Part I: ARE COLORS FOUND IN NATURE PURE OR A COMBINATION OF COLORS?
The world is very exciting because of its colors. Nature abounds with
color!! Everybody enjoys the striking colors of flowers when they walk
through a garden. Is that ripe tomato a pure red or is it a combination
of several colors? What about the carrot? Is its color pure or does
nature deceive us by mixing colors to make it look orange? This part of the
experiment will investigate a method for extracting color from fruits. You
will then try to separate the color by a process called column chromatography.
PROCEDURE
PART A. EXTRACTION OF THE
COLOR
Mass approximately 2 grams of tomato paste into a 50 mL beaker.
The exact mass is not important. Measure 5 mL of ethanol in a 10 mL
graduate cylinder. Pour the ethanol over your tomato paste.
Stir the ethanol-paste mixture for 1 minute. (The ethanol
is dissolving the water, salts and oils in the paste.) If the paste
dissolves all of the ethanol, add three more mL.
Decant the ethanol into a another beaker. Remove the last few
drops with a medicine dropper. Throw this ethanol away, in the location
specified by your teacher. Measure 5 mL of dichloromethane in your
graduated cylinder. Pour over the tomato paste. Stir the mixture for 1
minute. (The dichloromethane is dissolving the color from the tomato.)
Decant the dichloromethane into a clean 250 mL beaker. Remove
the last drops with a medicine dropper. Place the beaker in the hood
and allow the dichloromethane to evaporate to a volume of about 1 mL.
This is to concentrate the color. Do not try to speed the process by
using flames, water baths or electric burners.
PART B. PREPARATION OF THE COLUMN FOR CHROMATOGRAPHY
Obtain a Pasture pipet, a small piece of cotton, and a small sample of
sand. Use a dry cylinder and measure out 5 mL of alumina powder. Push the
cotton into the bottom of the pipet. Use a wire to help you. Pour sand
over the cotton to a height of 5 mm.
See a drawing of the Pasteur pipet
column
Obtain a slip of paper. Pour the alumina on the paper. Role the
paper and pour the alumina into the pipet until it reaches the indentation
near the top of the pipet.. (The alumina packing the column is called the
stationary phase. It will hold the plant color in the column).
Hold the pipet in one hand and gently tap the column with a finger of your
other hand. Make sure the alumina is evenly packed in the column. There
should be no air spaces! Stand your column in a test tube in the rack for
safe keeping.
Measure 20 mL of hexane into a cylinder. Pour into a beaker.
(This liquid is called the mobile phase. It will carry the plant color
through the column.) Clean and dry 2 medicine droppers. Obtain your color
extract from the hood. If all of the dichloromethane has evaporated, add 1
mL more, and stir to redissolve all of the color.
Leave your chromatography column in the test tube for easy handling.
Add 5 drops of hexane to the column. Add 5 drops of the color extract to the
column.
Fill the second medicine dropper with hexane. Just as the color
extract has dissolved below the alumina surface start to add drops of hexane.
Fill the chromatography column almost to the top with hexane. Keep this area full
of hexane.
Slowly lift the column out of the test tube after adding the hexane
to top for the first time. Note the movement of the color through the column.
Does it remain uniform? Note the time when the hexane first pours through
the bottom of the column. Keep the hexane on the column top full for two
more minutes.
After two minutes, stop adding hexane and let the column drain in
the test tube. Continue to make observation on the movement of the color.
Once the column has drained, make a diagram of the the results. Use colored
pencils to illustrate.
Note: This experiment can be repeated with other materials such as baby food
carrots, sweet potatoes, spinach, etc. You can also take fresh flower petals
and extract the color. To do this, you would tear the petals and place them
in a pestle. Cover with dichloromethane and grind with the pestle.
RESULTS and DISCUSSION
Now that you have completed the experiment, think about the process
and explain the role of each in the chromatography separation; dichloromethane
as the solvent; alumina as the stationary phase; hexane as the mobile phase
Explain your observations. What conclusion can you make about the
color of a red tomato? Are the colors in the different substances you tested
the same?
Adapted from an experiment developed by
ROBERT HOULE; BACON ACADEMY; NORWICH AVE.; COLCHESTER,CT 06485
Gwen Sibert
Roanoke Valley Governor's School
gsibert@rvgs.k12.va.us