Create fabric dye by synthesising an azo dye and using it to change the colour of a piece of cotton
In this experiment, students prepare an azo dye and use it to dye a piece of cotton. The synthesis is unusual in that whereas most organic syntheses require ambient or elevated temperature, this synthesis requires low temperatures.
This practical should take 20 minutes.
- Beakers, 10 cm3, x3
- Aminobenzene (aniline)
- Hydrochloric acid (concentrated)
- Sodium nitrite
- 2-Naphthol (also called β-Naphthol; naphthalene-2-ol)
- Sodium hydroxide solution, 2 mol dm–3
Health, safety and technical notes
- Read our standard health and safety guidance.
- Students must wear suitable eye protection (splash resistant goggles to BS EN166 3).
- This experiment should be done in a fume cupboard.
- Aminobenzene (aniline) is TOXIC by all routes, a carcinogen and mutagen, a skin sensitiser, causes eye damage and is toxic to aquatic life .
- Hydrochloric acid, HCl(aq), is CORROSIVE and a respiratory irritant (see CLEAPSS HazCard HC047a).
- Sodium nitrite, NaNO2, is an OXIDISER, toxic if swallowed and hazardous to aquatic life (see CLEAPSS HazCard HC093).
- 2-Naphthol is HARMFUL if swallowed or in contact with skin and toxic to aquatic life (see CLEAPSS HazCard HC070b).
- Sodium hydroxide solution, NaOH(aq), 2 mol dm–3 is CORROSIVE (see CLEAPSS HazCard HC091a).
- Ethanol is highly FLAMMABLE (see CLEAPSS HazCard HC040a).
- Urea is of low hazard (see CLEAPSS HazCard HC035a).
- Put eight drops of aminobenzene in a 10 cm3 beaker and add 30 drops of deionised water followed by 15 drops of concentrated hydrochloric acid. Swirl the beaker and then put it in an ice bath.
- Weigh 0.15 g of sodium nitrite into another beaker and add 1 cm3 of deionised water. Cool the beaker in the ice bath. Add one spatula of urea (this prevents side reactions occurring).
- Mix the contents of the two beakers together and keep in the ice bath.
- Weigh 0.45 g of 2-naphthol into another beaker and add 3 cm3 of sodium hydroxide solution. Swirl to dissolve.
- Take a piece of cotton cloth 2 × 2 cm2 and, using tweezers, dip it into the 2-naphthol solution. Allow the solution to completely soak the cotton.
- Dip the cloth completely into the diazonium salt solution. A red dye forms in the fibres, dyeing the cloth.
- Take the cloth out, wash it under the tap and leave to dry.
The orange-red azo dye forms in the fibres of the cotton, dyeing the cloth. The melting point of 1-phenylazo-2-naphthol is 133 °C.
The urea decomposes excess HNO2 formed and prevents many side reactions from occurring. A better ‘red’ dye is usually produced.
The microscale synthesis of azo dyes - student sheetExperiment | PDF, Size 0.16 mb
The microscale synthesis of azo dyes - student sheetEditable handout | Word, Size 95.05 kb
The microscale synthesis of azo dyes – teacher notesExperiment | PDF, Size 0.12 mb
The microscale synthesis of azo dyes – teacher notesEditable handout | Word, Size 53.24 kb
S. W. Breuer, Microscale practical organic chemistry. Lancaster: Lancaster University, 1991.
This resource is part of our Microscale chemistry collection, which brings together smaller-scale experiments to engage your students and explore key chemical ideas. The resources originally appeared in the book Microscale chemistry: experiments in miniature, published by the Royal Society of Chemistry in 1998.
© Royal Society of Chemistry
Health and safety checked, 2018
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