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The Chemistry of Indigo, page 2 of 2

Understanding pH in the Indigo Dye Vat

An indigo dye vat provides a reducing environment to keeping the indigo dye in solution. In this form indigo will penetrate and adhere to fabric fibres. A natural fermenting indigo dye vat operates at about pH 9.5. The efficiency of a vat can be improved with the use of a very strong reducing agent like sodium hydroxide. Ancient Blue Crystal Indigo is a ready made dye vat in a desiccated form. When water is added the instant dye vat is ready to be used. The pH of this type of dye vat can be very high indeed and could cause skin damage. The diagram below demonstrates how pH can be reduced using dilution and how much dilution is required for one unit reduction in the pH.

Figure 1. Two volume measurements are required 50 and 500 ml.

Figure 2. One gram of freeze dried indigo vat is placed in the 50ml measure and topped up with warm water. pH = 13

Figure 3. The first dilution is added to 450 ml of water. pH = 12

Figure 4. Take 50ml from the large cup and add this to 450ml of water. pH = 11

Figure 5. Take 50ml from the large cup and add this to 450ml of water. pH = 10

Figure 6. Take 50ml from the large cup and add this to 450ml of water. pH = 9

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