Material and chemicals:
| cotton yarn
| | Indanthren Yellow G (BASF)
| | sodium hydrosulfite (sodium dithionite)
| | sodium perborate
| | calcined soda (sodium carbonate anhydrous)
| | sodium hydroxide solution (32.5 %) | |
Apparatus and glass wares:
| hotplate
| | thermometer |
| glas stirring rod
| | metal forceps
| | crystallizing dish d = 12 cm
| | beaker 50 mL
| | 2 beakers 500 mL
| | 2 snap-cap vials 10 mL |
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Hazards and safety precautions:
 |
Sodium hydroxide solution is corrosive! Contact with skin can cause irritation or severe burns and scarring with greater
exposures. Swallowing may cause severe burns of mouth, throat, and stomach. |
| Sodium dithionite and sodium perborate are harmful if swallowed or ingested, and through skin contact.
Skin and eye irritant. |
Safety goggles and protective gloves must be worn.
Experimental procedure:
0.3 g of Indanthren Yellow G, 3 mL of sodium hydroxide soultion (32.5 %) and 1 g of sodium ditionite are dissolved in 200 mL of
dist. water heated to 50°C. After 10 minutes 10 g of cotton yarn are added to the solution. The dyeing takes place at about
50°C. After 30 minutes the yarn is washed. The washed substrate is placed in a beaker containing 200 mL of dist. water in
which 0.5 g
sodium perborate had been dissolved. The temperature of the solution is 50°C. After 15 minutes the impregnated yarn is washed
with cold water. Now the textile fiber is soaped in a solution of soap (0.5 g) and calcined soda (0.2 g) in 200 mL dist. water.
The yarn is washed again.
Result:
 Video clip (Download RealPlayer .rm file)
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Discussion and background:
"Vat dyeing" means dyeing in a bucket or vat. Vat dyes are insoluble in water and incapable of dyeing fibers directly. Reduction in
alkaline medium produces the water soluble alkali metal salt of the dye. In the reduced form the dye is applied to impregnate the
fiber. The dye absorbed on the fiber is converted to its original water insoluble form by a subsequent reoxidation. Vat dyes have
excellent wash and light fastness properties.
In the experiment described above the anthraquinone dye Indanthren Yellow is reduced with sodium hydrosulfite (sodium dithionite).
The reduction occurs at the keto groups (C=O) in the vat dye molecule. These groups are reduced to the acid form (C-OH) (1)
and, in the presence of excess alkalinity, form the soluble sodium salt (C-ONa).
Perborate is used as oxidising agent to regenerate and fix the insoluble dye on the fiber. Calcined soda (sodium carbonate), also
known as washing soda, helps the dye bond with the fibers.
References:
Demonstration Experiment on Video
Dyeing with Substantive Dyes
Vat Dye
Index of Lecture Experiments
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