Volume 1, Number 4
Oct 1988

pH and pH Pens

Several readers have sent in material for the comparison of pH readings by pH indicator, hot and cold extraction, and surface pH meter readings that was promised in the May issue, and it has all been helpful. There is too winch to incorporate in a single article, so it will be passed on in a series of items: tables, quotations, references and so on. We might even wind up having a column on pH, or a department, if the flow of information continues.

On the range of pH values: On page 653 of the second edition of Chemistry by John C. Bailar, Jr. et al. (Academic Press), there is a paragraph explaining that it is mathematical complexity rather than physical reality that sets limits to the range of pH values:

The values of pH or pOH for dilute solutions usually fall. between 1 and 14 and are therefore easier to cope with than the small H⁺ or OH^- concentrations. [For concentrated solutions, pH or pOH values are negative (e.g., pH < 0 for [H]⁺ >1 M) or greater than 14 (e.g., pH> 14 for ( [OH]^- >1 M). However, the use of pH and pOH is usually limited to dilute solutions where the values are between 1 and 14.]

Ellsworth Shriver summarized this in a letter last May said that a 0-14 range was almost universally used.

What Bertie L. Browning says: In the second edition of his Analysis of Paper (Dekker, 1977), on P. 173-174, he says: "The pH of paper can be estimated by spotting the surface with an appropriate acid-base indicator. Results are often in approximate agreement with those obtained by water extraction procedures, but frequently considerable differences are found. For some grades the indicator method can provide reasonable estimates of the pH of paper surfaces."

On the correlation between different measures of pH in paper: Most studies on this subject have compared cold and hot extraction methods with the flat head electrode. A good review of this work is in Richard Daniel Smith's "A Comparison of Paper in Identical Copies of Books from the Lawrence University, the Newberry, and the New York Public Libraries," Restaurator Suppl. 2, 1972. In that same year, 1972, two other articles on the subject appeared:

A. Joel et al. "The Measurement and Significance of pH in Paper Conservation," Bulletin of the American Group IIC 12(2), April 1972, p. 119-125.

George B. Kelly, Jr. "Practical Aspects of Deacidification," Bulletin of the American Institute for Conservation [a later name of the American Group - IIC] 13(1), 1972, p. 16-26.

A monograph supplement to the Abbey Newsletter is in press which will give comparative readings for 18 old and new papers: Helmut Bansa and Hans-H. Hofer, "Artificial Aging as a Predictor of Paper's Future Useful Life."

Comparisons of any of these three methods with pH indicators like the chlorophenol red in the pH pen sent to subscribers, however, are rare. The second and sixth booklets in Barrow's series entitled Permanence/Durability of the Book compare cold extraction-results with those from spot tests (chlorophenol red) for hundreds of printing and writing papers. The spot test results are given as color names, which are imprecise, but a correlation can still be observed in the following set of readings.

Table 3

Results of tests for pH (cold extraction) and of spot tests for acidity ... on 63 coated book papers*
Second and third columns, for books pH 5.5 and over]

^* From "Spot Testing for Unstable Modern Book and Record Papers," W. J. Barrow Research Laboratory, Richmond, 1969. (Permanence/Durability of the Book, VI) The indicator used was chlorophenol red.

How to get the most accurate readings from the felt-tipped chlorophenol red pen: Since the natural color of the indicator is red, it has to be applied in a thin enough layer for it to be affected by the paper's pH, and it has to be read as soon as it is dry. If it sits for an hour or overnight or longer, it will absorb C0₂ and S0₂ from the air and show the paper to be more acidic than it is.

The indicator does not lose efficacy with time, but the cotton wadding in the fillers of the pens we sent out before August 3 of this year were not very high quality and may deteriorate in time. Shelf life is not known, but replacement of those fillers is recommended before January, 1990. An opportunity will be provided to request one on the renewal form that will be sent out to subscribers in October.