WM - Thin layer chromatography

Question 1

What is chromatography used for?

Answer 1

To separate mixtures of molecules.

Question 2

What do all types of chrmoatography involve?

Answer 2

A mobile phase (a liquid or gas) that moves over a second material called to stationary phase (which doesn’t move).

Question 3

What is thin layer chromatography?

Answer 3

A simple way of separating mixtures.

Question 4

In TLC, what is the stationary phase?

Answer 4

A thin layer of silica (silicon dioxide) or alumina (aluminium oxide) fixed to a glass or metal plate.

Question 5

Explain the process of TLC

Answer 5

1) Draw a line in pencil near the bottom of the TLC plate (the baseline) and put a small drop or each mixture to be separated on the line.
2) Place the plate in a beaker with a small volume of solvent (this is the mobile phase). The solvent level must be below the baseline.
3) Leave the beaker until the solvent has moved almost to the top of the plate. Then remove the plate from the beaker and allow it to dry, Before it’s evaporated you should mark how far the solvent travelled up the plate (this line is called the solvent front).
4) As it moves up the plate, the solvent will carry the substances in the mixture with it - but some chemicals will be carried faster than other and so travel further up the plate. The result is called a chromatogram.
5) You can use the positions of the chemicals on the chromatogram to identify what the chemicals are.

Question 6

What is the mobile phase in TLC?

Answer 6

Solvent.

Question 7

What is the solvent front?

Answer 7

How far the solvent travelled up the plate in TLC.

Question 8

What is a chromatogram?

Answer 8

The finished plate in TLC where the mixtures have separated and the different chemicals have moved different distances.

Question 9

What can you use the positions of the chemicals on the chromatogram for?

Answer 9

To identify what the chemicals are.

Question 10

How are colourless chemicals revealed in TLC?

Answer 10

Using UV light or iodine.

Question 11

Why might UV light or iodine be used in TLC?

Answer 11

To reveal colourless chemicals.

Question 12

What would you see on the TLC plate if the chemicals in the mixture are coloured (such as the dyes that make up an ink)?

Answer 12

You’ll see them as a set of coloured dots at different heights in the TLC plate.

Question 13

What would you see on the TLC plate if the chemicals in the mixture are colourless?

Answer 13

No spots - you need to make them visible.

Question 14

What two ways can you make colourless chemicals visible on a TLC plate?

Answer 14

By using UV light or iodine.

Question 15

How can UV light be used to make colourless chemicals visible on a TLC plate?

Answer 15

Many TLC plates have a special fluorescent dye added to the silica or the alumina layer that glows when IV light shines on it. Where there are spots of chemical on the plate, they cover the fluorescent dye and don’t glow. You can put the plate under a UV lamp and draw around the dark patches to show where the spots of chemical are.

Question 16

How can iodine be used to make colourless chemicals visible on a TLC plate?

Answer 16

Another way of showing the position of the spots is to expose the to iodine vapour (leaving the plate in a sealed jar with a couple of iodine crystals does the trick). Iodine vapour is a locating agent - it sticks to the chemicals on the late and they’ll show up as purple spots.

Question 17

Other than to separate and identify molecules, how else can chromatography be used?

Answer 17

To purify substances.

Question 18

Does TLC involve small or large quantities off chemicals?

Answer 18

Very small quantities.

Question 19

What is TLC ideal for and why?

Answer 19

Identifying what makes up a mixture because it separates very small quantities of chemicals.

Question 20

How can chromatography be used to separate large quantities of a mixture in an organic synthesis?

Answer 20

You need larger-scale equipment, such as a glass column (e.g. a burette) packed with silica or alumina (the same stationary phase that is used in TLC). You then pour your mixture into the column and run solvent (the mobile phase) through it continually.

The different chemicals in the mixture move down the column at different rates, so they come out at different times, meaning you get pure chemicals.

Question 21

How can chromatography be used to purify substances?

Answer 21

You need larger-scale equipment, such as a glass column (e.g. a burette) packed with silica or alumina (the same stationary phase that is used in TLC). You then pour your mixture into the column and run solvent (the mobile phase) through it continually.

The different chemicals in the mixture move down the column at different rates, so they come out at different times, meaning you get pure chemicals.

Question 22

As well as to separate mixtures of chemicals, what else can chromatography be used to separate?

Answer 22

The desired product from unreacted chemicals or side products.

Question 23

What can the position of the spots on a plate help you do?

Answer 23

Identify substances.

Question 24

How can you identify substances from the position of the spots on the plate?

Answer 24

You calculate an Rf value.

The formula is:
Rf value = distance travelled by spot/distance travelled by solvent.

Rf values are always the same no matter how big the plate is or how far the solvent travels - they’re properties of the chemicals in the mixture and so can be used to identify those chemicals - But if the composition of the TLC plate, the solvent, or the temperature change even slightly, you’ll get different Rf values.

It’s hard to keep the conditions identical, so, if you suspect that a mixture contains a certain chemical then it’s best to put a spot of that chemical on the baseline nest to the mixture and run them both at the same time.

Question 25

If you just want to know how many chemicals are present in a mixture what do you have to do?

Answer 25

Count the number of spots that form on the plate.

Question 26

What is the formula for Rf value?

Answer 26

Rf value = distance travelled by spot/distance travelled by solvent.

Question 27

When are Rf values the same and when are they different?

Answer 27

Rf values are always the same no matter how big the plate is or how far the solvent travels - they’re properties of the chemicals in the mixture and so can be used to identify those chemicals.

But if the composition of the TLC plate, the solvent, or the temperature change even slightly, you’ll get different Rf values.

Question 28

What should you do if you suspect a mixture contains a certain chemical?

Answer 28

Then it’s best to put a spot of that chemical on the baseline nest to the mixture and run them both at the same time - as it’s hard to keep the conditions identical and this will ensure that the Rf values come out the same if the chemical is present.