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This week's LabZone activity
May 11, 2005
Paper Chromatography Analysis
Most commercial inks are mixtures of dyes and pigments. The vehicle for most of them is water. For these reasons, commercial inks can be analyzed by one of the modern chemist's most useful procedures, namely paper chromatography.
Chromatography means "writing with color," and it separates mixtures of colored chemicals into their separate components.
Materials and Equipment
- coffee filter paper (or white paper toweling)
- scissors
- pencil
- ruler
- tall jars (such as olive jars or pickle jars)
- cellophane tape
- samples of different inks
- water
Procedure
Cut strips of filter paper a little bit longer than the depth of the jar. They should be about an inch and a half wide. You will be hanging them over some tape stretched across the top of the jar in such a way that the bottom of the strip almost touches the bottom of the jar.
Draw a horizontal pencil line one inch from the bottom of the strip. Make a quarter-inch spot of ink on the line. Touch the ink samples to the paper on the line and the ink will spread in a circle. Allow it to dry and touch it again. You want to get an intense but small spot of ink on the filter paper. Allow it to dry thoroughly. (If you wish, you can use a hair dryer to blow it dry.) Be sure and label the top of the strip with the type of ink sample.
Put water in the bottom of the jar to a depth of about a half inch. Hang each strip over some tape stretched across the top of the jar. The bottom of the strip should rest freely in the water, but the spot should be above the water level. The filter paper acts like a wick and the water line will move up the paper.
Observations and Suggestions
The water moves up the paper, against gravity, by capillary action. This is the same principle that makes the water level in a straw higher than the water level outside the straw. Water is both attracted to the paper and to itself, thus the filter paper acts as a wick drawing water up its surface. Water soluble dyes will be carried along the surface of the filter paper by the moving water. Different dyes, however, will travel at different rates with the moving water. Thus they will be separated into separate bands.
If an ink spot does not move, you can conclude that the coloring agent is an insoluble pigment. Ball-point-pen ink is only moderately soluble in water. It is soluble in nail-polish remover. The coloring matter is very highly concentrated metallic dyes that will not move well in paper chromatography with a water solvent as the more soluble dyes found in felt-tip pens, markers, and fountain-pen inks.
Reprinted with permission from The Secret Life of School Supplies by Vicki Cobb. Illustrated by Bill Morrison. Published by J.B. Lippincott, New York. Text copyright © 1981 by Vicki Cobb ( www.vickicobb.com ).
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