Paper Chromatography
• This technique is used to separate substances that have di erent solubilities in a given
solvent (e.g. di erent coloured inks that have been mixed to make black ink).
• It is also used to identify unknown substances.
Chromatography relies on two di erent ‘phases’:
• the stationary phase, which in paper chromatography is very uniform, absorbent paper
• the mobile phase is the solvent that moves through the paper, carrying di erent
substances with it
Each component in the mixture will have a di erent attraction for the mobile phase and the
stationary phase:
- a substance with stronger forces of attraction between itself and the mobile phase will be
carried a greater distance in a given time
- A substance with a stronger force of attraction to the stationary phase will not travel as far in
the same time.
In chromatography, the mobile phase is the solvent chosen and the stationary phase is the paper.
Given an unknown organic solution, chromatography can usually identify a single compound or a
mixture.
- If the unknown sample is a mixture of compounds, there will be probably more than one spot
formed on the chromatogram.
- If the unknown sample is a pure substance, there will be a single spot formed on the
chromatogram.
Interpreting a chromatogram
Separation by chromatography produces a chromatogram. 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, or mixture, produces two or more spots
Chromatography can also be done when the di erent substances in the mixture are colourless.
The chromatogram can be exposed to a locating agent, which reacts with the invisible chemicals
so that they can be seen.
Identifying unknown substances using chromatography.
Di erent chromatograms and the separated components of the mixtures can be identi ed by
calculating the retardation factor (Rf). The Rf value is worked out by using this equation:
ff
ff ff ff ff ff ff fi
• This technique is used to separate substances that have di erent solubilities in a given
solvent (e.g. di erent coloured inks that have been mixed to make black ink).
• It is also used to identify unknown substances.
Chromatography relies on two di erent ‘phases’:
• the stationary phase, which in paper chromatography is very uniform, absorbent paper
• the mobile phase is the solvent that moves through the paper, carrying di erent
substances with it
Each component in the mixture will have a di erent attraction for the mobile phase and the
stationary phase:
- a substance with stronger forces of attraction between itself and the mobile phase will be
carried a greater distance in a given time
- A substance with a stronger force of attraction to the stationary phase will not travel as far in
the same time.
In chromatography, the mobile phase is the solvent chosen and the stationary phase is the paper.
Given an unknown organic solution, chromatography can usually identify a single compound or a
mixture.
- If the unknown sample is a mixture of compounds, there will be probably more than one spot
formed on the chromatogram.
- If the unknown sample is a pure substance, there will be a single spot formed on the
chromatogram.
Interpreting a chromatogram
Separation by chromatography produces a chromatogram. 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, or mixture, produces two or more spots
Chromatography can also be done when the di erent substances in the mixture are colourless.
The chromatogram can be exposed to a locating agent, which reacts with the invisible chemicals
so that they can be seen.
Identifying unknown substances using chromatography.
Di erent chromatograms and the separated components of the mixtures can be identi ed by
calculating the retardation factor (Rf). The Rf value is worked out by using this equation:
ff
ff ff ff ff ff ff fi
