Volume 2, Number 1
Apr 1989

Practical Aspects of Deacidification: pH and Alkaline Reserve

Talk presented at the IIC-AG Paper Conference at the American Philosophical Society, Philadelphia, May 30-31, 1972, and originally published in Bulletin of the American Institute for Conservation, Vol. 13, No. 1 (1972), p. 16-28, under the title "Practical Aspects of Deacidification." Reprinted with permission. The passages on effect of washing and deacidification on pH and alkaline reserve have been omitted.

Introduction

The detrimental effect of acidity in paper has been known for many years and many methods have been proposed to deal with this problem. In a previous paper by John C. Williams (1), the chemistry of paper deacidification and the principal methods used to deacidify paper have been discussed with extensive references.

The efficiency of deacidification is generally judged by the effect on the pH of the paper, deacidification being judged complete when the pH of the paper rises to 7.0 or above. This is subject to two difficulties. First, the measurement of pH of a solid such as paper is by no means simple. There are a number of methods in use but the agreement between them is less than reassuring in many instances. Second, the pH of the paper gives no indication of the alkaline reserve present in the paper which can protect the paper from future attack by acid from atmospheric pollution or other sources.

As an extension of John William' previous paper, the various methods of pH measurement have been compared with the actual acid or alkali content of the paper measured by titration against standard solutions of hydrochloric acid or sodium hydroxide.

The titrations also give a measure of the alkaline reserve left in the paper by the deacidification methods, and these have been compared as a measure of the efficiency of the methods for preserving paper.

Discussion

In order to evaluate the deacidification processes on a better basis than was possible by pH measurement, an independent measure of the acidity was desired. We, therefore, chose to titrate the paper samples with standard acid or alkali as appropriate. Since it was felt that extraction of acid or alkali from the paper sheet might be quite slow, we reduced each sample to a pulp in a Waring Blender with sufficient water to form a slurry of about 1% concentration. This consistency was easily poured and stirred, so that the titration could be carried out with, little difficulty using a pH meter to detect the end point. The pH meter was felt to be preferable to a color-type indicator as the slurry is frequently dark, particularly with old paper samples and printed material, so that the indicator color change would be difficult or even impossible to detect with accuracy.

Since we were aiming at a neutral paper, we decided to carry out all titration to a pH 7.0 end point.

A series of various papers was selected and measured for pH by several methods and compared with the pulped pH and titration results as shown in Table 1. The agreement between methods was quite good with some samples but varied widely in others. Note particularly that in Sample 5 the TAPPI cold extraction (2) shows a pH of 6.2 and the surface electrode a pH of 6.4, but the pulping procedure shows that the paper is actually alkaline and the titration indicates that there is a considerable amount of alkali present: 200 milliequivalents per kilogram of paper. This is equivalent to 1.0%. by weight of calcium carbonate in the dry paper. Only the TAPPI hot extraction (3) indicated a similar alkalinity. The discrepancy between the pH indicated by the various methods is quite large for this particular sample.

Table 1

pH of Paper: Comparison of Methods of Measurement

Note also that the titration can vary significantly with little variation in indicated pH. Examples 6 and 7 show identical pH values for the TAPPI cold extraction but different amounts of acid in the paper as determined by titration.

The discrepancies between pH indicated and the actual acid or alkali contents of the papers are not too bad in acid papers but can be very large in the alkaline papers, as indicated by Samples 8, 9 and 10 (Table 1). Of these three, the one with the highest pH has the lowest alkali content by titration.

The main reason for these discrepancies is that pH deals only with the ionized acid or alkali. Papers containing acids or alkalis of low solubility register only a fraction of their actual amount present with any pH measuring technique, whereas with a titration all of the acid or alkali is found. Similarly, in procedures based on soaking the paper, and particularly with surface pH Measurements, the acid or alkali may be so entangled in the cellulose net that extraction becomes impractically slow. The acid or alkali may also be chemically attached to the cellulose fibers, and therefore possible to extract without dissolving the cellulose or destroying it. Some of the difficulty is shown by the slow response of the surface electrode to paper pH. Even when the amount of water on the paper was restricted to one small drop, and migration retarded by backing the paper with a thin film of Teflon supported by urethane foam to insure close contact, at least two minutes were required for the pH meter to stop drifting and reach equilibrium. With a few samples, as much as five minutes was required. Considerable variation was also noted from point to point on the paper and the spread in values given in Table 1 for the surface pH results represents the variation among five different spots on the sample.

Now, it could be argued that only the hydrogen ions from the acidic substances in the paper are detrimental, as undissolved material should have no effect. This may be true; however, in deacidification, all of the acidic substances must be neutralized or the equilibrium will just shift to form more of the ionized material, and hydrogen ions will again be present to cause further degradation. Therefore it is important to remember that pH may not be a true measure of the total amount of acid or alkali present.

Of course, it is obvious that we cannot carry out a titration to determine the acid in the paper of a valuable book, but these titrations have shown the nature of the problem of measuring the acidity of the paper. The investigation is being continued with the goal of developing a reasonable nondestructive method which will give a reliable and accurate measure of the acid or alkali present.

It is readily apparent from these studies that pH measurement cannot be relied upon to estimate the "alkaline reserve" of a paper. This concept of "alkaline reserve!' is important in preservation, as it is a measure of the future protection against acid contact over a period of time. The acid contact may come either from atmospheric pollution (SO₂ or N₂0₄) or from gradual oxidation of cellulose to form acids or perhaps from spillage of acid beverages such as cola drinks which may have a pH of 4.0 or even lower.

The concept of "alkaline reserve" as an important factor in the preservation of paper was first noted by Hanson

(4), when he examined a book dated 1576. Some of the sheets were white and had good strength while others were brown and weak. The strong white sheets were found to contain 2.5% by weight of calcium carbonate while the brown sheets contained practically none. Hanson postulated that the white sheets were well preserved because they had an adequate "alkaline reserve!' in the form of calcium carbonate to protect then over the years....

Two to three percent calcium carbonate represents an alkaline reserve of 400 to 600 milliequivalents of alkali per kilogram of paper. Milliequivalents are a convenient form of measuring alkali as it puts all alkalies on an equal basis. One milliequivalent of any alkali will neutralize the same amount of acid as one milliequivalent of any other alkali and vice versa. For instance, one milliequivalent of sulfuric acid is 0.049 grams while one milliequivalent of hydrochloric acid is 0.036 gram. The weights are different, but the neutralizing power is the same.

Either of these would be neutralized by one milliequivalent of calcium carbonate (0.05 gram) or by one milliequivalent of sodium hydroxide (0.04 gram)....

Experimental

pH measurement:

1. TAPPI cold extraction: TAPPI method T509 Su 68

2. TAPPI hot extraction: TAPPI method T435 Su 68

3. Surface pH measurement: measured with Beckman Flat Bulb Combination Electrode No. 39182 and Fisher Accumet Model 320 pH meter. The paper sample was placed an a thin (0.005") Teflon film over 1" of polyurethane foam to provide an impervious flexible support for the sample. The sample was moistened with one drop of distilled water and the flat bulb electrode pressed to the moist spot firmly and held until the pH measurement became stable for at least 30 seconds (about two minutes). The pH electrode was previously standardized against a pH 7.0 buffer solution and checked several times each day. The electrode was rinsed with distilled water and dried with white facial tissue before each test.

4. Pulp pH procedure. A 2.5 grain sample of the paper was weighed to 0.01 grams and cut into small strips about me by two centimeters in size. These were transferred to a clean Waring Blender and blended to a pulp with 200 milliliters of distilled water for 45 seconds. The pulp was transferred to a clean, dry 400 milliliter beaker using a wash bottle with distilled water to flush the last traces of pulp into the beaker. About 50-75 milliliters of wash water was required for the flushing.

The pulp was stirred with a magnetic stirrer using a 1V' Teflon-covered magnet and the pH measured with a pH meter and glass electrode which was standardized against a pH 7.0 buffer.

Titration:

The pulp sample used for the pH measurement was titrated using 0.1000 N hydrochloric acid for samples with a pH above 7.0 or with 0.1000 N sodium hydroxide for samples with a pH below 7.0. The titrating solutions were added from a 25 ml burette with 0.1 milliliter subdivisions until the pH was exactly 7.0 and remained constant for one minute. The volume of titrant was read and the milliequivalents per kilogram of sample was calculated using the formula:

        ml titrant x normality x 1000
        -----------------------------  = meq/Kg
        wt of sample in g

(1) William, John C., "Chemistry of the Deacidification of Paper." Paper delivered at the annual meeting of the IIC-AG (International Institute for Conservation American Group, later called American Institute for Conservation) at Oberlin College, Ohio, June 5, 1971.

(2) TAPPI method T509 Su 68, Hydrogen ion concentration (pH) of paper extracts--Cold extraction method.

(3) TAPPI method T435 Su 68, Hydrogen ion concentration (pH) of paper extracts--Hot extraction method.

(4) Hanson, F. S., "Resistance of Paper to Natural Aging," The Paper Industry and Paper World, Feb. 1939, pp. 1157-1164.