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.  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. Commercial food colors are used as standards and the separated dyes can be identified.

 

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, distance component moves/distance solution moves, can be calculated for each component to aid in identification. The ratio also depends upon the paper, the developing solution and the amount of sample used.

Candies, such as M&M’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 the wool yarn. 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 ¾ to reduce the concentration of the H + and shift the equilibrium to the left releasing the dyes.  The yarn used in this experiment must be 100 % wool. The wool must be pretreated to remove fluorescent dyes.

 Two different developing solvents are used to illustrate the significant dependence of this technique on the solvent used. Vinegar usually better separates the yellow and blue dyes and ammonia better separate the red and yellow dyes.  Since it is difficult to apply comparable quantities of the extracted dyes and the food colors to the chromatogram, the students will only do a qualitative identification of components.


 

Materials Required

M&M's (not brown)

and/or Skittles (not “tropical fruit” as the colors are not intensive enough)

white household vinegar

clear, colorless household ammonia solution

food colors

red litmus paper

10 - 15 cm pieces of white undyed wool yarn

 

 

Equipment

Hot Plate

600-mL beakers

Large test tubes

10-mL graduated cylinder

stirring rod

evaporating dish

test tube holder

Test tube tongs

chromatography paper (paper, not plastic backed)

aluminum foil

boiling chips

ruler

 

Hazards

Care should be exercised when using boiling water baths and the ammonia.  Goggles should be worn at all times. 

 

 

Procedure

 

1.  Put 5 or 6 M&M's or Skittles (All the same type and color, eg.  5 blue M&M’s) into a large test tube.  Add about 5 mL of vinegar.  Make sure that there is enough vinegar to cover the candies.  Add more vinegar if necessary.  Heat the tube in a boiling water bath until the colored coating dissolves. Avoid dissolving the interior of the candies.

 

2.  Carefully decant (pour the liquid off of the solids) the solution which now contains the FD&C dyes, some sugar, etc. into another test tube.  Avoid transferring the sediment.

 

3.  To extract the dyes from the sugar etc. , add the prepared length of woolen yarn to the test tube containing the dye solution.  Add more vinegar to cover the yarn if necessary.  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.  You have just dyed the yarn with the food dyes from the candy coating and separated the dyes from the other components in the coating.

4.  To release the extracted dyes, place the dyed 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.

 

5.  Heat the tube containing the yarn and ammonia in a boiling water bath for about 5 minutes with occasional stirring. You have now released the dyes into solution..

 

6.   Remove the yarn and pour the solution containing the dyes into an evaporating dish.  Heat the evaporating dish over the boiling water bath 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.  Obtain two pre-cut 8 x 8 cm pieces of chromatography paper. Draw a pencil line with a ruler l cm from one long edge.  Mark with a pencil 5 equally spaced positions along the pencil line.  Label them blue, green, candy, yellow, red.  (See Below)  Use a capillary tube to place as small a drop as possible of the concentrated dye solution on the candy position on the pencil line.  Obtain some commercial food dye and place a small drop of each color of dye at the appropriate position on the pencil line. Allow the spots to dry.

 

 

8. The Chromatography chambers will have been prepared, by your instructor.  One chamber consists of 5mL of vinegar to a 600-mL beaker, and the other 5mL of clear ammonia solution in a 600mL beaker.  (Smaller beakers can be used)  Carefully staple each piece of chromatography paper (spot side out) into a cylinder and place one paper in each 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 mark the advancement of the developing solution and allow the paper to dry on a piece of paper towel.

 

9.  Compare the unknown to the commercially available dyes to identify the specific dyes present in the candy coatings.

 

10.  To be turned in: The type of candy and the color(s) used to dye the candy.

 

Disposal

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

 

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