Do Simple Chromatography

Mixtures are comprised of two or more components that have different properties. There are many methods used to separate mixtures into their basic components. For fluids, these methods are known as chromatography. Chromatography works a mixture (the mobile phase) moves through another substance (the stationary phase). Some components of the mixture move through more easily, while others are held back. This causes the mixture to separate into bands of different components.

Steps

Creating a Sample

  1. Make or buy a chromatography strip. A chromatography strip is a strip of material that the mobile phase mixture moves through. It serves as the stationary stage. You can by specially formulated chromatography strips for superior results, but for simply chromatography you can make your own. Cut a coffee filter into straight strips about one inch wide and five or six inches long.[1]
    • Paper towels, or any other paper, can be substituted for the coffee filter.
  2. Place a colored line near the bottom of the strip. Measure up about one inch from the bottom of the strip. There, use a black marker to draw a straight line across the width of the strip. Make sure that your line is up high enough that you can submerge the bottom of the paper without submerging the line.[1]
    • In place of a line, some people use a dot. This will work as well, but the bands will not be as clearly defined using this method.
  3. Secure the chromatography strip vertically. Use clothespins to hang the strip vertically. It should be hung such that the side with the marker line is nearest the ground. Make sure to place the clothespin as high on the strip as possible while still having a good hold. If the clip is too far down on the strip, it may interfere with the bands.[2]
    • You can hang the strip with paper clips, tape, or any other mechanism you see fit.

Introducing a Solvent

  1. Put water in a cup. When doing simple chromatography, your solvent (the liquid that carries the mobile phase across the stationary phase) will be water. Add a little water to a clear cup or glass. You only need enough for the chromatography strip to soak up, so just a few ounces is plenty.[2]
  2. Lower the chromatography strip into the water. Keeping your chromatography strip vertical, lower it into the water. Make sure that you have an apparatus set up to hold the strip there, as this process can take quite a bit of time. The bottom of the strip should be submerged, but the marker line should not be submerged. If you accidentally submerge this line, throw the strip away and make another.[1]
    • An example apparatus would be to have the clothespin that is holding the strip clamped to a pencil lying across the top of the glass. This way, the strip dangles down and just barely touches the water.
  3. Wait patiently. As the water moves up the strip, it will carry the different compounds in the marker with it. The lighter (smaller) compounds will move faster, and the heavier (larger) compounds will move slower. This will separate the compounds out into “bands” based on their size. This is a slow process, however. Watch the strip until you see the water reach about one inch from the top of the strip.[1]
    • The exact time it takes the water to reach one inch from the top of the strip will depend on what kind of strip you are using.

Observing the Bands

  1. Count bands you see. Once you have removed the strip from the water, your bands should stay in place. This will allow you to count how many different bands are visible on your strip. That will give you a rough idea of how many different sized compounds are in the marker’s ink.[1]
  2. Notice the color of each band. The marker’s black ink is made up of several different pigments. These pigments all have their own unique colors. When you separate them into bands, the bands will be the color of that individual pigment. By noting the color of each band, you can analyze what color pigments went into created the black ink in the marker.[2]
  3. List the bands by how far each of them traveled. Write the colored bands down in order from the top of the strip to the bottom of the strip. The bands near the top are your lightest compounds, and the bands near the bottom are your heaviest compounds. You are also likely to notice a color trend from top to bottom. Lighter colors are usually smaller compounds, and thus move farther up the strip, and darker colors will linger closer to the bottom because they are generally heavier compounds.[2]
    • The ratio of the distance a compound moved to the distance the solvent moved is called the Rf value. You can calculate the Rf value for any band by dividing the distance the band traveled from the original line to the distance the solvent traveled from the original line.[3]
    • For example, if you have a band two inches past above the line you made and the solvent traveled five inches above the solvent, you could use the equation Dband/Dsolvent = Rf. For this example, that means:
      • Rf = 2 inches / 5 inches
      • Rf = 0.4

Tips

  • If you wish to do this more scientifically, you can actually buy chromatography paper, which is similar to a coffee filter but creates a much more distinct and less blurry strip of colors when used properly. This is available through various lab equipment suppliers.
  • Try different colored markers, and notice which reaches the top first, which creates the prettiest strip of pigments, and so on.
  • Chromatography is used in actual scientific labs for various reasons, and can actually become quite complex. Chromatography includes a wide variety of scientific tests, and is not limited to this method.
  • Make sure the marker is water soluble before you do anything. Permanent marker doesn't usually work for this experiment.
  • Why does this work? Different colored pigments have different forms on the molecular level, and are pulled across the paper differently. This forms a long stretch of the pigments lined up in a line organized with the most similar colors near one another.

Warnings

  • Be careful to not submerge the mark on the filter. This will ruin the chromatography.

Things You'll Need

  • Clothespin
  • Coffee filter
  • Scissors
  • Water
  • Cup
  • Black marker

Related Articles

Sources and Citations

  1. ↑ 1.0 1.1 1.2 1.3 1.4 http://www.scienceprojectlab.com/paper-chromatography-experiment.html
  2. ↑ 2.0 2.1 2.2 2.3 http://www.birdandlittlebird.com/blog/2012/08/kitchen-table-science-simple-chromatography.html
  3. ↑ http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/covalent_compounds/seperationrev2.shtml
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