Paper chromatography is a modern method used separate mixtures. Paper chromatography uses paper as the stationary phase and a liquid solvent as the mobile phase. You will use paper chromatography to test food ings to see if the results from a single dye or mixture of dyes. If you cannot get food ing you can use ed felt pens or other water soluble ing pens. Your exercise is simple but uses very essential principles. Procedure: Cut a half inch wide (1.25 cm) strip of chromatography paper about 10 cm long. Make 5 or more of these strips. Make a start line with pencil mark at half an inch from one end of the paper strip; this will be the bottom. Do this with each strip. 1. Place about 5 drops of blue food in a disposable plastic cup, a small glass container or on a glass or ceramic plate. 2. Cut off one end of a toothpick. Dip the fresh cut end into the food ing. 3. Use the toothpick to place a dot of blue food on the pencil mark start line and allow it to dry. The dot should be about a millimeter across. 4. Attach a piece of tape to the top end of the strip of paper. Tape the paper to the pencil and lower the paper into the plastic bottle or tall glass jar. Check how far the paper projects into the container. On the outside of the bottle or jar, mark the position of the bottom edge of the paper by writing on a piece of Scotch tape stuck to the jar. Remove the pencil and paper. 5. Add water to container so the water level will touch the bottom of the strip of paper; the water level needs to be well below the pencil mark start line on your strip of paper and at least 1/4 inch below the blue spot. 6. Lower the paper into the bottle so the water touches the bottom of the paper. The water must NOT touch the blue spot of food. 7. Let the water wick (climb) up the paper. Note where the water wets the paper; the top of the wet area is the "solvent front". The water will climb the first few centimeters quickly. The food will probably trail behind the water. 8. When the front edge of the water reaches three fourths of the way up the paper remove the paper from the bottle or glass jar. Use a pencil (not a pen) to mark the front edge of the solvent. Allow the paper to dry. Use the pencil to mark the "center of gravity" of the dye spot. The "center of gravity" of the dye spot is its "average" position on the paper. 9. Note if more than one appears on the paper. If so, find the "center of gravity" for each dye. 10. Measure the distance between the start line (where the blue spot was placed) and the mark for the upper edge of the solvent front. Record this distance. Measure the distance between the start line and the "center of gravity" of the. Record this distance.
11. Repeat steps 1 through 10 using the yellow food ing. Repeat twice more, using the green food ing and then the red food ing. Record your observations and measurements on the Report Sheet. Separation of a mixture of all 4 food s Mix 5 drops of each of the food s in a disposable cup, small glass jar or in a puddle on your plate. Stir the mixture and allow it to stand for about ten minutes. This lets some of the solvent evaporate and the mixture will be more concentrated, making the s easier to see. Repeat steps 1 through 10 above to record a chromatogram for the mixture of the 4 food s. Record your observations and measurements on the Report Sheet.
Report Sheet Chromatography of individual food s or ed pens Brand name for food s used in experiment if not Schilling. Food Blue Yellow Green Red s observed in chromatogram for each dye(on paper) Food Mixture Yes/ No How many dyes are in the food? Blue Yellow Green Red List the food s that is/are mixtures of dye molecules? Explain your choices using the results of your chromatography experiments. Do you think that other substances like vegetable dyes or inks could be tested using this chromatography method? Justify your answer. Explain briefly what would have to be done if the dyes or inks would not dissolve in water? Could chromatography still be used to separate the mixture? Would a pure substance show more than one or SPOT in a chromatogram? Explain
Data and calculations for Rf values If a food contains only 1 dye, enter data for "fast dye" only. If a food contains only 2 dyes, enter data for "fast dye" and "slow dye" only. Blue food solvent front distance mm Yellow food solvent front distance Green food solvent front distance Red food solvent front distance Distance traveled by Solvent Front all distances in mm Data for the "Fast Dye" in food ing ( in dye that travelled greatest distance) Blue food Yellow food Green food Red food Distance traveled in mm Rf value Data for the "Middle Dye"in food ing ( in dye that travelled intermediate distance) "Middle Dye" Blue food Yellow food Distance traveled Rf value Green food Red food Data for the "Slow Dye" in food ing ( in dye that travelled shortest distance) Distance traveled Rf value Chromatography of mixture of four food s
How many dye s do you observe in your chromatogram of this mixture? What is the largest possible value for Rf? Solvent Front Distance traveled------------------ ----------------------------- "Fastest Dye in mixture" -------------------------- --------------------- ----------------------------------------------- Distance traveled----------------------------------- ------------ Rf value----------------------------------------------- "Slowest Dye in mixture " ------------------------- ---------------------- ----------------------------------------------- Distance traveled------------------------------------ ----------- Rf value----------------------------------------------- How does the Rf value for your "Fastest Dye" in this mixture compare to the Rf value for the same dye molecule obtained when you tested the individual food s? Give a numerical comparison. Does the Rf value seem to change? Are the very different or very similar?