Chromatography

Match these key words with the correct definitions:

Starter

SOLVENT

SOLUTEAQUEOUS SOLUTION

SOLUTION

NON-AQUEOUS SOLUTION

Match the key words with the correct definitions:

Starter

SOLVENT

SOLUTE

AQUEOUS SOLUTION

SOLUTION

NON-AQUEOUS SOLUTION

solution where a solute is dissolved in water

solution where the solute is dissolved in a solvent that isn’t water (e.g ethanol)

a liquid in which chemicals dissolve to make a solution (e.g. water, ethanol)

a substance that is dissolved in a solvent.

a solute dissolved in a solvent

Learning Objectives

• Understand when chromatography is used

• Describe the method of chromatography

• Know how to carry out paper

chromatography

• Analyse chromatograms

• Calculate Rf values

When is chromatography used?

Paper chromatography is a technique for separating components of a mixture.

Which kind of mixtures ?

Inks Colourings

agents Dyes/paints

Water soluble mixtures such as:

Simple Chromatography

• Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase

• The mobile phase moves, it can be a liquid or a gas.

• The stationary phase doesn’t move. It can be a solid or a suspended liquid.

How it works

• Some substances are more soluble than others.

• Substances that don’t dissolve, don’t move.

• Substances that dissolve the most move the farthest. origin

solvent front

Insoluble pigment

most soluble pigment

partly soluble pigment

How it works

• With a solid stationary phase, the pigments are adsorbed onto the surface of the solid particles or fibres.

• The relative forces of attraction between the stationary phase, the mobile phase and the pigment mean that separation happens Insoluble

pigment

pigment with weakest forces of adsorption

Pigment with strong forces of adsorption

How does it work?

• The water is drawn up by capillary action (sticky attractive force between water molecules and the paper).

• When the solvent reaches the ink spots they are dissolved and carried with the water as it moves up the paper.

• The most soluble ink travels the fastest, whilst the less water-soluble inks are left near the bottom of the paper.

chromatogramSolvent

front

Separated dyes

Base line (pencil line)

Start Finish

1) How many colours in the red dye?2) Which colours is the purple dye made

out of?3) Which dyes are made using yellow

colour?

Analysing chromatograms

1) How many dyes were used in Black?2) Are the yellows in brown and orange the same? How do you know?

Analysing chromatograms

Inks i, ii, iii and iv were made from colours A, B, C and DTry to work out which colours were mixed for each of the inks (i-iv)

Rf values

• The ratio between the distance travelled by the pigment and the distance travelled by the solvent front is a constant, Rf.

• (provided temperature, mobile phase and stationary phase are the same)

origin

solvent front

most soluble pigment B

partly soluble pigment A

Rf Values (retention factor)

• Distance travelled by component

• Distance travelled by mobile phase

• Specific to each plate• Comparable• Each component on

plate has unique Rf value

Calculating Rf values

• A chemical can also be identified by its Rf

(Retention Factor)• It is the ratio of the distance travelled by the

sample (spots) to the distance travelled by the solvent (water).

• The formula is:

The answer is never greater

than 1.

Calculate Rf for the three spots.

Substance

Distance moved by

sample (cm)

Distance moved by solvent

front (cm)

Rf

E120 7.2 10 0.72E133 4.1 10 0.41E124 9.5 10 0.95

Calculating Rf values

Past Paper Questions

Past paper question

Mark scheme

Challenge question - 1This question relates to the chromatogram shown in the earlier question. Refer back…

Mark scheme

Challenge question 2

Mark scheme – check your answer