What causes the colours in ink to move across absorbent paper?
Answers
Answer:
The pigments are carried at different rates because they are not equally soluble. A pigment that is the most soluble will travel the greatest distance and a pigment that is less soluble will move a shorter distance .
Explanation:
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Answer:
Paper chromatography
Paper chromatography is used to separate mixtures of soluble substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.
Explanation:
Phases
Chromatography relies on two different '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 different substances with it
The different dissolved substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.
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 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 (have the same Rf value)
Results of a chromatogram of three pure substances and brown ink
Interpreting the chromatogram for a brown ink
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 are at the same heights (and have the same Rf value) as the reference inks.
Rf values
Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value is always the same for a particular substance.
The Rf value of a spot is calculated using:
