Name: Michelle Jaconette
Message: Dear Dr. Burch:

My name is Michelle Jaconette and I just won a school prize for my 8th grade science project entitled “pH: It’s to DYE for!”, and I get to move on to my local County Science Fair. I need to fine tune my project and I am hoping you can answer a question or two for me.

My project involved changing the pH of six different colors of fiber reactive dye solutions from pH 5 to 13 to determine the best pH for dyeing cotton fabric. My results showed (beautifully) that pH 12 works the best. What I need to know is if changing the pH of the dye solution affected only the chemical structure of the cellulose, or does changing the pH somehow affect the chemical structure of the dye.

Also I would like to know why pH 12 produced for me the best color intensity as opposed to my pH 13 solution (in truth my pH paper went only up to 12, but I added so much more sodium carbonate to my pH 12 source that I assumed it was at least pH 13). Does the extra-high pH of 13 somehow degrade the cellulose fabric?

I have read in depth your website and have found lots of pertinent and interesting information. I would appreciate immensely any further information you could offer. Thank you so much for your time and consideration of my email. If you would prefer to answer me by telephone please let me know and we can set up an appointment for a phone call.

I used Procion dyes from the Grateful Dyes Co. located in Colorado. The colors I used are Fuchsia, Turquoise and Yellow. I dyed 100% cotton fabric that was not pre-soaked in any solution.

Congratulations on winning your school competition!

My son, who is in seventh grade, did a very similar experiment for science fair this year, dyeing cotton in a 4% solution of red MX-5B at various pHs, at 78 degrees Fahrenheit (25 C), for eighteen hours. The results were a little surprising to me, because I expected a pH of 8 to have quite poor results. His three swatches dyed at pHs of 2.5, 4, and 7 were all alike, a pale pink, as expected, but the one at a pH of 8 was only a little less bright than the best one. His pH 9 was almost identical to pH 10, which was the best pH he tried. The one with a pH of 12 was pretty intense, but a little spotty, and the one with a pH of 13 was of a medium intensity.

Our pH paper did go up to 14 (he used the pHydrion paper sold by Dharma Trading company). It is not correct to assume that a larger concentration of sodium carbonate will result in a significantly higher pH. The maximum pH that sodium carbonate will produce is likely to be around twelve. The Merck Index ("An Encyclopedia of Chemicals, Drugs, and Biologicals") gives its pH as 11.6, but Doug Wilson on the DyersLIST mailing list posted in 2003 that, while a one gram per liter solution of sodium carbonate has a pH of 11.1, and a 40 gram per liter solution has a pH of around 11.9, 100 grams per liter produces a pH of about 12.2. So, your "pH 13" dyebath was probably actually closer to a pH of 12. For his pH 13 dyebath, my son used trisodium phosphate (TSP), a powder sold in hardware stores for use as a caustic cleaner. (Please be aware that this substance is more hazardous to use than sodium carbonate.) I think he may have combined TSP with sodium carbonate for his pH 12 dyebath.

On mercerized cotton, the optimal reaction pHs of MX fiber reactive dyes that I have found in the scientific literature range from 10.2 to 11.1, depending on which individual dye you are considering. Rayon likes a higher pH, from 0.5 to 1.0 pH units higher (i.e., from three to ten times as basic). There's a optimum pH chart for several different MX dyes on the page on pH in the FAQ section of my web site. Unfortunately, red MX-8B, yellow MX-8G, and turquoise MX-G are not represented on this table, because I have not found this information anywhere. Dye retailers recommend a pH of 10.5 for all MX dyes, regardless of their individual properties, although the optimal pHs for the different color molecules varies a little, as you can see in my chart, whose data came from Ivanov's 'Reactive Dyes in Biology'.

Why does a pH that is higher than the optimum produce a poorer performance? One possible reason for the increased 'spottiness' my son saw at a pH of 12, as compared to a pH of 10, is that perhaps the dye reacted too quickly with the fiber, instead of diffusing along the fiber and spreading out a bit before reacting. (I also believe that I read somewhere that the substantivity of the dye is altered as the pH increases, but I am failing in my efforts to track this down anywhere; without better references, it would be best to skip this issue.) Perhaps the greatest effect of excessive pH on the dye reaction would be in the greater tendency of the dye to react directly with the high-pH water, before it can even reach the fiber. MX dyes do react with water, but the dye solution "goes bad" far more quickly once even the usual amount of sodium carbonate is added; it seems that the addition of an excessive amount of -OH- ions might easily increase this effect.

It is unlikely that the higher pH degrades the cellulose fiber. The mercerization process, which makes cotton dye more brightly and gives it a little sheen, involves the use of a 25% solution of sodium hydroxide, which is greater than pH 13.

Another question to consider is how much dye solution you added to your dyebaths - or did you add dye powder directly after measuring the pH of your dyebaths? If you used a dye concentrate, as my son did, of course the added volume of the liquid will reduce the pH slightly. Unfortunately, we have no idea how much of an effect the dye itself may have on the total pH, since it is impossible to use pH paper once the dye has been added.

Your dye source, Colorado Wholesale Dyes a.k.a. Grateful Dyes, prides itself on using no salts to dilute their dyes. Dye powders must be diluted in order to provide a consistent concentration of dye strength per gram of dye powder; Tamol, the product used instead of salts to dilute dye, is a lighter and fluffier powder than sodium sulfate, which is sometimes used by other dye sellers. Sodium sulfate can increase the color intensity of turquoise, and so could possibly confuse matters slightly. It's just as well that you know that you are not using it.

To recap,
What I need to know is if changing the pH of the dye solution affected only the chemical structure of the cellulose, or does changing the pH somehow affect the chemical structure of the dye.

We raise the pH of the dyebath in order to ionize the cellulose, that is, to encourage an H to come off of an -OH group on the cellulose, so that the dye can then react with that negatively-charged oxygen atom (losing a chlorine atom in the process). We do not intend to alter the dye chemical itself. However, there is no reason to suppose that a very high pH cannot affect the dye molecule itself, possibly harming it. In fact, I have often noticed that pouring a strong solution of sodium carbonate directly onto dyed material, in low water immersion dyeing, will temporarily cause a color shift in the dye on which it is poured. As the carbonate solution gets mixed in, the color seems to return to its previous value. What this really means, I do not know. Many colored materials exhibit some color shift with pH, the best of these being very useful for use as pH indicators. More likely to be important is the simple fact that dye will react with water and "go bad", that is, lose its ability to react with cellulose; this process occurs much more rapidly under basic conditions than at a neutral pH. It is reasonable to suppose that a very high pH will have this effect much more quickly still.

The way in which the dye reaction takes place is that a nucleophile, either a cellulosate anion or a hydroxide ion, attacks the carbon to which one of the chlorine atoms in the dichlorotriazine dye (MX dye) is attached. If the nucleophile is the cellulose fiber, the result is a covalent bond between the dye and the fiber; if it is a hydroxide ion, the result is hydrolyzed, nonreactive dye. (Taken from p. 194 of the book Cellulosics Dyeing, edited by John Shore, published in 1995 by the Society of Dyers and Colourists.)

I think that you should use the generic names of your dyes in your science fair presentation, explaining under your Materials section where you got them from. The MX codes and generic names for the fuchsia, turquoise and yellow dyes you used are as follows:

fuchsia
Procion red MX-8B
Colour Index reactive red #11

turquoise
Procion turquoise MX-G
Colour Index reactive blue #140

yellow
Procion yellow MX-8G
Colour Index yellow #86