Name: Julie

Region: Tennessee

Message: I do a lot of full immersion dyeing using Procion MX dyes and your site has been a tremendous help to me. I don't know that my question is very important, but it's something I've been curious about for a long time. I've noticed that at the stage I add my soda ash solution to the dye bath, the color of the bath often changes. Most frequently, it becomes more yellow than previously. As far as I can tell, the color of my fabric does not change when I put it back in the dye bath after adding the soda ash. Can you tell me what causes the color change? And does it affect the color of my fiber and I just can't tell, or does it only affect the dye bath itself?

I've noticed this myself. What's happening is that many dyes can act as pH indicators. They are a different color at a high pH than they are at a low pH. The exact pH at which the color changes is specific to each dye. Here's an example of a Procion MX dye that is two different colors at different pHs:


(from "Boysenberry MX dye turns red with high pH", a post I made three years ago in the Dye Forum.) The bluish red color on the left is PRO Chemical & Dye's Boysenberry (a "pure" unmixed single-hue dye, which I think is magenta MX-B, also known as reactive violet 13), in plain water, while the orangish red color on the right is the same dye with soda ash added. At the higher pH, the dye also seems a bit weaker in color. The color of the dye in the fiber is affected similarly to the color you see in the dyebath.

The reason for the color change is that a low pH allows a proton (a positively charged particle, which is a hydrogen ion) from the water to join the chemical structure of a dye. When the pH is increased above a certain point (a point which is different for different dyes), there are fewer available protons in the solution, so this extra proton leaves the dye molecule. The color of a molecule depends directly on what wavelengths of light are reflected by it. Any changes in the chemical structure of a dye molecule will also affect the energy of the light that it absorbs, and therefore the color (if any) that is reflected by the molecule. The loss or gain of an extra proton is enough to make big changes in the energy of the light that the electron bonds in the molecule can absorb, and therefore in the color of the dye.

The color that the dye changes to when the soda ash (or other pH changer) hits it is not permanent. The final color of the dye will depend on the final pH. Once you have washed out the soda ash, the pH will be low enough that the color of the dye in your fiber will go back to the original hue.

You can see the same sort of color change in many colored substances. It's the basis for pH indicators, such as pH paper, which changes color according to the pH of the solution you dip it in. You can even see it in some foods. For example, the anthocyanin pigments that color red cabbage, grapes, and blueberries all change color according to pH. If you add an acid (low pH) to an anthocyanin pigment, it will turn it red, but a basic (high) pH will turn it blue. Compare the color of a bit of red cabbage juice in vinegar, which is acid, to the color in water to which a bit of baking soda has been added. If you make the pH high enough, it will turn green. Add more acid and the color will return to a more reddish color.

Since the colors of the textile dyes you're using revert to the neutral-pH color once the pH is neutralized, the only practical aspect to this color change, in dyeing fabric, is that it means you cannot eyeball your color mixtures, for being the color you're trying to get, once you have added soda ash.

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