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Paper Chromatography Separation of Ink Pigments
Introduction
Many inks are actually mixtures made up of several basic pigments. Each pigment may
have a different color, a different structure, and different physical and
chemical properties. The pigments in an ink mixture may be separated using paper
Chromatography.
Concepts
Separation of a mixture
Adsorption
Mixture vs. pure substance
Chromatography
Background
The word Chromatography is derived from two Greek words meaning color (chroma)
and writing (graphein)—“color writing.” The term was coined by the Russian
chemist Michael Tswett in 1903 to describe a new technique he had invented to
separate the pigments in green plant leaves. Tswett found that in addition to
chlorophyll, the green main pigment, plant leaves also contained red and yellow
secondary pigments. The results were literally “written in color” when a plant
extract was passed through a column containing a clay-like absorbent solid.
One hundred years after Tswett’s discovery, Chromatography has evolved
into the most important tool chemists have for separating the components in a
mixture. Some examples of different types of Chromatography and their uses
include:
Gas Chromatography, which is used in forensics and toxicology to analyze
drugs and other substances in blood samples
Gel-permeation Chromatography, which is used to separate and purify
proteins
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Ion-exchange chromatography, which is used to remove ions from water
(water softening)
Paper Chromatography is an example of a more general type of Chromatography
called adsorption Chromatography. The paper acts as an adsorbent, a solid which
is capable of attracting and binding the components in a mixture (see Figure 1).
The mixture to be separated is “spotted” onto the surface of the paper and a
solvent is then allowed to seep or flow through the paper. If one of the
components in the mixture is more strongly adsorbed onto the paper than
another, it will spend a smaller fraction of time free in solution and will move
up the paper more slowly than the solvent. Components that are not strongly
adsorbed onto the paper will spend a greater fraction of time free in solution
and will move up the paper at a faster rate. This “partitioning” of the
components of a mixture between the paper and the solvent separates the
components and gives rise to different bands. If the components of the
mixture are colored, like the pigments in an ink, the bands are easy to see.
Experiment Overview
The purpose of this experiment is to determine if an ink sample is a pure
substance or a mixture. The pigments in different inks will be separated using paper
chromatography.
Pre-Lab Questions
1.
Read the Background section and the Procedure—why is a pencil
rather than a pen used to mark the “starting line” in Step 2?
2.
The solvent in this experiment will flow upward through the paper,
against gravity, via capillary action. What is capillary action and what causes
it?
3.
There are basically two types of pens—water-soluble versus
“permanent markers.” What type of pen will probably give better separation in this
paper chromatography experiment with water as the solvent? Explain.
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4.
Think back to your “coloring book” days as a child. What combination
of colors can be used to make black? Predict the color pigments that may be present
in black ink.
Materials
Beaker, 250-mL
Pens or markers, various colors
Chromatography paper
Scissors
Colored pencils (optional)
Stapler
Distilled or deionized water
UV lamp or black light (optional)
Metric ruler
Watch glass, large, or aluminum foil
Pencil
Safety Precautions
Although the materials in this activity are considered nonhazardous, please
observe all normal laboratory safety guidelines. Wash hands thoroughly with soap
and water before leaving the laboratory.
Procedure
1.
Cut a large rectangle of chromatography paper, about 17 cm wide x 8 cm
high.
2.
Using a pencil, lightly draw a line across the width of the paper, about
1 cm from the bottom (Figure 2). Mark three locations on the line with a small dot
or “X”. Space the marks evenly, starting about 4 cm from either side, as
shown in Figure 2.
3.
Using a separate pen or marker for each spot, place a small dot of ink
(about 1 mm wide) in the center of each “X”.
4.
On the data sheet, record the name and color of the pen or marker
used at each location on the paper.
5.
With the pencil line on the outside, roll the chromatography paper into a loose
cylinder and carefully staple the side edges together in two places (Figure 3).
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6.
Add about 10 mL of distilled or deionized water to a 250-mL beaker. Check the
water level in the beaker: Place the paper cylinder alongside the beaker (not in it at this
time). The water level in the beaker must be below the line marked on the paper.
Adjust the amount of water in the beaker as needed.
7.
Carefully place the chromatography paper into the beaker. The ink spots should
be above the water level in the beaker (Figure 4).
8.
Gently cover the beaker with a large watch glass or a sheet of
aluminum foil. (Do not jostle the beaker.) The water will be drawn upward
through the chromatography paper by capillary action. The pigments in the ink
will be carried up the paper at different rates, depending on their properties.
9.
When the solvent “front” is about 1 cm from the top of the
chromatography paper, carefully remove the cylinder from the beaker. Mark the
solvent line with a pencil.
10.
Air dry the chromatography paper for a few minutes, then gently
remove the staples and place the paper on a paper towel to dry.
11.
Using a pencil, outline the pigment bands. (The colors may fade over
time.)
12.
On the data sheet, draw a representation of each chromatogram,
noting the location and color of each pigment band and the location of the solvent
front.
13.
Attach the chromatogram to the lab report.
Post-Lab Questions (Use a separate sheet of paper to answer the
following questions.)
1.
Do all black inks contain the same pigments? Describe the composition
of black inks that appear to be made up of more than one pigment.
2.
Do any of the inks appear to be a single pigment? How could different
solvents be used to determine if these inks really contain only one pigment?