Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Transfer 0.5 g of FD&C Blue No. 1 dye into a 250-mL beaker.
2. Add 200 mL of DI water to the beaker containing the dye.
3. Mix thoroughly with the stirring rod.
4. Label the beaker properly.
5. Rinse the stirring rod with DI water.
6. Repeat steps 1–5 using clean beakers for the other three dyes.
7. Using the graduated cylinder, measure 100 mL of DI water and transfer it into a clean beaker.
8. Add one buffer capsule to the water.
9. Label the beaker properly including the buffer name
10. Repeat steps 7–9 using a clean beaker for the other buffer capsule.
11. Using the graduated cylinder, measure 100 mL of DI water and transfer it into a clean beaker.
12. Add the entire amount of calcium chloride dihydrate solid to the water.
13. Mix thoroughly with the stirring rod.
14. Label the beaker properly.

Safety Precautions

The pH 2 and pH 12 buffer solution are corrosive to skin and eyes. Bleach is a corrosive liquid that causes skin burns. It may also generate chlorine gas when mixed with acid and when heated. Sodium thiosulfate is slightly toxic by ingestion and is moderately toxic by ingestion and inhalation as well as a body tissue irritant. FD&C dyes are slightly hazardous by ingestion, inhalation, eye and skin contact. Red No. 40 may be absorbed through skin. Calcium chloride is also slightly toxic. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines. Review current Safety Data Sheets for additional safety, handling and disposal information.

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. Leftover buffer solutions may be neutralized and then rinsed down the drain with excess water according to Flinn Suggested Disposal Methods #24b and #10. The excess 10% bleach and 1 M calcium chloride may be disposed of down the drain with plenty of water according to disposal method #26b. The excess 1 M sodium thiosulfate solution may be oxidized according to Flinn Suggested Disposal Method #12b. Note: The bleach and the sodium thiosulfate may be used to oxidize/reduce each other.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The Prelab Questions may be completed before coming to lab, and the Post-Lab Questions may be completed the day after the lab.
• The dilution with DI water serves the purpose of a control. The dye is diluted with DI water to the same volume as the other samples for an accurate comparison.
• If desired, fill graduated pipets with the solutions that each group will need—6 mL of each dye solution and 4 mL of each test solution. Label all pipets carefully. Once the required amount is in the pipets, turn them upside down so they do not leak and secure with a rubber band. Store each grouping in a disposable cup for easy class distribution.
• During the 15-minute wait, other activities, discussions (see Teaching Tips) or demonstrations may be performed.

Teacher Tips

• This activity can be used at the beginning of the chemistry unit discussing observations or with units on chemical reactions and food chemistry.
• Food can be colored with natural dyes such as beet juice. Have student do an inventory at the grocery store or at home finding products that contain no dyes, natural dyes and also ones that contain FD&C dyes. Discuss as a consumer which items they would purchase and why. Many common grocery store items have products marketed in all three dye categories. Examples include popsicles, yogurt, candy, over-the-counter medicines, etc.
• Excess dye is provided for additional investigations if time permits.
• Other FD&C food dye kits available from Flinn Scientific include Food Dye Chromatography, Catalog No. AP7394, and Quantitative Determination of Food Dyes, Catalog No. AP7428.

Answers to Prelab Questions

1. What is the purpose of this experiment?

   The purpose of the experiment is to test different food dyes under varying food processing conditions, to determine stability.

2. What is used as the control in this experiment? Why is it an ideal control?

   The food dye with 1 mL of DI water added to it will be used for the control. This is a good control because the dilution factor of the DI water to dye is the same as the other test samples.

3. Give an example of a physical and chemical change for each indicator below.
   1. Bubbles forming

      Physical: boiling
      Chemical: any gas formed in a reaction

   2. Color change

      Physical: dilution—adding food dye to water
      Chemical: any new product formed of a different color

   3. Solid forming

      Physical: freezing
      Chemical: any precipitate formed in a reaction

Sample Data

Answers to Questions

1. Which of the four FD&C food dyes tested appeared to be the most stable, that is, resistant to change in the testing conditions? Why?

   Blue No. 1 appeared the most stable; the color did not change except in the bleach.

2. Which FD&C food dye appeared to be unstable or most reactive? Why?

   The Red No. 3 appeared the least stable; the solution reacted to almost every variable. It precipitated in both the pH 2 and calcium chloride solutions Yellow No. 6 was also reactive.

3. What variable caused the greatest amount of change with all four of the food dyes tested? What happened?

   In the bleach all four dyes discolored. It was the only variable to react with all four dyes.

4. If a food scientist has a product with a pH of 2, which food dye(s) would not be recommended for use?

   Red No. 3 could not be recommended for use since it formed a precipitate in pH 2 solution. Yellow No. 6 discolored— which may also be unacceptable for processing.

5. In a beverage packaging plant, which dye(s) may need a recommendation for use with softened water? Why?

   Both Yellow No. 6 and Red No. 3 formed a precipitate with calcium chloride, which was used to simulate hard water.

6. By accident, a person washed yellow socks in a load of white clothes. Bleach had been added to whiten the whites. If the socks were dyed with Yellow No. 6 what would you predict would happen to the yellow socks after the washing?

   Most likely the yellow socks would decolorize like all the dyes did when mixed with bleach.

References

Epp, Dianne N. The Chemistry of Food Dyes; Miami University Middletown: Middletown OH; 1995, pp 11–13, 18–20, 43–45.

Introduction

Dyes color our world, both in nature and in manmade materials. In the case of food items, only seven dyes are used to make most of the colors seen in food sold in the United States. Explore how these dyes and their colors change when introduced to different food or chemical conditions.

Concepts

• Food dyes
• pH
• Chemical reactions
• Chemical change

Background

Food dyes are subject to a variety of conditions. These conditions involve the food processing cycle as well as the properties of the food itself. The dyes may be put in acidic or basic foods, they may be heated or cooled, and they may come in contact with reactive foods and processes—ones that may oxidize, reduce or otherwise chemically react with the dyes. Is the dye stable enough to be used in food and what types of food processes may change the resulting color of the food? Due to the rigors of food processing, food dyes are usually added at the latest stage possible. The changing colors of the dye in different conditions may be part of the charm as well as part of the challenge of producing commercial food.

Color additives are used to make food more visually appealing to the consumer. Dyes may be added to give color to foods that otherwise do not have any, add the natural color back to food that has decolorized or been compromised by food processing, give varying-colored foods more even toning, and also to color imitation foods. In some cases, dyes may be used to mask poor quality or inferior food. For example, in the butcher section meat can be colored to appear fresh even after it would have naturally turned brown. Jams and jellies may be colored to give the impression of higher fruit content. Some food, such as imitation crab meat, is colored to resemble the natural product.

In the past, food colorants were added to food with little or no health testing. In 1906, to propagate the food safety effort, the USDA hired a consultant, Dr. Bernance Hess, to determine which dyes would be safe to consume in food. In 1907, the number of synthetic food dyes approved for use in the United States was reduced from 695 to just seven. As additional data were collected through consumer reports and laboratory testing, more dyes were eliminated or restricted. Only two of the original dyes from 1907 are still accepted for use today. Five others have been added between 1907 and 1971 when the last was approved. In total, only seven dyes color all U.S. food today. (See Supplementary Information in the Teacher PDF for the chemical structures of the USDA-approved food dyes). Food dyes can undergo chemical changes with food. To determine if a chemical reaction has occurred between a food dye and its surroundings, scientists must look for evidence of a chemical change.

A chemical change is defined as a change in the composition and properties of a substance. The transformation of old materials (reactants) into new substances (products) as a result of a chemical change is called a chemical reaction. Many types of observable changes are used to help identify that a chemical reaction has occurred. Signs of a chemical change include formation of a solid precipitate, release of gas bubbles that are not due to a physical change such as boiling, a color change that does not result from dilution or color mixing and temperature changes that are not caused by external heating or cooling.

Experiment Overview

Synthetic food dyes may change color under different chemical conditions. Four out of the seven FD&C food dyes will be tested with a series of other reactants, including acidic and basic solutions, simulated hard water as well as with oxidizing and reducing substances to determine if a chemical reaction takes place.

Materials

Prelab Questions

1. What is the purpose of this experiment?
2. What is used as the control in this experiment? Why is it an ideal control?
3. Give an example of a physical and chemical change for each indicator below.
   1. Bubbles forming
   2. Color change
   3. Solid forming

Safety Precautions

The pH 2 and pH 12 buffer solution are corrosive to skin and eyes. Bleach is a corrosive liquid that causes skin burns. It may also generate chlorine gas when mixed with acid and when heated. Sodium thiosulfate is slightly toxic by ingestion and is moderately toxic by ingestion and inhalation as well as a body tissue irritant. FD&C dyes are slightly hazardous by ingestion, inhalation, eye and skin contact. Red No. 40 may be absorbed through skin. Calcium chloride is also slightly toxic. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Place the 24-well reaction plate on a piece of white background paper as shown in Figure 1.
   {12156_Procedure_Figure_1}
2. Using a clean graduated pipet, add 1 mL (20 drops) of FD&C Blue No. 1 to each well A1–A6.
3. Using a clean graduated pipet, add 1 mL of FD&C Red No. 3 to each well B1–B6.
4. Using a clean graduated pipet, add 1 mL of FD&C Yellow No. 6 to each well C1–C6.
5. Using a clean graduated pipet, add 1 mL of FD&C Red No. 40 to each well D1–D6.
6. Using a clean graduated pipet add 1 mL (20 drops) of deionized or distilled water to each of the four wells in the first vertical columns (wells A1–D1).
7. Repeat the procedure from step 6 in the five remaining columns using the following test solutions and a clean, graduated pipet for each solution. Add 1 mL of pH 2 buffer solutions to wells A2–D2; pH 12 buffer solution to wells A3–D3; calcium chloride solution to wells A4–D4; bleach to wells A5–D5; and sodium thiosulfate solution to wells A6–D6.
8. Stir each well with a toothpick, rinsing the toothpick with distilled water between wells.
9. Allow the solutions to stand for 15 minutes and record all observations in the data table. Note that the first column A1–D1 contains only the dyes and DI water—use these wells as controls for color comparison purposes.
10. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12156_Student1.pdf