Chromatography

Starter Match these key words with the correct definitions: NON-AQUEOUS SOLUTION SOLVENT SOLUTION AQUEOUS SOLUTION SOLUTE

Starter Match the key words with the correct definitions: SOLUTION a solute dissolved in a solvent SOLUTE a substance that is dissolved in a solvent. SOLVENT a liquid in which chemicals dissolve to make a solution (e.g. water, ethanol) AQUEOUS SOLUTION solution where a solute is dissolved in water NON-AQUEOUS SOLUTION solution where the solute is dissolved in a solvent that isn’t water (e.g ethanol)

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 ? Water soluble mixtures such as: Colourings agents Dyes/paints Inks

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. solvent front most soluble pigment partly soluble pigment origin Insoluble 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 pigment with weakest forces of adsorption Pigment with strong forces of adsorption Insoluble pigment

How does it work? chromatogram 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. Solvent front chromatogram The colours separate out because they dissolve in the solvent (soluble) Separated dyes Base line (pencil line)

Analysing chromatograms Start Finish How many colours in the red dye? Which colours is the purple dye made out of? 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?

Inks i, ii, iii and iv were made from colours A, B, C and D Try 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) solvent front most soluble pigment B partly soluble pigment A origin

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

The answer is never greater than 1. 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: Rf = distance moved by sample distance moved by solvent The answer is never greater than 1.

Calculate Rf for the three spots. Calculating Rf values Calculate Rf for the three spots. Substance Distance moved by sample (cm) Distance moved by solvent front (cm) Rf E120 7.2 10 0.72 E133 4.1 0.41 E124 9.5 0.95

Past Paper Questions

Past paper question

Mark scheme

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

Mark scheme

Challenge question 2

Mark scheme – check your answer