PAUL ROBERTS: Hi, I'm Paul Roberts. I'm a chemistry teacher at Andover College, and
I've been teaching chemistry for eight years. 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 that 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. 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 2 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-- 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 just practice a 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're very, very small dots
in chromatography.
Now, we're going to suspend this over the beaker. And so I'm just going to attach 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. So 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.
Now, 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 2 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 in 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've 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 another chromatogram I prepared earlier, and this has dried out completely.
And I'm going to remove the 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 this solvent front. Now, the distance
of the solvent front from the baseline, 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 a 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've not just drawn 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 of distance is 59 and the
blue residue is again 62. The blue dye has traveled an identical distance on both samples.
That 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 can put to 0 there and a 0 there. That means
essentially, 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 value, we use the 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 a value
here-- 59 divided by 64-- of 0.92. Now, there are no units on an 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 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 Rf values.