What is the effect of pH in dyeing? What is the optimal pH?

Every chemical reaction works best at a certain pH, or degree of acidity.

Acid dyes

In the case of acid dyeing, a low pH helps to form the hydrogen bonds that attach acid dyes to protein fibers, such as silk and wool, as well as nylon. Acid dyes do not work on cellulose fibers such as cotton. Optimal pHs vary according to the specific type of acid dye. Some require only a very mild acid; others a more significantly low pH.

Silk can be dyed at low pH (acidic) or high pH as with cellulose fibers; wool can only be dyed at low pH because it is damage by high pHs. Cellulose fibers, such as cotton, linen, rayon, etc., cannot be dyed at all at low pHs.

Table I. pH range for dyeing wool with different classes of acid dyes

dye class	pH range
Leveling acid / Kiton / Strong acid	2.5 to 3.5
Milling acid / Weak acid	5.2 to 6.2
Super Milling / Fast Acid	5.5 to 7.0
Lanaset	4.5 or 5.0
Procion MX used as acid dyes	2.5 to 3.5

Sources: for the first four dyes in the table, the optimal pH ranges are found in various palces within the book Wool Dyeing, edited by David M Lewis. I am trying to track down where I obtained the figure for Procion MX dyes used as acid dyes, which is not found in the same book; certainly, ProChem's recipes call for more acid than their other acid dye recipes do, but they do not mention the optimal pH range.

Fiber reactive dyes

In the case of most popular fiber reactive dyes, a high pH actually activates the cellulose (cotton) fiber, forming a cellulosate anion, which can then attack the dye molecule, leading to a reaction that produces a strong, permanent covalent bond. Without a high pH, the dye will not fix permanently to the cellulose fiber. In dyeing cotton and other cellulose fibers with popular fiber reactive dyes such as Procion MX or Sabracron F dye, sodium carbonate is used for no other reason than to increase the pH of the dye reaction, so that the fiber will react with the dye. (See also What is soda ash, and what's it for?.)

Dichlorotriazine dyes (Procion MX type)

Ivanov's Reactive Dyes in Biology (see Books) gives the following information, for dyeing mercerized cotton with dichlorotriazine (Procion MX type) dye:

Table II. optimal pH for dyeing cotton

Procion MX Dye	Optimal pH for cotton dyeing	common names
brilliant red 2BS	10.2
brilliant red 5BS	10.4	"light" or "mixing" red
brilliant yellow 6GS	10.8
blue 3GS	10.8
yellow RS	10.9
brilliant orange GS	11.0
brilliant blue RS	11.0	"sky" or "basic" blue
red GS	11.1

Ivanov adds that the optimal pH values may shift either up or down for other materials; for rayon they are between 0.5 and 1.0 units higher. Most instructions say that a pH of 10.5 is ideal; others say above 10.5.

(Note that the "S's" in MX codes in the above table are superfluous; they merely indicate that the dye is sold at a standard concentration - as it nearly always is. When it is not, the difference ins strength is noted; for example Fuchsia MX-8B 150% is half again as strong as the standard grade, by weight.)

Vinyl sulfone dyes (Remazol type dyes)

The optimum pH for dyeing cotton with remazol dyes is probably 11.5. (Thanks to Dye Forum member Puffin for finding this.) On wool and other protein fibers, Remazol dyes can be used as true reactive dyes by simmering at low pH, specified by Batik Oetoro as being 5.5.

Lanasol dyes

The Lanaset line of dyes for wool, silk, and nylon includes some fiber reactive dyes which are designed for use on protein fibers, the Lanasol dyes. Although they are true fiber reactives, they are used at a mildly acidic pH, like acid dyes.