GCSE OCR Gateway Chemistry C2.1g–k Chromatography

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 
In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification. 

I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least. 

That written in italics is for the higher tier paper only.

C2 Elements, Mixtures and Compounds

C2.1g describe the techniques of paper and thin layer chromatography

I guess we all did paper chromatography in pre–school or kindergarten if not in primary school.

You had a jam jar with a coffee filter laid flat on top of it.

You put a spot of black ink in the centre of your filter and dropped water onto it the spot.

You watched the water spread out from the centre and take the different colours in the black ink with it.

You saw that black ink was a mixture of coloured inks.

i) Paper Chromatography.

• uses a special chromatography paper to separate compounds.

• works for coloured inks e.g. black ink and mixtures of amino acids.

• uses water or mixture of solvents to separate the mixture.

• diagram above shows the set up.

How to set up paper chromatography of black ink

1.   take a strip of chromatography paper that fits in your beaker.

2.   draw a pencil line about 2cm from the bottom of the strip. This is the baseline or starting point.

3.   leave a very small spot of black ink on the base line.

4.   hang the strip of paper in your beaker with the end of the paper just in the water.

5.   cover the top of the beaker with a watchglass.

6.   wait for the water to rise to near the top of the paper by capilliary action.

7.   Mark the height that the water rose (the solvent front)

Your chromatogram might look like this:

ii) Thin Layer Chromatography or TLC for short.

TLC uses a special powder coating (kieselguhr or silicon dioxide SiO₂) on a glass or thick foil plate.  These days the TLC plate, as it’s called, comes pre-prepared but some of us remember when we had to coat our own glass plates back in the day !!

Now the TLC plate though more fragile than paper is used in the same way as paper in paper chromatography.

You mark a base line starting point in pencil.

You add a very small spot of mixture say amino acids or black ink.

You stand the plate in water or a solvent usually in a gas jar and seal the jar with a cover slip. (You can use a tall jam jar or a specially made TLC glass tank)

You wait for the solvent to rise almost to the top of the plate then take it out of the solvent.

You mark the solvent front and calculate the Rf values for your coloured components of the black ink.  (see below for how to calculate an Rf value)

C2.1h recall that chromatography involves a stationary and a mobile phase and that separation depends on the distribution between the phases

How do paper and thin layer chromatography work?

The paper or the powder on the TLC plate is the stationary phase i.e. it does not move.

The solvent (e.g. water) is the mobile phase because it rises up the plate or paper by capillary action. 

If a part of the black ink is more soluble in the solvent (mobile phase) it will travel further up the paper or plate than a different part of the black ink.

This way the parts of the black ink get separated on the paper or plate.

C2.1i interpret chromatograms, including measuring Rf values and recall and use of the formula for finding Rf value.

Rf value is the Retention factor

It is measured on the chromatogram from paper or thin layer chromatography.

How measure the Rf value of a component on a chromatogram:

• measure the distance from where the mixture was placed to the middle of the component spot—call that B

• measure the distance from where the mixture was placed to the solvent front—call that A

The Rf value of that component is the fraction B/A expressed as a decimal.

See the diagram below of the paper chromatography of black ink:

C2.1j suggest suitable purification techniques given information about the substances involved

Mixture	Purification Technique	Example
Insoluble solid in water	Filtration	Muddy water
Two liquids with different boiling points (about 25oC apart)	Simple distillation	Alcohol and water
Several liquids with very different boiling points	Fractional distillation	Petroleum or crude oil
Solid impurities in an aqueous solution	Simple distillation	Salt water/ sea water
Salt crystals from an aqueous solution	Crystallisation	Copper sulphate solution

C2.1k suggest chromatographic methods for distinguishing pure from impure e.g. paper, thin layer (TLC) and gas chromatography

Use paper chromatography to identify mixtures of dyes in an unknown ink.

Use TLC to identify mixtures of amino acids from an unknown protein

Use gas chromatography to identify mixtures of metabolites from urine in a drug test sample.