Volume 4, Number 1
Feb 1991

An Annotated Bibliography on Variations in pH Measurement

Discrepancies in pH measurements are found from reading to reading with the same method, same sample and same lab; from me place to another on the same sheet of paper; from one method to the next; and from one lab to another even when they use the same method and paper from the same source. They can be reduced by careful attention to method, but they can never be eliminated entirely. It is best to resign oneself to this, and give up the fruitless search for the Perfect Reading, the Ultimate True Value.

These references are meant to give historical background to the results of round robin tests on 20 papers for the ISO group now working on a standard for paper permanence. Ten labs used the same method to measure pH m those 20 papers, but for any given paper, there was an average spread of 1.3 units. For three papers, the spread was under 1.0, but the others had spreads as wide as 1.9 units (e.g., from pH 8.0 to 9.9).

The references are arranged in historical order.

Joseph H. Flynn and Leslie E. Smith, "Comparative pH Measurements on Papers by Water Extraction and Glass Electrode Spot Tests." Tappi 44(3), Mar. 196, p. 223-227.

"Correlation between the spot tests and [cold] extraction pH's was rather poor, especially for papers of high extraction pH.... It appears that other variables [besides type of pulp or added chemicals] in the papermaking, blending, and pulping cause considerable variation in measured pH values of the paper..." The five papers with the highest pH differed by 1.5 units between the two methods of measurement, and the five with the lowest pH differed by 0.5. The surface pH readings were consistently lower for the alkaline papers, and higher for the acidic papers. They were taken with flat-head electrodes, which had recently been invented.

Ilpo Palenius, Paul Ålander and Arne Juselius, "Reliability of the Determination of the pH Value of Pulp, Paper and Board." Papper och Trä, No. 3, 1962, p. 85-90.

The authors' purpose was to find ways to minimize discrepancies between labs using the same methods. Eight labs tested 6 kinds of paper and board by cold and hot extraction, the French AFNOR method (cold extraction with NaCl solution), and flat-head electrodes. Everything was controlled, including the water used and method of calibrating the pH meters. "The variations in the results of pH measurements obtained by different laboratories are thus obviously made up from the following components: differences in water quality (0.4 units); different amounts of impurities in the air (0.2 units); differences in pH meters and electrodes (0.2 units); and differences in quality and manner of cleaning of the glass vessels and in sample handling (0.2 units) = altogether 1.0 pH unit. There must be added to this estimation differences occasionally caused by still unknown factors [including] unfamiliarity with the procedure of determination."

Charles T. Ray, "Flat Electrode Measurement of pH in Printing Papers." Tappi 55(3), Mar. 1972, P. 393-395.

In those days, two electrodes were used. The combination (single) electrode later came into use because it was more reliable. The pH of 16 kinds of paper was tested by hot extraction and flat electrode surface measurements; significant correlations were not found, except for book papers. The scatter plots tell the story, but just to nail it down, the author also gives chi square, standard deviation, correlation coefficient and the equation for the regression line. "It is believed that [the] difference in the type and quantity of sizing materials in the various papers is the main factor contributing to the lark of correlation between the two procedures."

A. Joel, N. Indictor, J. F. Hanlan, and N. S. Baer, "The Measurement and Significance of pH in Paper Conservation." Bull. Amer. Group-IIC 12(2), April 1972, P. 119-125.

Surface pH was compared with cold extraction, and an effort was made to find the causes of discrepancies. "It is recommended that the amount of water used in making a surface pH measurement be kept at a minimum. These data strongly suggest that the two different pH measurements actually measure quite different properties.... Conditioning at low pH caused decreased values of measured pH implying that the electrode experiences a memory effect, that that the buffer conditioning process (or similar prior contact) will affect any succeeding measurement." 13 references.

George B. Kelly, Jr., "Practical Aspects of Deacidification." Bull. Amer. Inst. Conservation 13(l), 1972, p. 1626.

Three methods were compared: cold and hot extraction, and surface measurement with a flat-head combination electrode. Cold and hot extraction varied by as much as 2.6 units m 11 types of paper and board, with an average difference of 0.6. The surface measurements were not consistently above or below the hot extraction measurements, but they differed from then by almost a whole pH unit. The biggest differences were found with the buffered papers.

Richard D. Smith, "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. 76 pp., 35 refs.

The author reviews the literature on the contact method of measuring pH, and presents conclusions from 15 older books tested for him at IPC and Chicago Paper Testing Lab. "Differences between the contact and the TAPPI cold extraction pH values were greatest when the buffering capacities were least.... The contact pH and TAPPI cold extraction pH values become more accurate estimators of the true pH as the age of the paper being tested increases.... Wetting time... has been related to the differences between the contact pH and cold extraction pH values."

Bertie L. Browning. Analysis of Paper. 2nd ed. Marcel Dekker, New York & Basel, 1977.

"The term 'pH of paper' is essentially undefinable because the heterogeneous system of fibers, additives, and adsorbed water do not conform to the basic definition of pH established for aqueous solutions.... In the absence of hydrolyzable salts, the hot and cold extraction methods may be expected to give about the same values." The author reviews the methods of finding the pH of the water extract by cold extraction, hot extraction, surface electrode with and without sodium chloride or potassium chloride, color indicators and the isohydric method. The isohydric method involves preparation of a series of solutions at pH 2, 3 and so on up, and measurement of the pH of each solution over time after squares of paper have been put into each--a lot of trouble.