Cleaning Wool Rugs and the Mystery of pH

September 14, 2004
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During the last few years, rug cleaners have received mixed signals on pH levels for their cleaning solutions. Is there something new under the sun?

Most cleaners have a general understanding of pH. As the difficult-to-understand textbooks say, "pH is the logarithm of the reciprocal of the hydrogen-ion concentration of a solution." The symbol pH, from the German, means the power of hydrogen. pH is a measure of the degree of acidity or alkalinity of a substance on a scale of 0 to 14, with 7 being the neutral point. The numbers increase as alkalinity increases and decrease as acidity rises.

The scale is logarithmic, meaning that as we move away from pH 7 (neutral) it is increasing by the power of ten. As you can see from the chart, pH 0 and pH 14 are 10 million times more acidic or alkaline than pure water.

The guidelines we have been given over the years for cleaning wool rugs or carpet say to stay within a pH level of 4.5 to 8.5 and there should not be a problem. Well, there is more to the story than just pH: buffers. Buffers have been discussed in training classes through the years and are now coming back into focus. The subject is well covered in the book "Fundamentals of Carpet Maintenance" by Dr. Eric M. Brown of the United Kingdom.

Buffers are additives that stabilize the pH of a detergent. As cleaners, we purchase detergents in a concentrated form and dilute them with water. Buffers give stability to the pH after dilution, allowing for higher dilution ratios. The key concern in cleaning wool rugs or carpets is causing dye bleed. The pH of a product is only a general guideline; it is the stability of the pH that is the most important factor, i.e. how well it is buffered.

Large amounts of a buffering agent (alkaline builders) in a cleaning product can cause a pH shift in a wool rug. Immediately or over time, this pH shift will result in bleeding. Just noting a cleaning product's pH will not tell you if it has been highly buffered.

In order to check the stability of the pH, a test can be carried out on the chemical. It involves a rather simple laboratory procedure known as titration, which you may have encountered in high school chemistry class.

One hundred milliliters of the cleaning chemical at working strength is placed in a conical flask, and the pH is measured. Dilute hydrochloric acid (0.1M) is then added progressively from a graduated tube known as a burette until the pH of the product/acid mixture reaches 7. The amount of acid necessary to reach this pH is recorded. Further acid is then added until the mixture reaches pH 5.5.

If a large amount of acid is required, then the chemical is well buffered and is likely to disturb the pH environment of the rug and its dyes. If very little acid is required, then the product should not cause dye bleeding.

It is impractical for a cleaner to perform such a test on all of their cleaning products. An organization called WoolSafe will test products for manufacturers and, if the product passes, will allow them to use the WoolSafe logo.

Wool does have an affinity for dye stuffs which react chemically and combine with the protein in the cell structure. However, the functional dye group for wool is almost always acidic. As dyeing take place, the acid group in the dye (+ charge) reacts with a basic group on the wool fiber (- charge) and the dye and fiber become attached to one another. Because the dye is acid, the reaction is carried out under acid conditions; the finished wool rug will be acidic in pH, often in the range of pH 4 to 5 but sometimes as low as pH 2.5.

Destabilization of the dye-fiber bond causes bleeding to occur in wool rugs and carpets. Destabilization is caused by altering the pH of the fibers and occurs when rugs are cleaned with highly buffered products. A cleaning product in the "safe" pH range can cause dye bleed, so do not rely on that number. To be safe, look for the WoolSafe logo on products, which will tell you an independent testing body has evaluated your cleaning agent.

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