3/12/24 Paper Chromatography
• We think that inks, for example black ink, is just one color. In
fact, black ink is a mixture of different colored inks. These inks
can be separated using a technique called paper
chromatography:
(a) A special kind of paper, like filter paper is used.
(b) A small drop of ink is placed on the paper. The paper is
placed in a beaker with a low volume of water.
(c) The paper soaks up the water. As the water moves up, the
different-colored inks separate out. The resulting image is
called a chromatogram.
(d) The colored inks separate because the water dissolves
them. As the water moves up the paper, it carries the ink
particles with it.
(e) Different kinds of inks are carried different distances
before they are left behind. This is because the inks have
different levels of solubility. The more soluble the ink, the
further its particles are carried.
• Some ink is not soluble in water, such as the ink in permanent
marker pens. To separate out the colours in these inks, you
would need to use a different solvent, such as alcohol.
• When comparing different substances, scientists may use a
large piece of chromatography paper and place many spots of
ink alongside each other. The paper must be placed in a large
chromatography tank.
• Chromatographs can be used to study the dyes used in food.
Some dyes contain only one substance: they are a pure
substance. Other dyes are mixtures. It is important to know
exactly what is being used when our food is processed - we
need to know if any substances could be a health risk.
• Public health scientists may also use chromatography to
check that the colourings being used in products such as hair
dye or the ink in pens are not harmful. To do this, they
compare chromatograms from the food, hair dye or ink and
those of the colorings that are permitted.
