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 ColorResults and DiscussionMass 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 about 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.Part C. Performing the Column Chromatography
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.
See schematic of a simple liquid chromatographic separation setup.
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.
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 phaseExplain 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?
