Paper Chromatography

Method for chromatography of single food colouring.

1. Draw a pencil line about 2cm from the bottom of the chromatography paper. Pencil will not mix with the dyes.

2. Use a capillary tube to place a spot of food coloring on the line

3. Pour about 1cm of water/oil into a beaker

4. Attach paper to glass tube with tape and hang from beaker. Make sure the paper is dipped into the solvent but solvent line is below the pencil line.

5. Place a lid on the beaker to prevent evaporation of water

6. Remove the paper when the solvent reaches about 3/4 up the paper and mark solvent front by drawing a straight line

7. Hang up to dry

Why do we use a capillary tube?

To place small dots so dyes do not mix with eachother

How to find the Rf value?

Rf= distance travelled by substance/distance travelled by solvent

Mobile phase

where molecules can move

Stationary phase

where molecules can’t move

What happens to a substance which is more soluble in the solvent?

1. will spend more time in the mobile phase

2. move further up the paper

What happens to a substance which is more attracted to the paper?

1. will spend more time in the stationary phase

2. stays lower down the paper

How does the number of spots change with different solvents?

distribution of chemicals will change

How to calculate Rf value?

distance travelled by substance/ distance travelled by solvent

How to know if the reference value is the same as the compound value?

Test in different solvents and see if Rf values are the same

How are different solvents at different positions on the chromatography paper?

solvent moves through paper

different dyes have different solubilities in solvent

and different attractions for the paper

and so are carried different distances

What is gas chromatography used for?

separate complex mixtures

What is the mobile phase in gas chromatography?

inert gas

What is the stationary phase in gas chromatography?

inert liquid