pH pens and chlorophenol red

Chlorophenol Red
Information from Abbey Publications
abbeypub [at] aol__com

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.  At the time of writing
(July 1995), 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 Pensr (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.

Reference Sources for Chlorophenol Red

    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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                   Conservation DistList Instance 9:8
                  Distributed: Tuesday, July 11, 1995
                        Message Id: cdl-9-8-001
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