TSG Chapter V. Analysis and Testing Methods for Textiles - Section A. Determining pH

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Contributors: Originally drafted by Meredith Montague. Contributions from: Alicia Bjornson, Deborah Bede, Lucy Commoner, Martha Grimm, Teresa Heard, Jane Hutchins, Marlene Jaffee, Mary Kaldany, Joan Marshall, Denise Migdail, Meredith Montague, Deborah Trupin.
Editors: M. Cynthia Hughes, Jane Lynn Merritt, Deborah Trupin, Sara J. Wolf. Date: January 1995.
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Copyright: 2024. The Textile Wiki pages are a publication of the Textile Specialty Group of the American Institute for Conservation of Historic and Artistic Works.
The Textile Wiki pages are published for the members of the Textile Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.

Determining pH: Introduction[edit | edit source]

pH: A logarithmic measurement of the hydrogen ion concentration used to express the acidity or the alkalinity of a solution. The unit of measurement derives from a scale whose values run from 0–14 with 0 representing the most acid, 14 representing the most alkaline, and 7 indicating neutrality.

pH measurement may be used in the initial examination of a textile to give an indication of its acidity or alkalinity. This information is useful when developing a treatment strategy. It may also be used before, during, and after wet-cleaning, to aid in the detection of soluble organic acids in the rinse baths. A third purpose may be to test materials to be used in the treatment of objects, such as detergents, spot-cleaning agents, etc. A fourth purpose may be for obtaining a pH reading to test storage or exhibition materials.

Factors to Consider[edit | edit source]

It must be possible to wet an inconspicuous area of the object with water in order to take a pH reading.
The inconspicuous area to be tested should be chosen carefully. Surface testing involves wetting a local area, which can result in tide-lines or local color changes.
The slight pressure necessary to obtain good contact with electrodes or pH strips when surface testing may not be acceptable given the characteristics of some textiles. Deformation or damage may result.
Begin any pH test with a measurement of the water to be used in the test to provide a control for comparing sample pH values.
When choosing materials to be used in conservation treatments, their pH values should be considered along with the optimum pH range of the object. Cotton and linen (and other plant fibers) are best at a neutral or slightly alkaline pH, while silk is best kept at a slightly acidic pH (6) and wool tolerates a pH range between 5–8.
Temperature is an important factor in pH measurements when using an electrically controlled pH meter. Temperature influences the voltage output of a probe; therefore, pH readings must be taken at a standardized temperature.
As the pH of a fabric may vary greatly over its surface, pH measurements maybe more useful as indications of change during treatment than of the actual acidity or alkalinity of the fabric.

Materials and Equipment[edit | edit source]

Materials to be used in testing[edit | edit source]

Blotter paper
Purified water (deionized or distilled)
Mylar®
Weights (suitable for fragile fabrics)
Buffer solutions (pH 4, 7, and 10 are most commonly needed)
Sodium chloride (for extraction)

pH test equipment[edit | edit source]

pH meter with combination flat electrode or,
ColorpHast® pH Indicator Strips: strips of paper or plastic in which indicator dye is chemically immobilized either by covalent bonding to the cellulose fiber, or by copolymerization with acrylamide and methacrylamide (choose non-bleeding type) or,
Bromothymol Blue pH Test Kit: provides equipment necessary to make a relative pH determination of an aqueous solution, including an indicator dye in a dilute alcohol solution or,
pH pens: can indicate degrees of acidity depending on the color of the mark which is made directly on the material to be tested. Note that the marks are permanent. These pens should not be used directly on artifacts.

Test Methods[edit | edit source]

Surface pH Reading.[edit | edit source]

Surface readings attempt to quantify the pH of a very small area. Widely used in textile conservation, they are fairly quick to perform and are non-intrusive. As the test reads only the surface pH of a particular area, a better indication of the approximate pH of the textile may be obtained by evaluating readings from several different locations. For the procedure used in taking standard readings of paper, see Tappi T 529 pm-74).

pH meter with combination flat tipped electrode[edit | edit source]

1. Calibrate the meter in the buffer solution according to manufacturers instructions with the value closest to the reading of the anticipated pH value. Do a two buffer calibration, if possible, for accuracy. Place a piece of Mylar® under the area to be tested. A piece of blotter or paper towel should be placed under the Mylar® if working on a hard surface (to protect the tip of the electrode).

2. With an eye dropper, dispense a droplet or two of purified water onto the area. (If the electrode is stored in purified water, it may carry enough water for the test, but it must be fresh.)

3. Place the tip of the electrode onto the fabric sample with gentle, even pressure. Allow meter reading to stabilize before recording the value.

4. Dry the area in a method suitable to the artifact, such as blotting or air drying.

5. The results of the test are dependent on the amount of organic acids dissolved in the test solution, and are therefore subject to a great number of inconsistencies.

Finishes may prevent dissolution of organic acids.
Neither the amount of time nor the temperature of the test solution required to maximize the dissolution of organic acids may be appropriate for the artifact.
The small amount of water used in these tests is quickly affected by the dissolution of CO₂ from the air, causing the pH to steadily drop. Therefore, the sample should be measured quickly.
Inaccuracies due to the difficulties in measuring low ionic strength solutions may occur (see below).

ColorpHast® pH indicator strips[edit | edit source]

1. Place Mylar® beneath area to be tested.

2. Apply a small amount of purified water to the area to be tested with an eye dropper.

3. Place strip face down into wetted area, cover with Mylar® and gently weight to ensure good contact. After a minimum of two minutes, remove strip and determine pH value by comparing color of strip to the chart provided on the package.

4. Place blotter paper above and beneath test area and weight it until dry.

The same points of evaluation for the pH meter apply to the strips.
Errors can occur in weakly buffered solutions. The indicator is itself an acid or base, and can alter the reading of the test solution if there is not sufficient buffering capacity present.

pH pens[edit | edit source]

When testing for treatment or exhibition materials, draw a line on the item to be tested, which may need to be wetted before testing. For an artifact, use only if fiber samples can be removed for the test. Magnification of the fibers may be necessary to see the results. Check the resulting color according to the instructions provided.

Any color in the materials to be tested may interfere with the results and give false readings.

Rinse water pH reading[edit | edit source]

The rinse water of successive textile baths is measured with a pH test instrument. This monitors the dissolution of organic acids during wet cleaning.

pH meter with combination electrode

1. Calibrate the meter in a buffer solution with a value closest to the anticipated pH value. Always do a two-buffer calibration if the meter allows it.

2. Remove a beaker of rinse water, place the electrode, without stirring, deep into the water so that the measurement is made before being affected by the CO₂ dissolving at the surface of the sample.

ColorpHast® pH Indicator Papers

1. Remove a beaker of rinse water and immerse strip into water for a minimum of two minutes.

2. Obtain value by comparing color on strip with chart provided.

Bromothymol Blue pH Test Kit

1. Fill two test tubes (provided) with 5 ml of rinse water each.

2. Add specified number of drops of Bromothymol blue indicator to first tube and place tubes in holder.

3. Use color-coded wheel to match color of the indicator solution.

Extraction method[edit | edit source]

This is a destructive test and is suitable only for treatment or storage materials. It is rarely, if ever, used for conservation purposes.

1. Cut the sample to be tested into small pieces.

2. Soak pieces in solvent to extract the maximum amount of organic acids.

3. The solvent, either purified water or a sodium chloride solution, can be used at room temperature or heated to boiling.

4. Obtain pH value by using a pH meter or indicator papers as described above.

The industry standards that detail this method include the following:

ASTM D2165–78. "Standard Test Method for pH of Aqueous Extracts of Wool and Similar Animal Fibers."

AATCC 81–1989. "pH of the Water Extract from Bleached Textiles."

Tappi T 435 OS 77. "Hydrogen Ion Concentration (pH) of Paper Extracts-Hot Extraction Method."

Tappi T 509 SU 68. "Hydrogen Ion Concentration (pH) of Paper Extracts-Cold Extraction Method."

Measurement of Pure Waters[edit | edit source]

Due to low conductivity*, pure waters (which include distilled and deionized) frequently pose difficulties in obtaining steady pH readings when using galvametric instruments. pH meters produce readings by measuring the flow of ions across a reference point in an electrode. Since pure water is a poor conductor, the flow of ions is impaired, resulting in slow and erratic readings. Several solutions to this problem are discussed:

Select dilute standard buffer solutions when calibrating pH meters that are of similar low ionic strength to that of the water being tested.
Do not stir a high purity sample as it may result in a "noisy" measurement (slow and erratic). This is due to the large difference in the ionic strength between the reference filling solution and the sample (Westcott, 1978).
As pure water is susceptible to large pH changes from exposure to contaminants, rinse down the electrode completely with distilled water after standardization in a buffer and between each sample (Westcott, 1978).
Difficulties in measuring low ionic strength solutions may be due to malfunction at the reference junction in many combination electrodes when it is of a porous material such as ceramic (see Illingworth, 1980; Kopelove, Franklin, and Miller, 1989). To reduce resistance and contamination, a calomel sleeve may be used.
There is available a Pure Water Test Kit (Orion Research) that consists of low ionic strength buffers for calibration, and an ionic strength adjuster, that is added to the sample to be tested. The adjuster consists of a dilute potassium chloride solution which boosts the ionic strength of a sample without significantly altering the pH of the sample (according to research done by the company). There will, however, be a minimal loss of accuracy.

^*Low conductivity is considered less than 10 micro-ohms, or, in the more current terminology, less than 10 microsiemens. This can be expressed as resistivity greater than 100,00 OHMS-CM.

Discussion[edit | edit source]

As a measure of acidity or alkalinity, pH has been frequently used by textile conservators for evaluating the condition of a textile. External sources of acidity from manufacturing processes, environmental conditions, handling and storage units and materials act as catalysts for the internal chemical processes of oxidation and acid hydrolysis. These processes attack the molecular structure of the fiber and produce acidic by-products that continue the reactions. These by-products are often associated with fiber degradation and increase the acidity of a textile.

The goal of many textile conservation treatments, such as wet-cleaning, is to interfere with this cycle by returning extreme pH values to more nearly neutral, thereby slowing the rate of deterioration. To the extent that acidic by-products of degradation or alkaline residues are water-soluble, their presence can be monitored by surface pH measurement, and by measurement of rinse water baths. However, it should be understood that in this context, pH measurement reveals limited information about a particular textile. pH is an aqueous measurement of OH- and H+ ions and thus only measures the pH of those functional groups soluble in water.

Solubility is affected by many variables, including the wettability of the sample. As one researcher states. "The concept of pH was developed to describe the physiochemical properties of dilute aqueous solutions and has no fundamental significance in application to a heterogeneous solid material containing only adsorbed water." (Browning, 1970). It follows further that a local surface measurement gives a very small piece of information about the entire textile, as pH can vary significantly from the surface to the interior of a yarn or even from handling edge to the center of the textile. A conservator must rely on additional sources of information, such as knowledge of manufacturing techniques, prior care and handling or the presence of stains and discoloration, to draw conclusions about the condition or needs of a textile.

References[edit | edit source]

Browning, B.L. 1970. "The Nature of Paper." Deterioration and Preservation of Library Materials. Chicago University Graduate Library School, 34th Conference. 18-38.

Kopelove, A., S. Franklin, and G. McGaha Miller. 1989. "Low Ionic Strength pH Measurement." American Laboratory. June: 40-47.

Tappi T 529 pm-74. https://ipstesting.com/find-a-test/tappi-test-methods/tappi-t-529-ph-surface/ (Accessed February 28, 2017.)

Tappi T 435 OS 77. http://www.tappi.org/content/SARG/T435.pdf (Accessed February 28, 2017.)

Tappi T 509 SU 68. https://ipstesting.com/find-a-test/tappi-test-methods/tappi-t-509-ph-cold-extract/ (Accessed February 28, 2017.)

Westcott, C. Clark. 1978. pH Measurements.New York: Academic Press.