Paper chromatography
Paper Chromatography is used to separate different substances dissolved in a liquid. is another method that can be used to test if a substance is A substance that consists of only one element or only one compound. or A substance, usually unwanted, that is present in another substance.. It can also be used to separate mixtures of Able to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution. substances in order to find out what substances are in the mixture. These are often coloured substances such as food colourings, inks, dyes or plant pigments.
Image caption, 1. Spots of ink or plant dye are placed on a pencil line
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Paper chromatography can be used to analyse colourless substances. A A substance used to detect another substance by making it visible. reacts with these substances to form coloured products, or products that glow under ultraviolet light. For example, iodine vapour reacts with fats and oils, turning them brown.
Phases
Chromatography relies on two different 鈥榩hases鈥:
- the Phase in chromatography that moves, usually a solvent or mixture of solvents. is the The liquid in which the solute dissolves to form a solution. that moves through the paper, carrying different substances with it
- the Phase in chromatography that does not move, for instance, the paper in chromatography. is contained on the paper and does not move through it
The different A substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.
Water is often used as a solvent but if the substances being tested do not dissolve in an A water-based solvent used to dissolve substances that are water soluble. then an alternative A non-water-based solvent used to dissolve substances that are not water soluble. must be used such as alcohol.
Interpreting a chromatogram
Separation by chromatography produces a The results of separating mixtures by chromatography.. A paper chromatogram can be used to distinguish between pure and impure substances:
- a pure substance produces one spot on the chromatogram
- an impure substance produces two or more spots
A paper chromatogram can also be used to identify substances by comparing them with known substances. Two substances are likely to be the same if:
- they produce the same number of spots, and these match in colour
- the spots travel the same distance up the paper compared to reference spots (have the same Rf value)
In this chromatogram, the brown ink is made of a mixture of the red, blue and yellow inks. This is because the spots in the brown ink have travelled the same distance (and have the same Rf value) as the reference inks.
Rf values
This distance a spot travels is partly dependent upon the time it is left in the solvent. This distance travelled compared with the distance travelled by the solvent is the same for a particular substance.
The Rf value of a spot is calculated using:
\(Rf = \frac{distance~travelled~by~substance}{distance~travelled~by~solvent}\)
The Rf value is always the same for a particular substance using the same stationary phase and mobile phase. Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances.
Rf values vary from 0 (the substance is not attracted to the mobile phase) to 1 (the substance is not attracted to the stationary phase).