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Jacquard Soda Ash Dye Fixer

Soda ash is a mild alkali that promotes the chemical reaction between fiber reactive dye and cellulose fiber. Soda ash contains 98% to 99% sodium carbonate

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Arm & Hammer Washing Soda

Washing soda contains about 37% sodium carbonate

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Procion MX Fiber Reactive Cold Water Dye

When mixed with soda ash, Procion dyes are permanent, colorfast, and very washable. You can easily create a palette of brilliant colors ranging from light pastels to deep, vibrant hues.

I'm afraid that that page of mine ("What is soda ash, and what's it for in dyeing?") is a little misleading, because it really does not matter that much exactly how much soda ash you use. Sodium carbonate is a wonderfully forgiving material for the purpose of setting the pH for a reaction with fiber reactive dyes; a wide range of concentrations will work just fine.

The usual amount of soda ash used in the presoak solution for tie-dyeing ranges from nine tablespoons per gallon (ProChem's recommendation) to one cup or sixteen tablespoons per gallon (Dharma's recommendation). ProChem says that nine tablespoons of their anhydrous soda ash is 80 grams; it's about 99% pure sodium carbonate. 

Assuming that the ProChem sodium carbonate is 100% pure, their soda ash presoak works out to be 80 grams per 4 liters, or about 0.75 mole in 4 liters, or about 0.2 molar (the unit used by chemists to describe the concentration of a solution). Doug Wilson on the DyersLIST mailing list calculated that this concentration produces a pH of 11.7. What you need is a pH that, after further dilution of the sodium carbonate soak in the fabric by the water you mix with your dyes, will be around 10.5 to 11, so this is certainly sufficient. The Dharma recommendation, for one cup of sixteen tablespoon of soda ash per cup of presoak, contains almost twice as high a concentration of sodium carbonate, but it still works fine.

The wonderful thing about sodium carbonate is that it produces a close-enough pH over a wide range of concentrations. This is very unlike a strong base, such as sodium hydroxide (NaOH), which produces one full unit change in pH for every ten-fold increase or decrease in concentration. Given that a certain amount of sodium hydroxide will produce a pH of 11.7, using one-tenth as much will produce a pH of 10.7. If you use only one-tenth the amount of sodium carbonate that will produce a pH of 11.7, though, you will get a pH of 11.2, not nearly as big a difference. 

This is because sodium carbonate is a weaker base, so not all of the sodium carbonate you add to water is present in its fully ionized form; some of it is in the ionized form, producing 2 Na⁺ ions and one CO₃^-- ion (which in turn reacts with water to produce OH^-), but some remains in the combined form as Na₂CO₃. If it is more dilute, a larger fraction of the sodium carbonate becomes ionized. In contrast, all of the sodium hydroxide you add is fully ionized, producing only Na+ and OH-, with no remaining NaOH still in combined form.

If the room in which your dye/fiber reaction is occurring is nice and warm, and the amount of dye sufficient, so that all the other reaction conditions are just right, then you can get by with a range of pHs, too. The optimum pH for the reaction of Procion MX dye with mercerized cotton is about 10.5 (see "What is the effect of pH in dyeing? What is the optimal pH?"). A pH that is a full pH unit down, however, at 9.5, will still work pretty well. Remember that, in the extreme case of a strong base such as sodium hydroxide, one pH unit expresses a ten-fold change in OH^- ion concentration. This means that if you use only one-tenth as much NaOH as you need, the reaction will still go. With sodium carbonate, since it's a weaker base, you can vary the concentration you use by even more for that same one-pH-unit change. So, even a small amount of sodium carbonate will probably still be sufficient to make the reaction work.

In conclusion, all of the concentrations that you mention of washing soda, or sodium carbonate decahydrate, will work. I would recommend using 200 grams of washing soda per gallon for the tie-dye presoak, but if you use twice as much, or half as much, it should work fine.

Good luck to your daughter. Tie-dyeing is a lot of fun.

By the way, since there is sometimes confusion on this matter, I must point out that we need to use sodium carbonate only with fiber reactive dyes, such as Procion MX dyes or Remazol dyes. Soda ash and washing soda do not work at all with all-purpose dyes, such as Rit or Tintex dye. Whether or not we use sodium carbonate, all-purpose dye will bleed from one part of the tie-dye to another when it is first rinsed, and it will continue to bleed whenever it is washed, so that it fades very quickly, and ruins other laundry that it is washed with. Fiber reactive dye, when properly fixed with sodium carbonate, will last for years, and, after the unattached excess dye is washed out, is safe to wash with any color of laundry at any temperature.

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