CANDY CHROMATOGRAPHY

PURPOSE

The purpose of this experiment is to separate and identify the FD&C dyes from M&M's or Skittles using paper chromatography.

DESCRIPTION

This experiment is appropriate for general, first-year college prep, or AP classes and could be adapted for use with middle school physical science classes. Wool yarn is used to separate the FD&C dyes present in the coatings of M&M's and/or Skittles from other components of the coatings. The resulting dyes are then separated using paper chromatography. If commercial food colors are used as standards, the separated dyes can be identified.

TIME REQUIRED

One and one-half to two lab periods.

MATERIALS

Chemicals:

M&M's and/or Skittles
white household vinegar
clear, colorless household ammonia solution
food colors
red litmus paper

Equipment:

Bunsen burner
ringstand, ring and wire gauze or hot plate
600-mL beakers 25-cm × 150-cm test tubes
l0-mL graduated cylinder
stirring rod
evaporating dish
test tube holder
beaker tongs
crucible tongs
wool yarn
chromatography paper (Whatman #1)
tooth picks
stapler
scissors
aluminum foil boiling chips
ruler

HAZARDS

Care should be exercised when using boiling water baths.

PROCEDURE

1. Place a test tube containing a 10-15 cm length of white woolen yarn and 10-15 mL of household vinegar in a boiling water bath and heat for 4-5 minutes to remove any fluorescent dyes which could interfere with the separation of the FD&C dyes. After cooling, remove the yarn from the vinegar and let it drain.
2. Place 5 or 6 M&M's or Skittles in a test tube with enough household vinegar to cover the candies. Heat the tube in a boiling water bath until the colored coating dissolves. Avoid dissolving the interior of the candies.
3. Carefully decant the solution which now contains the FD&C dyes, some sugar, etc. into another test tube. Avoid transferring the sediment.
4. To extract the dyes, add the prepared length of woolen yarn and 3 mL of vinegar to the test tube containing the dye solution. Heat this tube in the boiling water bath for about 5 minutes with occasional stirring. Remove the yarn and rinse it with a little tap water.
5. To release the extracted dyes, place the yarn and about 5 mL of clear household ammonia solution in a clean test tube. Mix with a stirring rod and then test the resulting solution with red litmus paper to make sure that it is basic. If not, add a bit more ammonia solution.
6. Heat the tube containing the yarn and ammonia in a boiling water bath for about 5 minutes with occasional stirring to release the dyes. Remove the yarn and pour the solution containing the dyes into an evaporating dish. Heat the evaporating dish gently to concentrate the solution. Stop just short of dryness. If all the liquid evaporates, add a drop or two of distilled water and stir.
7. Cut two l0-cm × 20-cm pieces of chromatography paper. Draw a pencil line l cm from one long edge of each piece of paper. Mark six or seven equally spaced positions along the pencil line. Use toothpicks to place as small a drop of the concentrated dye solution as possible on two of the positions on the pencil line on each piece of paper. Allow the spots to dry and spot them again in exactly the same positions to increase the amount of sample. Spot a third time and a fourth time if the color 13 not very intense. Use toothpicks to place one small drop of each food color sample on the remaining positions on each piece of paper.
8. Add 5 mL of vinegar to a 600-mL beaker. Carefully staple one piece of chromatography paper into a cylinder and place the paper in the beaker with the spots at the bottom of the cylinder. Be careful that the paper does not touch the sides of the beaker. Cover the beaker with aluminum foil and allow the chromatogram to develop until the developing solution has climbed to about l cm from the top of the paper. Remove the paper from the beaker, open it out and allow the paper to dry on a piece of paper towel.
9. Repeat step 8 using the second piece of chromatography paper, but develop with 5 mL of clear ammonia solution.
10. Use the following information to identify the specific dyes present in the candy coatings.

DISPOSAL

Remaining interiors of candies may be disposed of with solid waste. Solutions that remain may be flushed down the drain.

DISCUSSION

Paper chromatography is an important separation technique that depends upon differences in how strongly the dyes are adsorbed onto the paper (stationary phase) and how soluble the dyes are in the developing solvent (mobile phase). In paper chromatography, a small amount of the mixture to be separated is placed close to the edge of a piece of paper. The edge of the paper is then immersed in a developing solution. As the developing solution ascends up the paper by capillary action, the. components of the sample are carried along at different rates. To prevent evaporation of the developing solution, this process is carried out in a closed container.

Each component of the mixture will move a definite distance on the paper in proportion to the distance that the solvent moves. This ratio, H_f = distance component moves/distance solution moves, can be calculated for each component to aid In identification. H_f values are dependent upon the paper, the developing solution, and the amount of sample used.

Candies, such as Mix's or Skittles, contain FD&C dyes, sugar, and other organic and Inorganic substances In their coatings. If the coating is dissolved in an acidic solution, the FD&C dyes can be adsorbed from the solution by wool. The dyes can be released from the wool in an alkaline environment. Thus the dyes can be separated from other substances in the coatings. This simplified equation represents the equilibrium involved:

Wool + Dye + H⁺ <====> Dyed Wool⁺

A solution of ammonia can be used to supply sufficient OH^- to reduce the concentration of the H⁺ and shift the equilibrium to the left releasing the dyes.

TIPS

1. The yarn used In this experiment must be 10O% wool. To save time, the teacher can pretreat the wool to remove fluorescent dyes.
2. Two different developing solvents are used to illustrate the significant dependence of this technique on the solvent used and because the vinegar separates the yellow and blue dyes better while the ammonia separates the reds and yellows better.
3. Since it Is difficult to apply comparable quantities of the extracted dyes and the food colors to the chromatogram, It is likely that the H_f values will not be very reproducible. The separations are sufficiently distinct that students can identify the components qualitatively.

REFERENCES

Jenkins, C., Science and Children, April 1986, p. 25. -- This article describes a chromatography experiment for young children using Kool-Aid and lists the FD&C dyes In various brands of food colorings.

McDuffie, T.E. Jr. and Anderson, J., Chemistry Experiments from Daily Life, J. Weston Walch, Publisher, 1980, p. 77. -- This work describes a similar experiment which uses non-consumer solutions for the extraction and for developing the chromatogram.

Submitted by John Hnatow