Chlorophenol red is a pH indicator, a kind of chemical that changes color when the pH (acidity or alkalinity of the solution it is in) goes up or down. Eastman Kodak is one of many companies that supply these indicators. They have 67 of them in their chemical catalog. Each indicator changes color at some point between zero and 14 on the pH scale. Actually, they change gradually from one color to another over a range called a "visual transition interval." This interval may be narrow (one pH point) or broad (over five pH points). For chlorophenol red it is narrow (5.2-6.8 or so) when the solution is applied to dry paper, and somewhat broader (4.8-6.4 or so; different suppliers give different figures) when the solution is in a beaker to which solutions or slurries are added. William J. Barrow, the pioneer of document lamination, deacidification and permanent paper from the 1940s to 1967, tested many naturally aged book papers using a chlorophenol red solution, and reported a visual transition interval of pH 6.0 to 6.7. Two years after his death, his lab published a booklet, "Spot Testing for Unstable Modern Book and Record Papers" (Permanence/Durability of the Book, VI). The booklet described four spot tests: for pH, alum, rosin and groundwood.
After pH meters were invented, pH indicators were little used. The heyday for indicators was from the 1920s to 1960. Many of them are used for specialized jobs not well suited to pH meters. Chlorophenol red, for instance, is used for biological research to stain certain kinds of cells, as well as to identify alkaline paper.
All of the supply sources and most of the literature assume that pH indicators are used to test the pH of solutions, rather than of dry materials like paper. This is why they describe the color of chlorophenol red as "red." It actually shows purple or lavender when it is applied to dry alkaline paper. It has other colors as well. The crystals are red to green or brown; a distilled-water solution is yellow, orange, or dark red; and when it dries to a thin film on a plate, it is yellow. The colors that are most useful for dry-paper testing are the purple it shows above a pH of 6.7 or 6.8, and the yellow it shows below a pH of 5.5 or 6.0, immediately after it dries. In its visual transition interval, between pH 6.0 and 6.8, it shows an intermediate color between yellow and purple.
Unlike some pH indicators, chlorophenol red reliably changes color at the same pH, regardless of the concentration of the solution. This is important for accuracy. This characteristic is also handy for storing and shipping the indicator in a dried form. A rag is prepared by dipping it into a concentrated solution and letting it dry. (This is an ancient way of storing and transporting dyes.) A piece of the cloth can be put into a teaspoon or other container, a drop or two of water added, and a toothpick or small cotton swab used to draw a line on the paper with it. There is a good supply of this kind of cloth at Abbey Publications, which we would be happy to send without charge to anyone who wants to play with it.
Chlorophenol red's chemical name is dichlorophenolsulfonephthalein, and it is classified as a sulfonephthalein. A variant spelling of chlorophenol is chlorphenol. Either the sodium salt or the "straight acid" can be used; both forms have a pH of their own, slightly on the acid side, and are chemically stable and nontoxic in the concentrations used in the Abbey pH Pens (1 g/L). These pens are sold by Abbey Publications both directly and through distributors. Two well-known library suppliers, Demco and Gaylord, carry them. There are distributors in Canada, Australia, Belgium and France too.
The chlorophenol red solution can stain clothing if it is not washed off before it dries. Plain water will remove it.
Phloroglucinol Graff's C-Stain and other indicators for lignin are available in small amounts from Integrated Paper Services, in Wisconsin (414-749-3040) and from the Institute of Paper Science and Technology, in Atlanta (404-894-5369). Talas, in New York City (212-219-0770), can supply phloroglucinol.
Complete and clear instructions for the use of a variety of spot tests are in chapter 10 of the Paper Conservation Catalog available from the American Institute for Conservation in Washington, DC. Chapter 10 costs about $11 postpaid (202-452-9545). It lists twelve suppliers for a number of different indicators.
Eleven other kinds of pH pens, are listed with their sources & prices & evaluated in the Alkaline Paper Advocate, Nov. 1990, p. 55.
A large variety of pH test papers is sold by Micro Essential Laboratory in Brooklyn (718-338-3618).
1. J. M. Kolthoff and V.A. Stenger, Volumetric Analysis. v.1. 2nd ed. Interscience, NY, 1942. Dewey classification number at Brigham Young University Library: 545.5
2. R.G. Bates, Determination of pH. Wiley, NY, 1954. 541.372
3. E. Bishop. Indicators. Pergamon Press, Oxford, 1972, 543. In 8 v.51.
4. I.M. Kolthoff. Acid-Base Indicators. MacMillan, 1937. (Detailed) 545.6
5. _________. Acid-Base Indicators in Analytical Chemistry. 1959. 543
6. _________. Indicators, Their Use. . . 1926. 545.5
7. _________. pH and Electro Titrations. 1941 541.37
8. _________. Potentiometric Titrations. 1926. 541.37
9. William Mansfield Clark, The Determination of Hydrogen Ions, 3d ed. Ch. 3-8. Williams & Wilkins, 1928.
10. Barnett Cohen, various articles in the 1910s and 1920s. He did the early research on chlorophenol red.
11. CAS Registry Handbook - Common Names, 1988. There are three entries under chlorophenol red:
1. (4430-20-0) C19 H12 Cl2 05 S. This is for Chlorophenol red.
2. Chlorophenol red ion (1-): (52762-31-9) C19 H11 C12 05 S
3. Chlorophenol red radical: (38885-61-9) C19 H12 C12 05 S
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