Author: Maddie Van Beek
Chromatography is the separation of a material into its individual components. Sometimes a mixture appears homogenous, but is actually made up of several different substances. Chromatography allows us to separate those substances. Why would we want to separate components of a material in the first place? Chromatography allows us to view the individual parts of a mixture, thus making it easier to analyze. For example, investigators sometimes use chromatography to identify unknown substances at crimes scenes. Another example of separating a mixture using chromatography would be separating common table salt, NaCl, into Sodium (Na) and Chlorine (Cl). The separation is completed by passing the mixture through a solution using a medium.
How is this done?
Chromatography includes a stationary phase and a mobile phase. The stationary phase is the component that DOES NOT move, and the mobile phase is the component that DOES move. There are four different types of chromatography, but today, we will be focusing on paper chromatography. In paper chromatography, the stationary phase is a strip of paper, and the mobile phase is a solvent. The solvent (mobile) travels through the paper (stationary) using capillary action.
Capillary Action is what causes the water to “climb” up your paper. Because of cohesion within liquid molecules, and adhesion between the liquid and the solid, capillary action allows liquids to move against the forces of gravity.
The spread of the water helps separate the different components that make up the mixture. Each component may have slightly different properties, which is what allows them to flow at different rates or travel varying distances from the original starting point.
How is this done?
Chromatography includes a stationary phase and a mobile phase. The stationary phase is the component that DOES NOT move, and the mobile phase is the component that DOES move. There are four different types of chromatography, but today, we will be focusing on paper chromatography. In paper chromatography, the stationary phase is a strip of paper, and the mobile phase is a solvent. The solvent (mobile) travels through the paper (stationary) using capillary action.
Capillary Action is what causes the water to “climb” up your paper. Because of cohesion within liquid molecules, and adhesion between the liquid and the solid, capillary action allows liquids to move against the forces of gravity.
The spread of the water helps separate the different components that make up the mixture. Each component may have slightly different properties, which is what allows them to flow at different rates or travel varying distances from the original starting point.
Rf# = D#/DT
D# = Distance one of the components traveled from its starting point.
DT = Total distance traveled by the solvent.
The important thing to remember when measuring is to measure from the starting line to the MIDDLE of the ending point.
D# = Distance one of the components traveled from its starting point.
DT = Total distance traveled by the solvent.
The important thing to remember when measuring is to measure from the starting line to the MIDDLE of the ending point.
As the solvent moves through the paper, the components of the mixture are separated, because they travel at different rates. For example, some components are more soluble, so they travel more easily with the solvent. Other components are more absorbable, so they do not travel as far. Eventually, all components cease moving and are absorbed by the paper. When the components are absorbed, they have separated from one another and can be clearly identified.
At this point, you can calculate the retention factor (Rf) of each component in the mixture.
At this point, you can calculate the retention factor (Rf) of each component in the mixture.
For example, in the diagram below, the component #1 traveled 2cm. The total distance that the solvent traveled was 8 cm. Rf1 = 2cm/8cm. Rf1 = 0.25
Try using chromatography to separate the different colors in a black marker!
YOU WILL NEED:
Paper towels
Water soluble black markers--Ex. Crayola. Bring three different brands to compare.
Scissors
A clear cup
Water
Pencil
Tape
YOU WILL DO:
1. Cut a strip of paper towel about two inches wide and as tall as the cup you are using.
YOU WILL NEED:
Paper towels
Water soluble black markers--Ex. Crayola. Bring three different brands to compare.
Scissors
A clear cup
Water
Pencil
Tape
YOU WILL DO:
1. Cut a strip of paper towel about two inches wide and as tall as the cup you are using.
2. Wrap one end of the paper towel around the pencil and tape it in place. When you lift the pencil horizontally, the paper towel hangs from it.
3. Fill the clear cup with about one inch of water.
4. Draw a line with one of the black water-soluble markers about two inches from the bottom of your strip of paper towel.
4. Draw a line with one of the black water-soluble markers about two inches from the bottom of your strip of paper towel.
5. Set the pencil across the mouth of the cup so the paper towel is now dangling inside the cup
6. MAKE SURE the paper towel reaches the water, but that the black line is NOT submerged.
7. Watch the water climb up the paper towel--what happens?
8. Write down your observations as the colors separate.
9. When the solvent reaches the top and the colors are fully absorbed, measure the Rf.
Your results might look something like this:
7. Watch the water climb up the paper towel--what happens?
8. Write down your observations as the colors separate.
9. When the solvent reaches the top and the colors are fully absorbed, measure the Rf.
Your results might look something like this:
Try using different colors and compare the results!
Extension: Mystery Marker!
Have a friend select one of the three black markers and draw a line on a strip of paper towel, just as you did before. Now, it’s your job to solve which marker your friend used! First, go through the chromatography process (above) with the mystery mark.
Next, go through the same process with all three markers. Compare the Rf and range of colors to identify the mystery mark!
Fall Chromatography
You can even use chromatography to predict the colors that leaves will change in the fall! Just like black markers have more than one color, leaves have more than one pigment. Use chromatography to separate the pigments and find their “true” colors.
Let’s try it!
YOU WILL NEED:
1. Green leaves (make sure they haven’t changed colors and aren’t too dry)
2. Rubbing Alcohol
3. Plastic Wrap
4. Paper towels
5. Pencils
6. Tape
7. Scissors
8. Clear Cup
9. Tap Water
10. Shallow pan (a brownie pan would work well)
YOU WILL DO:
1. Tear the green leaf into tiny pieces and drop the pieces into the clear cup.
2. Pour enough rubbing alcohol into the cup to cover the leaf pieces.
3. Cover the top of the clear cup with plastic wrap.
4. Fill the shallow pan with hot tap water.
5. Place the covered clear cup in the pan full of hot tap water.
6. Wait for the alcohol to absorb the leaf pigments. It may take about 30 minutes for the alcohol to turn green.
7. Just as you did in the marker experiment, cut a strip of paper towel the same height as the clear cup and tape the end of the paper towel to the pencil.
8. Remove the plastic wrap from the cup and place the pencil over the mouth of the cup so that the paper towel is dangling inside the glass. Make sure the end of the paper towel is barely touching the green alcohol.
9. Check back periodically for about 90 minutes and write down your observations.
Repeat the experiment using leaves from different trees so you can compare the pigments, just as you compared different brands and colors of markers.
Extension: Mystery Marker!
Have a friend select one of the three black markers and draw a line on a strip of paper towel, just as you did before. Now, it’s your job to solve which marker your friend used! First, go through the chromatography process (above) with the mystery mark.
Next, go through the same process with all three markers. Compare the Rf and range of colors to identify the mystery mark!
Fall Chromatography
You can even use chromatography to predict the colors that leaves will change in the fall! Just like black markers have more than one color, leaves have more than one pigment. Use chromatography to separate the pigments and find their “true” colors.
Let’s try it!
YOU WILL NEED:
1. Green leaves (make sure they haven’t changed colors and aren’t too dry)
2. Rubbing Alcohol
3. Plastic Wrap
4. Paper towels
5. Pencils
6. Tape
7. Scissors
8. Clear Cup
9. Tap Water
10. Shallow pan (a brownie pan would work well)
YOU WILL DO:
1. Tear the green leaf into tiny pieces and drop the pieces into the clear cup.
2. Pour enough rubbing alcohol into the cup to cover the leaf pieces.
3. Cover the top of the clear cup with plastic wrap.
4. Fill the shallow pan with hot tap water.
5. Place the covered clear cup in the pan full of hot tap water.
6. Wait for the alcohol to absorb the leaf pigments. It may take about 30 minutes for the alcohol to turn green.
7. Just as you did in the marker experiment, cut a strip of paper towel the same height as the clear cup and tape the end of the paper towel to the pencil.
8. Remove the plastic wrap from the cup and place the pencil over the mouth of the cup so that the paper towel is dangling inside the glass. Make sure the end of the paper towel is barely touching the green alcohol.
9. Check back periodically for about 90 minutes and write down your observations.
Repeat the experiment using leaves from different trees so you can compare the pigments, just as you compared different brands and colors of markers.
Original image by Mattia Luigi Nappi, used in accordance with the GNU Free Documentation License, Version 1.2.
References
• http://chemlab.truman.edu/CHEM100Labs/Older%20Versions/PAPER%20CHROMATOGRAPHY.pdf
• https://soinc.org/sites/default/files/uploaded_files/crimebusters/Chromatography1.pdf
• http://www.exploratorium.edu/afterschool/activities/index.php?activity=172&program=952
• http://www.chemguide.co.uk/analysis/chromatography/paper.html
