Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the fabric test strips provided in the kit into ½–1" wide strips, depending on how many students are dyeing. If one end of the test strip is marked with a permanent marker, it will make fabric identification easier. The wool fabric is beige and is a good marker. Use sharp scissors and pull the fabric taut to facilitate cutting the fabric.
2. Cut the cotton fabric into strips 1–2" by 8–12", depending on the number of students in your class and the size of the dyeing beaker. An old, white cotton T-shirt or pillowcase is a good source of cotton fabric.

Safety Precautions

Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Indigo dye and especially the leuco base are very permanent dyes; they will stain clothes, paper, wood products and hands; wear gloves when performing the dyeing process. Sodium dithionite is a strong reducing agent; the solution is corrosive. Always wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please 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. The aqueous solution of leucoindigo can be disposed of by flushing down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• 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. Enough materials are provided to dye several pieces of fabric.
• Other cotton items like socks, headbands or handkerchiefs can also be brought in and dyed.
• Be very careful with the indigo dye—it is a very permanent dye and will readily dye body tissue, books, clothes, carpeting and any other organic-based item. Wear gloves and keep other material off the lab table.
• Warn the students to make sure the cap is firmly on the vial before shaking and placing it in the water bath.
• To save space and set-up time, three to four groups can use the same hot water bath. Make sure students label their vials.
• Covering the beaker that contains the luecoindigo dye solution will slow the development of the blue indigo dye layer on top of the solution. The blue layer will start to form almost immediately and will not affect the dyeing process. Cover the beaker with a watch glass, weighing dish, or piece of Parafilm^®.
• When concentrated in the reaction vial, the yellow leucoindigo will appear dark amber; this is normal. When “diluted” with tap water, the yellow color appears.

Answers to Prelab Questions

1. Describe the solubility and color of indigo and leucoindigo.

   Indigo is insoluble in water and is a deep blue color. Leucoindigo is soluble in water and is yellow in solution.

2. What is the approximate molar concentration of the final vat dye solution (step 13)?

   Indigo has a formula weight of 262.26 g/mol. If 0.1 g of indigo is used in the reaction and the final volume of solution is 50 mL, then the molarity is:

   {11987_PreLab_Equation_1}

   This is a very dilute solution and should provide some idea of how potent dyes can be.

3. Why are gloves necessary during this laboratory procedure?

   The sodium hydroxide solution is corrosive to body tissue and sodium dithionite is a body tissue irritant. In addition, the indigo dye is a very strong dye and will stain fingers and many other organic materials.

Sample Data

The fabric test strip is included in this kit to introduce textile science to your students. Clothes are very important to many students and they may not be aware that chemistry is very important in the preparation of the fabric and in the developing of dyes that work well with the different families of fabrics. A fabric test strip will be used to determine how well indigo is able to dye various types of fabrics.

The mechanism by which the dye molecules absorb onto the fibers depends on the chemical composition and physical structure of the polymeric chains that comprise the fibers. The porosity, or the packing of the polymer chains, determines how easily the dye molecules penetrate the fibers. Chemical attractions, such as hydrogen bonding, salt formations, or Van der Waal forces help keep the dye molecules absorbed on the fibers.

The fabric test strips consist of six different types of fabrics and are identical to those used in the textile industry to study dyes and their color fastness. The fabrics in the test strip represent major families of polymers used in fibers.

The indigo will dye the six fabrics differently depending on how well the fabric accepts or holds on to the dye. Cotton is easily dyed because of the relatively open structure of the cellulose polymer. In polyester, the polymer chains are closer, resulting in a dense, more closely packed structure. The dye molecules are simply too large to penetrate the polyester fiber and hence, polyester is not easily dyed. When dying the test fabric, cotton will be the darkest, followed by wool. The acetate and nylon are a lighter blue. The polyester and acrylic will be the lightest blue.

In addition to dying the fabric test strips, we recommend dyeing 100% cotton fabric. Indigo is a great dye for cotton and the result looks very much like a new pair of blue jeans (the unwashed type). An economical way to obtain cotton fabric is to ask your students to bring an old white T-shirt, towel, sheet, or pillowcase. Cut the item into 2" x 10" strips, which will fit nicely into the 100-mL beaker. By combining several solutions into a larger beaker, large items like socks can be dyed.

Answers to Questions

1. Describe the color of the reaction solution during each step of the experiment.

   The initial “solution” of indigo and sodium hydroxide is blue but it is more of a suspension than a solution.
   
   The solution becomes more greenish-yellow after the sodium dithionite is added to the reaction. After more dithionite is added and the solution heated for a few minutes, the solution is a clear, deep (amber) color.
   
   When the concentrated leucoindigo solution is added to the tap water containing sodium dithionite, the solution becomes yellow. However, the surface of the solution starts to turn blue due to air oxidation.

2. Describe the colors associated when dyeing cotton fabric. Does all the cotton turn the same color at the same time?

   The fabric is yellow in the leucoindigo solution. There are some blue splotches due to air oxidation on the surface of the solution.
   
   As the fabric is removed, it immediately begins to turn yellowish-green, then bluish-green and then a deep blue color. The areas of the fabric that are exposed to air turn blue quicker. The fabric that is folded over on itself does not turn blue as quickly. In the end, the color is fairly uniform through out the fabric.

3. Does indigo dye all six fabrics on the fabric test strips equally? Describe the color differences.

   No, cotton and wool accept indigo the best and are deep blue. The acetate and nylon accept some dye and are a lighter shade of blue. The polyester and acetate accept the dye the least and are a very pale blue.

4. Why might some fabrics be dyed by indigo easier than others?

   The are many possible answers. The simplest answer is “like dissolves like.” Fabrics that have a similar polarity to indigo will be more willing to accept the indigo dye. Think of the fabric like a solvent and the dye as a solute. Some fabrics are hydrophobic and are not easily wetted. If the fabric is not easily wetted, the dye will not come in contact with the fabric. Yet another explanation may be that the fabric may not have any functional groups that will hydrogen bond to the indigo molecule. The hydrogen bonding and other attraction forces help retain the dye molecules. Lastly, some fibers are very dense and the dye molecules cannot get into the fiber.

5. Why does bleach remove the indigo dye from blue jeans?

   Bleach is a good oxidizing agent and oxidizes the indigo to a variety of smaller molecules. These smaller molecules lose their conjugated, carbon-carbon double bond systems that provide the color. They are also water soluble and may be removed from the fabric.

6. Why do blue jeans fade over time?

   As the fibers break down, the trapped indigo dye molecules can escape the fabric. This leads to the faded color.

7. Why is the leuco base of indigo water soluble?

   Like dissolves like. The polar dianion is easily dissolved in the polar water solvent.

8. How could the manufacturing process for dyeing clothes with indigo be converted into a continuous process?

   Answers will vary. One possibility is to have one vat of leuco base indigo and one vat of an oxidizing agent. Clothes could be continuously added to the leuco base vat and then removed and placed into an oxidizing vat.

Introduction

Blue jeans are an American fashion staple. The chemistry behind the color is rich in history and is a practical application of redox and organic chemistry.

Concepts

• Dyes and dyeing
• Organic chemistry
• Consumer chemistry
• Oxidation/reduction

Background

The preparation of dyes and the process of dyeing fabric are two of the oldest chemical processes developed by humans. The use of dyes is an ancient art that was first practiced in Egypt, Persia, China and India more than 5,500 years ago. The earliest dyes include madder, a red dye, and indigo, a blue dye. Indigo was originally extracted from the Indigofere sumatrana plant found primarily in India and later from the Isatis tinctoria plant found in Europe. The Indigofere plant produced a richer dye and during the Roman Empire, a vigorous trade route was established between India and Europe to supply this rich blue dye to Europe’s growing textile market.

The extraction and purification of natural dyes continued as a growing industry until the late-1800s when synthetic dyes were developed. The discovery of synthetic dyes was due to scientists beginning to understand more about the structure of molecules, chemical reactions and the synthesis of common materials, such as dyes. Much of the early history of organic chemistry involved the study and synthesis of dyes which led to the development of the modern chemical industry.

The first synthetic dye was prepared in 1856, when William Henry Perkin, a student at the Royal College of Chemistry in England, accidentally discovered a synthetic mauve dye. This led to the first commercial development of an organic compound. Not long afterwards, German chemists developed additional dyes and the synthetic dye industry grew rapidly in Germany. In the 1880s, Adolph von Baeyer undertook the study of indigo and determined its molecular structure and the chemical reactions to synthesize it. For his work on indigo, von Baeyer received the Nobel Prize in chemistry in 1905. However, because of the difficulties in providing the dye in large quantities (scaling up chemical reaction is not always easy), synthetic indigo was not commercially available until after 1900. By World War I, almost all the world’s manufactured dyes were produced in Germany and synthetic dyes had almost entirely replaced the extraction of dyes from natural sources.

Today, almost all dyes are still synthetically produced and are an important part of the chemical industry. The thousands of available dyes come in all colors and are used to color paper, plastics, leather, foods, cosmetics and fabrics, such as those used for clothes, linens and carpets.

Science of Dyes

Dyes are composed of intensely colored molecules that strongly absorb light in the visible range (400–700 nanometers). Only organic molecules with several connected (also called conjugated) double bonds are capable of absorbing enough light in this region to be effective dyes. Conjugated systems connected to aromatic rings, with electron-withdrawing and -attracting groups, are a common feature of dyes. Many dyes belong to similar families and have the same general molecular structures, differing only by a different electron-withdrawing or -attracting group. Some common dyes are shown in Figure 1.

Indigo belongs to a class of dyes called vat dyes. Vat dyes are the oldest known dyes and the term “vat” applies to the vessel used to extract and ferment the dye from its natural sources. A key feature of vat dyes is that they are water-insoluble dyes that are reduced to a water-soluble species known as a leuco base. There are many different types of vat dyes but all vat dyes share one common trait: they contain one or more carbonyl groups. The carbonyl group in a vat dye is reduced to the sodium salt by treatment with a reducing agent in the presence of a base. The sodium salt is then soluble in water.

Leuco bases are attracted to cellulose fibers, such as cotton and paper, and form strong hydrogen bonds to the cellulose structure. The dye molecule also penetrates the fiber structure and attaches to the cellulose molecule inside the fiber. Leuco bases are oxidized back to an insoluble dye when exposed to air. By this time the insoluble dye molecule is trapped within the molecular structure of the cellulose polymer and is not easily removed. This physical trapping and chemical insolubility of the dye molecule gives vat dyes their unusual resistance to fading. The most common vat dye is indigo which is used to give the traditional blue color to blue jeans.

Dyeing with Indigo

Indigo is insoluble in water but is easily reduced by sodium dithionite (sodium hydrosulfite, Na₂S₂O₄) in a strong alkaline solution to produce a water-soluble leucoindigo (see Figure 2). This leuco base is strongly attracted to cellulose. After the reduced dye has been absorbed on the fiber, the original insoluble dye is reformed by oxidation with air or chemicals. The colors from this dyeing process are very resistant to washing because the dye is insoluble in water. A fabric test strip will be used to determine how well indigo will dye the various types of fabrics. The mechanism by which the dye molecules absorb into the fibers depends on the chemical composition and physical structure of the polymer chains that comprise the fibers. The porosity, or the packing of the polymer chains, determines how easily the dye molecules penetrate the fibers. Chemical attractions, such as hydrogen bonding, salt formations or Van der Waal’s forces, help keep the dye molecules absorbed on the fibers.

The fabric test strip consists of six different types of fabrics (see Figure 3). These fabrics represent six major families of polymers used in fibers.

Acetate: Cellulose acetate polymer; prepared by acylating the hydroxy groups on cellulose.
Cotton: A natural organic polymer consisting of >95% pure cellulose, a linear polymer of glucose.
Nylon 66: Polyamide polymer; prepared by the condensation reaction between 1,6-diaminohexane and adipic acid or adipoyl chloride.
Polyester: Polyester polymer; prepared by the condensation reaction between ethylene glycol and terephthalic acid.
Acrylic: Polyacrylonitrile polymer; prepared by the polymerization of acrylo nitrile. Other olefins may also be added.
Wool: A natural fiber obtained from the fleece of sheep; consists of protein chains bound together by disulfide linkages. (Wool is light beige in color.)

Materials

Prelab Questions

1. Describe the solubility and color of indigo and leucoindigo.
2. What is the approximate molar concentration of the final vat dye solution (step 13)?
3. Why are gloves necessary during this laboratory procedure?

Safety Precautions

Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Indigo dye and especially the leuco base are very permanent dyes that will stain clothes, paper, wood products and hands; wear gloves when performing the dyeing process. Sodium dithionite is a strong reducing agent; the soluton is corrosive. Always wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. Setup

1. Prepare a hot water bath using 250 mL of water, a 400-mL beaker and a hot plate, Bunsen burner setup, or immersion heater. Note: Water should be just below boiling (80–90 °C).
2. Place about 0.1 g of indigo dye and about 5 mL of 1 M sodium hydroxide solution in the reaction vial. Note: The indigo is a dye and will stain fingers, clothing, and many other organic materials. Handle with care.
3. Seal the vial with the cap. Make sure the cap is on tight. Shake the reaction mixture. The indigo will not completely dissolve in the sodium hydroxide solution.
4. Optional: Wrap a 6-inch piece of copper wire around the cap to serve as a handle.
5. Place the sealed vial in the hot water bath. Note: These are special vials designed to be heated while sealed.
6. After 5 minutes, remove the vial from the hot water bath and place the vial on the counter. As soon as the vial is cool enough to touch (a few seconds), carefully open the reaction vial. Wear chemical-resistant gloves and safety goggles because the reaction vial may be pressurized and some solution may squirt out. Note the appearance of the solution.
7. Add about 0.9 g of sodium dithionite to the reaction vial. Seal the vial again with the cap. Make sure the cap is on tight. Vigorously shake the reaction mixture. Note: Water should be just below boiling (80–90 °C).
8. Place the sealed vial in the hot water bath.
9. Continue heating the indigo solution until the indigo completely dissolves and a clear, dark amber solution is produced. The amber solution indicates the formation of the leucoindigo form of the dye. If the solution turns orange dark amber but a blue solid remains on the bottom or top of the vial, remove the vial and shake the vial again. Heat until the solution is clear.
10. Optional: If after 6–8 minutes the solution is still dark blue, remove the vial from the hot water bath and place it on the counter to cool. As soon as the vial is cool enough to touch (a few seconds), carefully open the reaction vial. (Note: Wear chemical-resistant gloves and safety goggles because the reaction vial may be pressurized and some solution may squirt out.) Add another 0.1–0.2 g of sodium dithionite to the vial. Seal the vial again and shake vigorously before placing it in the hot water bath. Heat until the solution is a clear amber. Note the appearance of the solution.
11. Continue to heat the solution for 2–3 minutes after a clear yellow solution has been produced. Use tongs to remove the vial from the hot water bath and let it cool for a minute.
12. Place 40–50 mL of tap water and a small spatula (about 0.2 g) of sodium dithionite in a 100-mL beaker. Stir briefly with a glass stirring rod or metal spatula.
13. Carefully open the reaction vial. Wear chemical-resistant gloves and safety goggles because the reaction vial may be pressurized and some solution may squirt out.
14. Quickly add the amber leucoindigo solution to the beaker containing dilute sodium dithionite solution. The solution will be clear and yellow but may turn blue at the surface due to air oxidation.
15. Cover the beaker with a watch glass or weighing dish to prevent anything from falling in the solution.

Part 2. Dyeing the Fabric

16. Place the beaker containing the leucoindigo dye on a lab spill mat or several thicknesses of paper towels. Note: Indigo dye is messy. Use lots of paper towels or a laboratory spill mat.
17. Carefully place the fabric test strip into the leucoindigo dye solution. Using a glass stirring rod or metal spatula, work the fabric into the solution until it is completely saturated with the leucoindigo solution. As the fabric soaks in the solution, it will turn a bright yellow. The top of the solution will be blue due to air oxidation of the leucoindigo.
18. After the fabric has soaked for about 2–3 minutes, it is ready to be removed. Wear chemical-resistant gloves and safety goggles and carefully remove the fabric test strip using a glass stirring rod, metal spatula or tongs and hang it somewhere to dry. Indigo dye solution will drip from the fabric. Be careful not to drip any indigo dye solution on any fabric or other cellulose-type materials (e.g., carpeting, books, papers); it will stain. Place several paper towels below the dripping fabric to catch any drips. Wear chemical-resistant gloves to avoid getting any dye on your hands.
19. As the fabric is exposed to air, the yellow leucoindigo is oxidized back to blue indigo. This reaction can be followed by watching the fabric turn from yellow to green to blue-green and finally to a deep blue color. Make sure all the fabric is exposed to air and turns blue.
20. After 5–10 minutes, the fabric should be completely blue. Once completely blue, wear chemical-resistant gloves and thoroughly wash the fabric with cold water until the water is no longer blue.
21. With your instructor’s permission, other items or pieces of fabric may be dyed.
22. The aqueous solution of leucoindigo can be disposed of by flushing down the drain with excess water.

Post-Lab Questions (Answer on a separate sheet of paper.)

1. Describe the color of the reaction solution during each step of the experiment.
2. Describe the colors associated when dyeing cotton fabric. Does all the cotton turn the same color at the same time?
3. Does indigo dye all six fabrics on the fabric test strip equally? Describe the color differences.
4. Why might some fabrics be dyed by indigo easier than others?
5. Why does bleach remove the indigo dye from blue jeans?
6. Why do blue jeans fade over time?
7. Why is the leuco base of indigo water soluble?
8. How could the manufacturing process for dyeing clothes with indigo be converted into a continuous process?