Dyeing – three colours from the same dye-bath

Read our standard health & safety guidance

Lesson organisation

This is likely to be performed as a teacher demonstration, in the context of the topic of organic dyestuffs, either as an introductory attention-grabber, or later in the topic to stimulate discussion of the chemical interactions involved in dyeing. If the materials for the demonstration have been fully prepared, the demonstration itself should take around 30 minutes. The demonstration can be scaled up as required for larger audiences.

Apparatus and chemicals

Eye protection

Beakers (400 cm³), 4
Large watch glasses or petri dishes, 4
Tongs or forceps
Scissors
Sample tubes, stoppered, 6
Bunsen burner
Tripod
Gauze
Heat resistant mat
String and paper clips, crocodile clips or clothes pegs (see note 3)

Access to:

Top pan balance

For one demonstration:

Samples of the following fabrics in white (see note 2):
wool, silk, nylon, cotton, polyester, cellulose acetate (‘triacetate’), polyester/cotton mix.

Dyes:
Acid blue 40, 0.06 g (Low hazard)
Disperse yellow 7, 0.04 g (Irritant)
Direct red 23, 0.04 g (Irritant)

For all three dyes refer to CLEAPSS Hazcard 32 and see notes 3 and 4
Hydrochloric acid, 2 mol dm^-3 (Irritant), small quantity

Technical notes

Hydrochloric acid, (Irritant). Refer to CLEAPSS Hazcard 47A and Recipe Card 31

1 Prepare a ‘clothes line’ using string between laboratory stands, long enough and with enough ‘pegs’ (crocodile clips, paper clips or miniature clothes pegs) to hang out the dyed fabric samples.

2 About 100 cm² of each fabric, or a few cm of thread will be sufficient. As a minimum, samples of cotton, either polyester or cellulose acetate, and either wool, silk, or nylon are required. White nylon, 100%, can be difficult to obtain and it may be necessary to try a second-hand clothes shop. Cut four strips of each fabric (about 4 cm x 4 cm is suitable). Each fabric should be easily identifiable in some way, for example by cutting different shapes.

3 Preparing the dye samples:

Bottles should be opened in a fume cupboard. The dilute solutions are low hazard but will stain skin and clothes. Protective gloves (preferably nitrile) should be worn. Weigh out into stoppered sample tubes, two samples of 0.02 g of each of the red and yellow dyes, and two samples of 0.03 g of the blue dye. Label each sample tube. These dyes are available from Philip Harris Ltd (www.philipharris.co.uk) or from Kemtex Educational Supplies (http://www.kemtex.co.uk - see link to Dyes for Salters Advanced Chemistry).

4 If the teacher requires the dye bath to be made ready before the lesson, dissolve 0.02 g of each of the red and yellow dyes and 0.03 g of the blue dye in 200 cm³ of water in a beaker, add a few drops of dilute hydrochloric acid and heat to boiling.

Procedure

a If the dye bath has yet to be prepared, dissolve 0.02 g of each of the red and yellow dyes and 0.03 g of the blue dye in 200 cm³ of water in a beaker, add a few drops of dilute hydrochloric acid and heat to boiling.

b Place a sample of cotton, cellulose acetate and either wool, silk or nylon in the dye bath and simmer gently for about 10 minutes.

c Remove the fabrics with forceps or tongs, rinse under running water, and hang up on the ‘clothes line’. Cotton will be dyed red, acetate yellow and wool, silk or nylon blue-green. (Some of the yellow direct dye will take to these materials as well as the blue acid dye.)

d Try other materials as well if desired. Polyester will be dyed yellow and polyester/cotton will become orange.

e Now examine the effect of the dyes individually. Make three dye baths, the first containing 0.02 g of red dye in 200 cm³ of water, the second containing 0.02 g of the yellow dye in 200 cm³ of water and the third containing 0.03 g of the blue dye in 200 cm³ of water. Add a couple of drops of hydrochloric acid to each dye bath and heat to boiling.

f Place a sample of each fabric in each dye bath and treat as before, ie simmer for 10 minutes, remove the samples and rinse. Typical results are shown in the table.

Teaching notes

Different dyes bond to fabrics in different ways.

• Acid dyes contain acidic –CO₂H and –SO₃H groups which bond to the basic –NH groups in the amide linkages of wool, silk and nylon.

• Direct dyes bond by hydrogen bonding and take well to cellulose-based fibres such as cotton, viscose and rayon which have many –OH groups.

• Disperse dyes are not water-soluble. They exist in the dye-bath as a fine suspension (hence the name), and are absorbed as a solid solution by hydrophobic fabrics such as polyesters.

After the demonstration, students who have already studied the chemical structures of different types of fibre could be asked to predict the effects on other fabrics, for example on silk and nylon, which are polyamides like wool. They could also be asked to predict the effect of the dyes on a mixed fabric, such as cotton-polyester. They may now be able to offer a possible explanation for some odd effects in washing machine accidents, where labels and trim may emerge a different colour to the rest of the garment.

This demonstration could be followed up by various investigations, for example:

1 Devising mixed dye-baths to produce different colours to the ones demonstrated using the chemical principles fabrics described above.

2 Investigating the effect on the colours produced in different fabrics of:
• mordants such as salt or alum (Low hazard, refer to CLEAPSS Hazcards 47B or 2B)
• pH of the dye bath
• time in the dye bath
• temperature of the dye bath

3 How fast (resistant to washing out) are the dyes to a variety of treatments?

A good account of which types of dyes dye which fabrics is given in The Essential Chemical Industry, p 42. University of York: The Chemical Industry Education Centre, 1989.

Kemtex Educational Supplies (see website address below) provide kits of dyes and fabrics for similar demonstrations.

Health & Safety checked, May 2008

Web Links

Kemtex Educational Supplies (http://www.kemtex.co.uk) provide a range of information sheets about dyes and dyeing, and related processes such as transfer printing, as well as supplying a wide range of dyes to school Art & Design and Textile Departments.

The Bakken Library in Minneapolis, USA (
http://www.thebakken.org/education/SciMathMN/plant-dyes/dyes1.htm ), provides a cross-curricular unit called ‘A Lesson to Dye For’, which also highlights investigative skills. The level of chemistry is not high, but the range of ideas is very wide, and teachers may find some useful ideas for follow-up work.

The University of Regensburg, Germany (http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Video-e.htm), provides a wide range of video clips of chemistry experiments at an advanced level, including a number relating to dyes and dyeing.

(Websites accessed June 2008)