AIC’s 45th Annual Meeting – Book & Paper + Research & Technical Studies group talks, May 31, 2017 – “Revisiting paper pH determination: 40 years of evolving practice in the Library of Congress Preservation Research and Testing Laboratory” by Cindy Connelly Ryan

Cindy’s talk was a mightily condensed summary a few of the techniques for measuring the pH of paper that the Preservation Research and Testing Division (PRTD) at the Library of Congress has investigated over the last 4 decades. Her introduction was a summary of the challenges presented by this task. Due to the chemical structure of cellulose and the nature of paper, most methods can only approximate the pH of paper. The method of sample preparation can impact the results of measuring. How paper ages means that there may be a different pH in different regions. The ions that dictate the pH may not be soluble in water, making measuring pH harder. And atmospheric carbon dioxide can react with your solution and affect your results. Notice that I said “solution.” Cindy ended her introduction by noting that you can’t measure the pH of a solid. But you can approximate it, and the PRTD has been trying to identify the best way to do this for decades.

The PRTD’s focus on the pH of paper began in 1971 with the deacidification program. Chemist George B. Kelly used titrated alkalinity and titrated acidity as an “external yardstick”, and four different extraction methods: TAPPI cold method, TAPPI hot method, surface pH measurement, and “pulped” pH method. Kelly determined that for acidic papers, the method of measuring pH didn’t matter much, but the alkaline papers had an acidic pH despite their 1% alkaline reserve. The hot extraction method was shown to be much more accurate with alkaline papers, as it was likely better at getting all the ions into solution. The pulp method came close. Cindy then went on to talk about the uncertain origins of the pulp method (i.e. it’s not discussed in any published literature, but is mentioned frequently in internal documents from the PRTD). (I do wish that Cindy had gone into detail about the process of each method of extraction, because I wasn’t too sure about how each process worked. She doesn’t mention pH strips, gels, or pH meters at all in this talk. And TAPPI stands for the Technical Association of the Pulp and Paper Industry.)

Then Cindy skipped to the late 1990’s (she does mention that a few papers had been published in the decades between). By this time, the PRTD ramps up its documentation efforts, as well as its protocols for sample collection and homogenization. Most of these protocols were put on their website. During the renovation of the instrumental suite in 2007, the lab’s emphasis shifted to developing non-destructive and micro-invasive techniques, which were more appropriate for art objects rather than circulating collections materials. This meant that the sampling methods had to adjust accordingly.

To address the new challenge of micro samples (or none at all), the PRTD tried to make surface pH measurement work, but found that tideline formation and sensitive media made that difficult. “Miniaturization” was another method the PRTD tried. For this technique, sample size can be a few milligrams to a few micrograms, depending on the paper to water ratio and other details of sample preparation. They found that slurrying helps, but filtration makes no difference in pH measurement.  In addition, controlling the amount of carbon dioxide was key to getting an accurate reading with acidic papers. Both purged bags and sealed vials were tried, with comparable standard deviations but slightly different pH readings. The pH readings from the micro methods agreed fairly well with macro methods.

One of the best takeaways from Cindy’s talk was when she shared that during their renovation, the PRTD was sending out samples to a contractor for pH measurement, and papers that had an alkaline reserve were coming back with an acidic pH. Their conclusion was that not every method is appropriate for every type of paper, and that sample preparation can also affect results.

Here were some tips about each of the methods, taken from Cindy’s recap slide: The Hot Method is closest to objective measurement, but takes two hours per sample. The Pulped/Blender method generally agrees with the hot method, but is faster. The ISO cold method has a much higher standard deviation than the TAPPI cold method. Thea Surface pH method has the highest standard deviation of any method tested, and is difficult with alkaline papers, thick boards, and boards with adhesives. This method also causes tidelines. And the Mini Method is also difficult with thick boards, but the results comparable in repeatability to large scale extraction methods.

So, what does it take to accurately measure the pH of a piece of paper? A focus on repeatability and an optimistic attitude! The scientists and preservation specialists at the PRTD struggle with many of the same challenges that the rest of us do, albeit with fancier equipment. It sounds like just getting a ballpark figure for pH is as close as we can hope for for now. The PRTD is still investigating methods, and we should all look forward to their results!

Finally, one cool tip: You can make your own micro blender with a homemade Mylar blade attached to a Dremel tool!