pens. And another two are felt tip pens. It's very important
that they are non-permanent markers. Permanent markers won't run in the water solvent.
So as with chromatography, all chromatography, less is more. It's very important to put a
small amount of residue on the chromatography paper. And so what I can do first is practice
on another piece of paper. And this practice, that small dot there, you can see the size
of the dot there. It's really quite small. Students will be tempted to put a big dot
like that. Unfortunately, if they do that, they're not going to get good results. The
residues will run into each other.
So I want a very small dot. And I'm going to put them equal spaced on the chromatography
paper, 1, 2, 3, 4. So you can see, they are very, very small dots in chromatography.
Now, we're going to suspend this over the beaker. And so, it's going to attach to the
chromatography paper to a pencil. Now, this can actually be done beforehand for the students.
You can always give them the paper already with the pencil attached.
What I'm going to do now is check the height of the paper in the beaker. And you can see
that now, the paper isn't touching either the bottom or the side. And this will give
good results in the chromatogram.
I'll take the paper out. The temptation is to add the water with the paper in the beaker.
Unfortunately, if you do that, and you accidentally get water on the face of the paper, you'll
have to start the chromatogram again. So take it out. And you're only going to need to put
in around about two centimeters depth. Just checking that the height looks good, yeah,
that's nice, and popping the chromatogram in the beaker.
Now, the temptation is to move the beaker now. It's very important that we leave the
beaker nice and still. If you move it and the paper touches the side, or the water washes
over the baseline, you're going to get really quite bad results. So just leave the paper
nice and still for 20 minutes.
So it's been 20 minutes now. And you can see now in the chromatogram that there's been
quite good separation. You can see on this paper here how we've had no separation of
the dots either side. But the two dots have completely separated. And you can actually
see on the chromatogram that these two black inks are made from two different mixtures,
both though containing a blue and a pink.
Now, at this point, it's important to leave the chromatogram to dry. And I'm going to
leave the chromatogram to dry. You can pop it an oven or a windowsill. But don't lay
it flat. If you lay it flat, then the inks will start to smudge.
You'll also notice that I removed the chromatogram with about a centimeter from the top of the
ink to the paper. And that's because the residual moisture in the paper will still continue
to rise up the chromatogram. And it's very important that you don't let it go right to
the top. Otherwise the chromatogram will continue to rise until it's left, the dyes have left
the paper.
So here's a another chromatogram I prepared earlier. And this has dried out completely.
And I'm going to remove that chromatogram from the pencil. Now, on the chromatogram,
we're going to mark some key areas.
First of all is the solvent front. And you can see the solvent front is the furthest
point at which the water has risen up the chromatogram. And I'm going to mark that across
the top. So I've now drawn my solvent front. And I can measure the solvent front.
Now, the distance of the solvent front from the baseline-- it doesn't really matter what
units you measure it in. But this is a millimeters ruler. And so this is 64 millimeters, OK.
So we call this the solvent front, distance of 64 millimeters.
Now, I'm going to mark the key dyes points of each residues in the chromatography. You
can see there's a blue residue there. And then I've got below it kind of a pinky residue.
And on the other one here, I have a blue residue there, and again, a slight pink residue. And
you might be able to make out a very faint, another kind of pink residue there.
And I put my cross at the darkest points of the residues. I'm not just drawing a circle
around them. What we do now is measure the distance of the residues in millimeters. And
this distance here is 42.
The distance of this residue is 59. The distance of the blue residue is
62. And then for the other residues of the other ink, this residue distance is 59. And
the blue residue is again, 62.
The blue dye has traveled an identical distance on both samples, it suggests that it is an
identical chemical, and the same for the pink one. Notice, I haven't measured the residue
distance of the first and the fourth sample I put down. That's because they haven't left
the baseline. I mean, I can put a 0 there and a 0 there. That means essentially the
these two dyes are insoluble in water.
So now I've marked up my chromatography paper. I can calculate the RF values. Now, to calculate
an RF failure, the easy equation, RF is equal to the residue distance, residue distance
divided by solvent front. So if I'm going to calculate the RF value for the residue
at 59 millimeters, that will equal 59 divided by my solvent front which is 64 millimeters.
And that gives me my value here, 59 divided by 64 of 0.92.
Now, there are no units on RF value. That's because you're dividing millimeters by millimeters.
And if you're looking at your significant figures, it's important that the same significant
figures are used as they are in the measurements. So I'll give my answer to two significant
figures. So what we've done today in this core practical is to prepare the chromatography
paper to run the chromatogram accurately and precisely, and then to calculate the RF values.