Volume 3, Number 3
Aug 1990

An Investigation [1969] into the Use of pH Indicators on Paper

Reprinted with permission from the ICA Newsletter 7(l), June 1969, P.1-4. (ica=International Communication Association, Oberlin, Ohio) This was the second reference in the 1970 technical note reprinted on p.62 of the issue of this newsletter.

Scientists presently concerned with the conservation of paper warn that acidity is a most damaging factor, causing the gradual and irreversible breakdown of cellulose molecules. Many a work of art on paper has been injured by being placed in prolonged contact with acid materials, notably mats and containers made of impure wood-pulp cardboard. Since acidity migrates from one material to another, the valuable paper, which may not originally have been acid, is eventually affected with consequent weakness and embrittlement. One of the reasons why all-rag cardboard is used for mats and for the lining of solander boxes is because the general belief is that it is nearly neutral. However, only too often all-rag board is acid and its acidity may exceed the limits considered tolerable for the safekeeping of the paper it encloses. Acidity may also be introduced by secondary mounting materials--slip sheets, adhesives, tapes, backing boards, etc. Therefore it would be a great benefit to the curator if there were a reasonably sure yet simple method, not involving elaborate equipment, for gauging the acidity or alkalinity of the materials used with the valuable papers in his care.

Acidity or alkalinity is measured by chemists in term of the concentration of the hydrogen ion. Thus pH (literally potential of Hydrogen) is a symbol representing quantitatively the hydrogen ion concentration. Thus pure water at normal temperature has a value of 7. pH values from 0 to 7 indicate a decreasing sequence of acidity, pH values from 7 to 14 an increasing sequence of alkalinity.

Although there are various ways used by chemists to measure the pH of a water solution of a substance, the measurement of acidity in a dry or near dry material is less easy. One method that appears to be promising is by the use of acid-base indicating dyes. Litmus is a sell known example. Many of these dyes are available and, in a water solution, a drop of colored solution on paper will Indicate by its color the approximate pH value. Several specific dyes are now being suggested by researchers in paper chemistry for our particular purpose.

The ICA laboratory decided to investigate the method from the curator's viewpoint to see if there were difficulties in reading indicator spots on various kinds of papers and to see how the readings compared with the pH determination of the same papers obtained by the standard extraction method prescribed by the Technical Association of the Pulp and Paper Industry (TAPPI). In this method, a comparatively large specimen is macerated in cold distilled water, after which the pH is read by a sensitive electrical pH meter. In this endeavor, we sought and generously received the aid and advice of Professor Renfrow of Oberlin College and his wife, also a chemist. They began by encouraging us to believe in the responses of the spot tests and went on to make several indicator solutions for us, as well as six buffer solutions which enable us to observe how the indicators behave at known values.

We tried several indicator solutions:

Chlorophenol red (obtained through the kindness of Barrow Research Laboratory, Richmond, Virginia)

Turns yellow if more acid than 5.7,
Turns purple if more alkaline than pH 6.4,
Turns brown or grey if the pH falls between 5.8 and 6.4.

Cresol red

Turns yellow if more acid than pH 7,
Turns red if more alkaline than pH 7.5.

Brom thymol blue

Turns yellow of more acid than 6.0,
Turns green if more alkaline than 6.5.

Methyl red

Turns red if more acid than 5.0,
Tums greenish yellow if more alkaline than 6.0.

Eleven papers were tested, most of which were, or could be used for, mounting materials--Japanese mulberry tissues, all-rag papers and cardboards, "acid-free" non-rag board, etc. Since the size of the spot was not of consequence, the manner of application which we preferred was to place a drop of indicator at the end of a glass rod and to stroke it onto the paper so that a mark like an elongated comma was formed with a little pool at one end and tapering off at the other. Three observers watched during the drying of the strokes of the indicator on one after another of the papers and made individual estimates of the acidity or alkalinity indicated by the color changes. In no case did the estimates vary much from one another. The methyl red solution gave disappointing results. It was not only a faint stain, often difficult to read, but its indication did not always agree with answers given by the other solutions. Be the other indicators however, there was satisfactory agreement. It was found that the spots should be read a short tire after application but while still wet. All of them turned color upon drying and had to be rewet for pH determination if allowed to dry except chlorophenol red which did not undergo this second color change. For the thin, absorbent tissues, the best results were obtained when they were folded to four or six thicknesses before application of the indicator spots.

Duplicate samples of the test papers were submitted to the Chicago Paper Testing Laboratory for pH determination by the TAPPI extraction method and these answers were compared with the average estimates formed by the observers of the spot tests. For seven of the eleven papers agreement was very close, surprisingly so when it is realized that the spot indications cannot be read as precisely as with the cold extraction method. In three cases disagreement was decided, 2 pH units apart; for each the spot tests indicated acidity below 6.0 pH whereas the extraction method showed neutrality.

The reasons for these disagreements have not been determined. One of the papers was of the thickness, density and surface characteristics of manila card, not of all-rag content but designed to be neutral for insulation in electrical uses. The other two were Japanese tissues. Repeated spot tests on these tissues at ICA and the pH determination of a second sample submitted to the Chicago Testing Laboratory established the fact that the disagreement was not a matter of error. It my just be coincidence that of the eleven papers tested, the three specimens where there was disagreement were the only old ones.

As far as w know, the other papers were not more than a year old, but the two tissues were obtained in 1956 and the "neutral" paper in 1959.

Conclusion. As a result of this incomplete investigation, we conclude that pH indicators are beneficial in determining the acidity-alkalinity measure of fresh papers.

Therefore we recommend that the paper curators of our member museums equip themselves with solutions of chlorophenol red in order to be able to determine whether or not new stock intended for use with art papers is acid. Chlorophenol red is suggested in preference to other indicators because it has several advantages, not the least of which is that the Barrow Research Laboratory recommends it and in Vol. VI of their Permanent-Durable Series, soon to be published, will describe the preparation of the solution most suitable for use with papers.

Other advantages are: the colors to which it changes are strong and easy to read; the colors do not change, at least not immediately after drying, and so continue to indicate the pH value for some time after application; and, when prepared according to the Barrow Research Laboratory directions, the color changes occur in the pH range which is crucial to paper. It is important that the pH of paper is 6.0 or higher. Therefore, a spot of chlorophenol red solution applied to material being tested should turn purple or slightly purple, indicating a pH of 6.4 or better. If it turns greyish brown, a nondescript color, the pH is 5.8 or better, indicating an acidity which may have to be acceptable from a practical point of view, especially for materials not in direct contact with valuable papers. If the color becomes a decided yellow, the stock should not be used or should be deacidified before using. Tiny drops or smears applied near edges are all that is required for the determination, so that new stock can be returned if found faulty. In the case of cardboard, the outside edge should be tested as well as both surfaces.