Microscale Synthesis of Indigo in Vat Dyeing

Last Updated on 04/09/2021

Dyes preparation and fabric dyeing 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 5500 years ago. Indigo has been used for coloration of textiles since antiquity. Indigo was originally extracted from the Indigofere Sumatrana plant found primarily in India and later from the Isatis Tinctoria plant in Europe. Due to the demand of this supply of blue dye, a trade route between India and Europe was established. Natural indigo extracted from the Indigofera tinctoria plant species maintained its dominant position in the international market until the synthesis of indigo in 1897. In this article we will know the details of microscale vat dyeing with indigo.

Microscale Vat Dyeing

Despite its long history and current importance, it is surprising that indigo must be classified as a vat dye, because the present application methods for indigo and the fastness properties of indigo dyeing are not typical of vat dyes as a whole. Vat dyes show outstanding colorfastness properties, particularly to light, washing and chlorine bleaching; while indigo dyeings have poor washing and chlorine fastness and poor abrasive resistance

The first synthetic dye was generated in 1856, when William Henry Perkin, a student at the Royal College of Chemistry (England) accidentally discovered a synthetic mauve dye. This in turn, led to the first commercial development of an organic compound. In the 1880’s, Adolph von Baeyer undertook the study of indigo and determined its molecular structure and synthetic routes. For this work, von Baeyer received the Nobel Prize in chemistry in 1905. It was commercially available until after 1900 due to the problems faced when trying to mass produce the invention. However, by WWI, 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.

Microscale Vat Dyeing with Indigo:
To fully understand the process of microscale vat dyeing with indigo, we must first look at the chemistry behind it. Indigo dye (indigotin) is derived from indican, which is a water-soluble, colorless substance present in indigo-bearing plants. When indigo-bearing plants are crushed and placed in water the indican is released into the solution through bacterial action. When air is beaten into the solution, the indican combines with oxygen to produce indigotin or “indigo blue.” Indigo blue is insoluble and settles out of the solution to produce the blue indigo dyestuff.

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. This leuco base is strongly attracted to cellulose because they have large pores into which the dye molecules easily fit. Upon exposure to the oxygen in the air, the dye molecule rapidly oxidizes to its insoluble form, but by then it has already been trapped inside the cellulose fiber; due to its immense size, it cannot be removed at all easily. When a fiber is placed in the indigo white solution, the indigo white penetrates into the fibers and makes loose chemical combination which is oxidized back to the insoluble indigo blue. The colors from this dyeing process, the vat method, are very resistant to washing because the dye is insoluble in water. However, synthetic fibers are not good candidates for vat dyeing because they have tightly packed fibers not easily penetrable by dye molecules. For this lab we will be using cotton which is a natural organic polymer consisting of >95% pure cellulose, a linear polymer of glucose.

Dyeing with Indigo

Equipments and Materials:

  • 1 Paper Clip
  • 1 25-mL graduated cylinder
  • 1 Sheet of Weigh Paper
  • Pairs of Gloves
  • 1 Large Weigh Boat
  • 1 Stirring Rod
  • 1 2 X 3 inch piece of aluminum foil
  • 1 600-mL beaker
  • 4 Shoestrings
  • 1 Small Weigh Boat
  • 1 125-mL Erlenmeyer Flask
  • 1 Hot plate with stirring rod

Working Procedure:

  1. Put on your gloves, Indigo will stain your skin.
  2. Fill a large beaker 2/3 full with water. (Warm if Available)
  3. Add both pairs of shoestrings to the water.
    Note: This will remove any sizing material that could interfere with the dye adhering to the fabric.
  4. Add 75 mL of water to the Erlenmeyer flask.
    Note: For the second set of shoe strings, you may need to replenish the water; make sure there is the full amount of water in the Erlenmeyer flask each time.
  5. Add 15 mL sodium hydroxide solution to the same Erlenmeyer flask.
  6. Mass 0.12-0.14 g of indigo on weigh paper and add it to the flask.
  7. Carefully add a magnetic stirrer. Do this by tilting the flask and allowing the stirrer to slide down the wall of the flask until it is on the bottom.
  8. Cover the top with a small square of foil.
  9. Straighten out the paper clip. Use the end to poke two small holes in the foil top.
  10. Mass 1 g of sodium dithionite on the small weigh boat and set aside.
  11. Turn on the hot plate and stirrer, and gently heat the flask to near boiling over medium heat (5-10 minutes). If necessary, turn up the heat.
  12. Remove the foil from the Erlenmeyer flask. From the small weigh boat, add the sodium dithionite until the indigo dissolves and a clear yellow solution appears.
    Note: If the solution does not turn yellow, add more sodium dithionite in 0.05 g increments.
  13. Immediately replace the foil.
  14. Continue heating for 2 minutes. Then, remove, the flask from the heat.
  15. Remove one pair of shoestrings from the beaker. Wring any excess water out of the shoestrings.
    Note: For a “tie dye” effect, loosely tie slipknots along the length of the shoestring before immersing it in the dye bath.
  16. Turn off the magnetic stirrer. Do not turn off the heat.
  17. Gently swirl the flask, then remove the foil cover from the flask and add one set of shoestrings.
  18. Leave the shoestrings in the dye bath for 5 minutes.
  19. Label the weigh boat with your names.
  20. Remove the shoestrings from the dye bath and place them in the weigh boat.
    Note: If the solution turns blue during the dyeing process or after you remove the shoestrings, and then add more sodium dithionite solution in 0.5 g increments until the solution becomes clear and yellow. Also, slightly increase the heat.
  21. Allow the shoestrings to dry overnight.
  22. Repeat steps 15-21 for the second set of shoestrings. Add more water if necessary as noted in Step 4.

Additional Notes: 20% sodium dithionite solution may be used but it must be fresh.

In step 10, use 5 ml.
In Step 11 and 20, add more in 0.1 ml increments.

Questions on This Experiment:

  1. What type of compound is indigo?
  2. Could you dye fabric using a mixture of indigo and water? Explain.
  3. What do you think would happen if you did not soak the shoestrings prior to dyeing them?
  4. What color is the dye bath just before you add the shoestrings?
  5. What is the significance of this color?
  6. What happens to the shoestrings after you remove them from the dye bath?
  7. What is the significance of this color?

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