GCSE Chemistry Notes: Describing & explaining methods of chromatography
Mixture separation using CHROMATOGRAPHY analysis and interpretation of chromatograms
METHODS OF SEPARATING MIXTURES - using Paper chromatography, thin layer chromatography and gas chromatography
(Suitable for AQA, Edexcel and OCR GCSE chemistry students)
PARTS 2.3 and 2.6 Methods of separating mixtures are described e.g. paper chromatography, thin layer chromatography, gc gas chromatography, glc gas-liquid chromatography
Remember, in the physical separation processes of paper chromatography and gas chromatography, no chemical reaction changes are involved, so no new substances are made.
Alphabetical list of KEYWORDS for Parts 1-3: atom * balancing equations * centrifuges/centrifuging * chemical reaction/change * chromatography (paper/thin layer and theory) this page * compound * covalency * crystallisation * decanting/decantation * displayed formula * distillation (simple or fractional) * element * equations * evaporation * filtration * formula * gas chromatography * impure/pure * insoluble * ionic equations * ionic valence * iron-sulphur separation and heating experiment * magnet * mixture * molecule * naming compounds and ions * particle pictures of elements/compounds/mixtures * physical change * precipitation * products * pure substance * purification * reactants * sand/salt separation * separating funnel * separating mixtures * soluble/solution/solvent/solute * solvent extraction * symbols (for elements, formula, in equations) * state symbols * working out formulae *
2.3 Paper Chromatography or Thin Layer Chromatography
This method of separation is used to see what coloured materials make up e.g. a food dye analysis (e.g. smarties), separating the different coloured dyes in an ink e.g. felt tip pen inks.
Chromatography can be used to identify substances and check on the purity of a substance.
For example you could then investigate whether a pen ink is made up of one or more colours by paper chromatography.
The different food colourings in confectionary products e.g. in the icing top of a cake, the sugar coating on smarties etc. can all be separated and identified using paper chromatography.
The coloured material mixture to be separated e.g. a food dye (6 on the diagrams below) is dissolved in a solvent like ethanol ('alcohol') and carefully spotted onto chromatography paper or a thin layer of a white mineral material on a glass sheet (immobile or stationary phase).
The paper is carefully dipped into the solvent (mobile phase) and suspended so the start line (baseline) is above the liquid solvent, otherwise all the spots would dissolve in the solvent!
A DIAGRAM TO ILLUSTRATE the principles of PAPER CHROMATOGRAPHY
For best results:
For accurate work the distance moved by the solvent is marked on carefully with a pencil and the distances moved by each 'centre' of the coloured spots is also measured.
These can be compared with known substances BUT if so, the identical paper and solvent must be used (See Rf values below and diagrams above).
The Rf values is the ratio of how far the spot travels (D on diagram) relative to the distance moved by the solvent front (S on the diagram) and must have values of >0 and <1 to be of any use.
Each dye has its own reference values Rf for a particular set of experimental conditions
Chromatography Rf vales depend on the substance, the type of paper (stationary phase) AND the solvent (mobile phase).
To get consistent chromatography results you must use identical conditions, including temperature, and identical chemical reagents. These criteria is an example of 'fair testing' methodology.
THEORY of thin layer/paper chromatography e.g. for a series of dyes
The final result is the vertical separation of the spots up the paper which is now referred to as the chromatogram.
Any colour which horizontally matches another is likely to be the same molecule i.e. red (1 and 6), brown (3 and 6) and blue (4 and 6) match, showing these three coloured substances are all in the food dye (6), the dye that's being analysed by paper chromatography.
It is possible to analyse colourless mixture of chemicals if the 'spots' can be made coloured by some further chemical or light treatment e.g.
If a substance is pure, only one spot will appear on the chromatogram, impurities may show up as other faint spots.
Thin layer chromatography (t.l.c, in chemistry, is not 'tender loving care'!) is where a layer of paste is thinly and evenly spread on e.g. a glass plate. The paste consists of the solid immobile phase like aluminium oxide dispersed in a liquid such as water (thick paste and dried out) or silica gel. The mobile phase or solvent is just the same as paper chromatography e.g. ethanol, other alcohol etc.
Important note: If the starting spot moves up the paper and remains as a single spot, it means that substance must be pure and not a mixture. If it was a mixture, then at least two spots would be seen.
Gas-liquid chromatography is described further down the page
For more on the chromatography of amino acids see:
GCSE notes AMINO ACIDS and natural polymers
Advanced A Level Notes Amino acids - molecular structure, preparation and reactions
A plant material extraction process using chromatography
The experiment described below simulates one way in which drug companies extract plant material to develop products in the pharmaceutical industry.
1. Take some suitable plant material and crush it with a pestle and mortar.
2. Scrape the crushed plant material and gently heat with a small volume of suitable solvent like water or alcohol to dissolve and extract some of the soluble coloured plant material.
3. Filter off the residue to give a clear but coloured concentrated solution.
4. Take the solution and spread it along the start line of some chromatography paper (diagram A).
5. Dip and suspend the chromatography paper into a suitable solvent in a covered container so the start line is above the solvent surface and let the solvent be absorbed by the chromatography paper and move upwards (diagram B).
6. When the solvent front has reached near the top of the paper, stop (diagram C) and hang up the chromatogram to dry.
7. You can then separate the mixture 'physically' by cutting strips off for each band of coloured material that has separated out on the chromatogram (diagram D).
8. You can then extract each separated product by re-dissolving it by placing the strips in the solvent.
9. The different solutions can then be carefully evaporated to produce the separated coloured solid materials.
Note: In practice in the pharmaceutical industry large scale thin layer chromatography would be used.
Another example of how to interpret a chromatogram where paper chromatography has been used to analyse coloured materials.
Lets assume we have pure red, blue and yellow dye molecules.
These three rise to different heights on the chromatograms.
Three different Rf reference values (yellow > red > blue).
By matching the spots horizontally, you can deduce:
(i) The purple ink contains red and blue dye molecules.
(ii) The green ink contains blue and yellow molecules.
(iii) The brown ink contains red, blue and yellow molecules.
TOP OF PAGE and sub-index
2.6 GAS CHROMATOGRAPHY
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