Expt 001 -- Paper Chromatography

Description

Chromatography is a technique in which components of a mixture are separated from one another on the basis of differences in solubility and attraction for two phases -- a moving phase and a stationary phase. The components of water soluble dye pigments are separated using water as the mobile phase and paper as the stationary phase.

Safety

No special hazards are associated with this experiment. The pens can stain clothing.

Procedure

  1. Obtain five pieces of chromatography paper. Draw a pencil line (graphite pencil only; no pen) perpendicular to the long axis of the paper and about 2 cm from one end. Using markers filled with water soluble pigments provided by the instructor, place a small dot on the center of the pencil line, one marker per strip. At the other end of the strip, write a pencil label indicating which pen was used.
  2. Select a 24-well plate. Place 0.5 cm of water in each of five adjacent wells on the side of the plate. Bend the ends of the paper strips 90º just above the dot of dye. Place the short end in one of the wells; drape the paper across the plate. Use a different well for each of the strips. The strips should not touch. The dye dot should be at least 0.5 cm above the top level of the liquid water.
    !!!Click here to See Movie.
  3. Note any changes with time.
  4. The movie is speeded up by a factor of 10 by using time lapse techniques.
    !!!Click here to See Movie.
  5. See the same separation after 5 more minutes.
    !!!Click here to See Picture.
  6. Remove the paper after 30 minutes, or when the moving solvent boundary as seen by the paper appearing wet advances to within 1 cm of the end of the paper opposite from the well.
  7. Measure the distance from the dot (pencil line) to the edge of the solvent boundary for each strip. For each color, measure the distance from the center of the pencil line to the edge of the color nearest the solvent boundary. For each color, calculate the ratio the distance moved by the color to the distance moved by the solvent boundary. This ratio is called the Rf value.
  8. A second set of separations is also shown. Makeup students may use this set to calculate Rf values. Use a ruler to make measurements on the computer monitor.
    !!!Click here to See Picture.
  9. Place a coin ($0.25 US) in the center of a circle of filter paper, and draw a circle in pencil. Punch a small hole at the center of the paper. Using colored pens, mark alternating patterns of dyes on the circle. (Using matched sets of 5 or 6 dots gives excellent results.)
    !!!Click here to See Picture.
  10. Tear a piece of chromatography paper in half. Roll one half into a tight cylinder, and insert it into the hole.
    !!!Click here to See Picture.
  11. Fill a center well of a 24-well plate with 1 cm of H2O. Place the circle over that well so the rolled piece enters the water and serves as a wick.
    !!!Click here to See Movie.
  12. Allow 20-30 minutes of a chromatogram to develop. Remove the circles and permit it to dry. Note the pattern.
  13. The movie is speeded up by a factor of 10 by using time lapse techniques.
    !!!Click here to See Movie.
    !!!Click here to See Picture.
  14. The results of another chromatography are shown also.
    !!!Click here to See Picture.
    !!!Click here to See Picture.
    (If large papers are used, places the paper in a covered, sealable, see-through container. The cover prevents water evaporation into the air, and permits large papers to be used.)

Questions

  1. In a given series of experiments, the following data are observed for two yellow pigments: boundary distance = 8.3 cm; front edge of yellow-1 spot = 4.8 cm; boundary distance = 7.4 cm; front edge of yellow-2 spot = 3.2 cm. Calculate the Rf values of these dyes.
  2. A green dye is separated in a chromatogram. The boundary moves 8.9 cm. The front edge of a blue spot appears at 6.1 cm, and that of a yellow spot appears at 3.8 cm. Is the yellow dye likely to be yellow-1 or yellow-2?

Handout

Name ___________________________ Class ________

Teacher__________________________

SmallScale 001 Paper Chromatography

  1. Makeup students may use the picture below to answer the questions and calculate Rf values. Use a ruler to make measurements on the computer monitor.
  2. Which dye is most strongly attracted to the paper?
  3. Which dye is most strongly attracted to the solvent?
  4. Calculate the Rf values of each dye in one of your chromatograms.
  5. Color Distance Rf

Curriculum-

Separation is usually taught early in a chemistry course, when the notions of mixtures, compounds, and elements are introduced. This experiments fits in early with these discussions. It has many applications, and can be used wisely nearly anytime during an introductory chemistry course. Because the ideas are so powerful, and the treatments of the theory usually cursory for general student audiences, the earlier, the better for chromatography.

Safety-

No special hazards are associated with this experiment. The pens may stain clothing.

Time-

Teacher Preparation: 10 minutes

Class Time: 30 minutes

Materials-

Disposal-

The liquids may be discarded safely at the sink. Discard solid wastes with ordinary solid trash.

Lab Hints-

Black pens work very well; they usually contain a variety of pigments, and the mixtures used vary from manufacturer to manufacturer.

Answers-

Q1. In a given series of experiments, the following data are observed for two yellow pigments: boundary distance = 8.3 cm; front edge of yellow-1 spot = 4.8 cm; boundary distance = 7.4 cm; front edge of yellow-2 spot = 3.2 cm. Calculate the Rf values of these dyes.
A1. Rf yellow-1 = 4.8/8.3 = 0.58; Rf yellow-2 = 3.2/7.4 = 0.42.
Q2. A green dye is separated in a chromatogram. The boundary moves 8.9 cm. The front edge of a blue spot appears at 6.1 cm, and that of a yellow spot appears at 3.8 cm. Is the yellow dye likely to be yellow-1 or yellow-2?
A2. Rf yellow = 3.8/8.9 = 0.43; this implies that yellow-2 is the pigment.

Reference-

See M. Kandel, J. Chem. Educ. 1992, 69, 988-989; R. Becker, J. Ihde, K. Cox, J. L. Sarquis J. Chem. Educ. 1992, 69, 979-980.

Key Words 1-

solvent, solute, chromatography, paper chromatography, stationary phase, mobile phase, Rf, separation, pigment, chromatogram

Elements-

C