Teacher Notes

Chromatography Challenge

Guided-Inquiry Kit

Materials Included In Kit

Cups, polypropylene, 30-mL, 30
Filter paper, 12.5-cm diameter, 100 sheets
Markers, water-soluble, set of 8
Page of color photos of sample art chromatograms

Additional Materials Required

Paper clip (optional)
Paper towels
Pencil
Ruler, metric
Scissors
Tap water
Water soluble black pens or markers, additional (optional)

Prelab Preparation

This activity will require about 2 hours of teacher preparation time the day before the lab to test the pens or markers and to create the art chromatograms for the chromatography challenge. See the included page for color photos of the art of radial chromatography. Three examples are shown in Figure 3, along with the pens and the ink patterns used to create them. Note: Manufacturers may change ink formulations without notice—please test the pens or markers beforehand.

Black ink codes:

  1. Liquid Expresso™
  2. EF®
  3. Dixon® Vis-aid® Overhead Projection Marker
  4. Mr. Sketch® Scented Water Color Marker
  5. LePen®
  6. Paper Mate®
  7. Vis-à-Vis®
{12611_Preparation_Figure_3}

Safety Precautions

Although the materials in this activity are considered nonhazardous, please observe all normal laboratory safety guidelines. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All of the materials may be disposed of in the solid trash according to Flinn Suggested Disposal Method #26a.

Lab Hints

  • Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students to each make five radial chromatograms. One hundred filter paper circles are provided—75 for student chromatograms, 15 to make wicks and 10 for teacher samples.
  • This laboratory activity can reasonably be completed in two 45- to 50-minute class periods. The prelaboratory assignment may be completed before coming to lab. One class period is usually needed to identify the pigments present in different inks and to determine the effects produced by different types of ink “spots” (e.g., dots, lines, arcs). The second class period may then be devoted to duplicating the art chromatogram produced by the teacher.
  • Experiment with a variety of water-soluble black markers or felt tip pens to determine the composition of each. Many different brands are commonly available at local stores. Some of the brands we have tested include Expresso, Prang, Vis-Aid, Mr. Sketch, LePen, Paper Mate Flair, Vis-à-Vis, Dick Blic and EF. The specific brands of pens included in the kit may vary depending on availability. Test the pens and markers beforehand to determine how the inks will separate using water as the solvent.
  • Manufacturers may change ink formulations without notice. Do not assume that the sample art chromatograms described in the Prelab Preparation section and shown on the color photo page can be duplicated without prior testing. Two Paper Mate Flair pens were tested for this activity with vastly different results. One showed (from least mobile to most mobile) purple, gray, yellow, blue and lavender, the other showed lavender, pink, turquoise, green and yellow.
  • Number or code the markers in some way and place them in a central area for students to share.
  • Coffee filters are a suitable substitute for the filter paper. The ruffled sides of the coffee filter should be removed with scissors and can be used as “wicks.” Water, however, is more quickly absorbed by coffee filters than by filter paper. This will reduce the separation and resolution of the ink pigments.
  • If a chromatogram is running too slowly or not at all, check to make sure that the wick has been inserted snugly into the hole and that there is good contact between the wick and the inside edge of the hole. Typical running times for the chromatograms are 15–17 minutes.
  • Avoid excessive handling of the filter paper. Oils from the skin can interfere with capillary action that draws the water through the paper.

Teacher Tips

  • Laminating the sample teacher art chromatograms will increase their durability and protect them from spills during the student procedure. These can be saved for future classes.
  • To save laboratory time, wicks may be cut by the teacher in advance, using only 10 filter paper circles. This would leave 5 extra pieces of filter paper for the teacher to experiment with.
  • If an ultraviolet (UV or “black”) light is available, shine it on the art chromatograms. Some pigments will fluoresce under UV light.
  • The following student laboratory kits can be use to further study the technique of chromatography: AP4503—Introduction to Paper Chromatography Kit, AP5992—Chromatography Centrifuge Kit and FB0586—Plant Pigment Chromatography Kit.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Asking questions and defining problems
Analyzing and interpreting data

Disciplinary Core Ideas

MS-PS1.A: Structure and Properties of Matter
MS-PS2.B: Types of Interactions
HS-PS1.A: Structure and Properties of Matter
HS-PS2.B: Types of Interactions

Crosscutting Concepts

Patterns
Cause and effect

Answers to Prelab Questions

  1. What will be the adsorbent in this activity? What will be the solvent?

    The filter paper will be the adsorbent and water will be the solvent.

  2. Why is it important that only the wick and not the filter paper circle be in contact with the water in the cup?

    If the filter paper is in direct contact with the water in the cup, some of the inks on the paper may dissolve in the water. This will dilute the ink spot on the paper and may change the composition of the ink spot that remains. The separation (resolution) of the pigments on the paper may also be affected—the chromatogram may appear “muddy.”

  3. What are some of the variables that will affect the pattern of colors produced on the filter paper?

    Some of the variables that will affect the pattern of colors produced on the filter paper are (a) the pen brand and the ink composition in each pen, (b) the type of ink spot that is marked on the paper, (c) the thickness or absorbency of the filter paper, (d) the purity of the water solvent, (e) the length of time that the chromatogram is allowed to develop, (f) the distance away from the center hole where each ink spot is placed and (g) the thickness or heaviness of the ink spot.

  4. Working with a partner, write a general outline describing an “action plan” to reproduce the art chromatogram created by the teacher.

    Note: Ask to see the students’ plans. In the first round of this experiment, students should identify the pigments that are in each ink and determine the radial band patterns that will result from different types of ink spots (e.g., dots, lines, arcs, wedges) Trial and error will then be required to discover where the ink spots must be placed, relative to the wick in the center of the paper, to duplicate the art chromatogram created by the teacher.

Sample Data

This is a guided-inquiry activity requiring original art chromatograms prepared by the teacher. See the color photo page for ideas and the Prelab Preparation section for instructions for preparing the chromatograms. The pens tested in this experiment and their ink compositions are described in the Answers to Post-Lab Questions.

Answers to Questions

  1. List the brand of each pen tested in this activity and record the colors observed in each radial band pattern. Fore each pen, list the colors (pigments) from least mobile (greatest attraction to the paper—closest to the center) to most mobile (least attraction for the paper—closest to the outer edge).

    Colors shown in parentheses may be indistinct or blend in with other pigments. Some colors are only seen when the chromatogram is allowed to run longer than 15 minutes. Remember, manufacturers may change formulations and pigments.
    Dick Blick: Black, gray, (green) blue, violet, lavender, yellow, pink
    EF: Lavender, pink, yellow, turquoise
    Le Pen: Brown, (yellow) purple, aqua
    Liquid Expresso: Dark midnight blue, gray, dark gray
    Mr. Sketch: Yellow, orange, rose, pink, turquoise
    Paper Mate: Purple, gray (yellow), blue, lavender
    Vis Aid: Gray, yellow-green, blue, lavender, purple
    Vis-à-Vis: Yellow-green, rose, purple, turquoise

  2. Why does an ink separate into different pigment bands? Each ink is composed of different compounds.

    Each compound (pigment) has characteristic physical properties that will determine its relative attraction for the adsorbent (paper) versus the solvent (water) in this chromatography experiment.

  3. Do any of the pens or markers appear to contain common pigments? How can you tell if similar-colored pigments from different pens are actually the same compound? Do any similar-colored pigments appear to be different compounds?

    Several of the pens and markers appear to contain similar pigments. The turquoise in the Vis-à-Vis, Mr. Sketch, and EF pens may be the same compound because they seem to have the same mobility—this pigment traveled the farthest in all three pens. The blue pigment in the Paper Mate, Vis Aid and Dick Blick also may be the same compound with moderate mobility. The yellow in the EF pen is quite mobile whereas the yellow in the Mr. Sketch pen has much greater attraction to the paper—these pigments are most likely different compounds.

  4. Why were only water-soluble markers or pens used in this activity? How could the experiment be modified to separate the pigments in “permanent” markers or pens?

    The ink must dissolve in the water in order to migrate with the water through the paper. If the ink does not dissolve, the ink spot will remain at the origin and will not separate into different color bands. The experiment may be modified to separate the pigments in permanent markers by running the chromatograms with different solvents, for example, an alcohol solvent.

Teacher Handouts

12611_Teacher1.pdf

References

This activity was adapted from Flinn ChemTopic™ Labs, Vol. 2, Elements, Compounds and Mixtures; Cesa, I., Editor; Flinn Scientific, Inc; Batavia, IL (2006).

Student Pages

Chromatography Challenge

Introduction

Black is the absence of color—or is it? The challenge in this activity is to reproduce beautiful multicolor art patterns with a variety of black ink pens and markers using paper chromatography.

Concepts

  • Paper chromatography
  • Separation of a mixture
  • Physical properties

Background

Many common materials are made up of mixtures of compounds. Separating mixtures to determine the identity of one or more compounds has many practical applications in the fields of medicine and law enforcement, for environmental studies, and in manufacturing. It is often difficult to separate mixtures if the compounds are chemically similar. Chromatography is a technique used to identify and analyze components of a mixture.

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 Mikhail Tswett (1872–1919) in 1903 to describe a new technique he had invented to separate the pigments in green plant leaves. Tswett found that in addition to the main green pigment, chlorophyll, plant leaves also contain 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.

Paper chromatography is a type of chromatography called adsorption chromatography. The paper acts as an adsorbent, a solid which is capable of attracting and binding with the components in a mixture (think of the word adhere). The mixture to be separated is “spotted” onto the surface of the paper and a solvent is then allowed to seep through the paper by capillary action. As the components of the mixture dissolve in the solvent, they will travel up the paper at different rates depending on their characteristic physical properties. If one of the components in the mixture is more strongly adsorbed onto the paper than another, it will spend less time in solution and will move up the paper more slowly than the solvent. Components that are not strongly adsorbed onto the paper will spend more time 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, depending on their physical attraction for the paper versus the solvent. If the components of the mixture are colored, the bands are easy to see.

Radial chromatography is a technique using a paper circle with a hole in the center. Ink is spotted onto the circle. A paper “wick” is inserted into the hole and then placed in a cup of water, making sure the paper circle does not come in direct contact with the water (see Figure 1). The water seeps up the wick, then outward through the paper. The different pigments making up the ink mixtures will separate in a circular (radial) pattern. The resulting pattern is called a chromatogram.

{12611_Background_Figure_1_Radial paper chromatography}

Experiment Overview

The purpose of this activity is to make a reproduction of a teacher-created “art chromatogram.” Students will first investigate the color patterns produced by radial chromatography using different types of black ink to determine which pens or markers were used and how they were spotted (e.g., dots, lines, arcs). The results will then be used to reproduce the chromatogram assigned by the teacher.

Materials

Cups, plastic 30-mL, 2
Filter paper, 12.5-cm diameter, 6
Markers and pens, black, water-soluble, various types
Paper clip (optional)
Paper towels
Pencil
Ruler, metric
Scissors
Tap water

Prelab Questions

Read through the lab and answer the following questions on a separate sheet of paper.

  1. What will be the adsorbent in this activity? What will be the solvent?
  2. Why is it important that only the wick and not the filter paper circle be in contact with the water in the cup?
  3. What are some of the variables that will affect the pattern of colors produced on the filter paper?
  4. Working with a partner, write a general outline describing an “action plan” to reproduce the art chromatogram created by the teacher.

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. Obtain a piece of filter paper and fold it in half three times to make 8 equal pie-shaped sections. Using scissors, cut along the folds to make 8 wedge-shaped pieces.
  2. Obtain another piece of filter paper and lay one wedge on top of the circular piece to determine the center of the filter paper. Mark the center with a pencil (see Figure 2).
    {12611_Procedure_Figure_2}
  3. Using a sharp pencil or unbent paper clip, poke a small hole in the center of the filter paper.
  4. Starting at least 5 mm from the center hole, place a small but concentrated spot of ink from a water-soluble black marker or pen onto the paper. The “spot” may be a dot, a wedge, a line, an arc, etc. See Figure 1 for an example.
  5. Roll up one of the filter paper wedges into a tight cone and insert the cone-shaped “wick” into the hole in the center of the filter paper circle. Enough of the wick should be above the filter paper so that there is good contact between the wick and the filter paper. Make the hole slightly larger, if necessary.
  6. Set the prepared filter paper circle on top of an empty 30-mL cup. The filter paper should rest on the rim of the cup. If the wick is too long, trim the bottom so the wick will fit in the cup, allowing the circle to rest on the rim of the cup.
  7. Remove the prepared filter paper from the cup. Fill the plastic cup with tap water to within about 1 cm from the top. Make sure the rim of the cup is dry.
  8. Set the prepared filter paper circle on top of the water-filled cup.
  9. As the chromotogram is developing, a second one may be started by repeating steps 1–8 with another paper circle, wick, and cup.
  10. When the water has advanced to within 1–2 cm of the outer edge of the filter paper on the first chromatogram (about 15 minutes), carefully lift the paper, remove the wick, and set the chromatogram on a paper towel to dry.
  11. Repeat as many trials as needed with a new piece of filter paper and wick each time to reproduce the art chromatogram created by the teacher.
  12. 12. Attach the desired art chromatogram to the Chromatography Challenge Worksheet.

Student Worksheet PDF

12611_Student1.pdf

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