Paper pH level

The reaction or acidity/alkalinity of paper is characterised by concentration of hydrogen ions (pH).

pH<7   [ H+ ] > [ OH- ] acidic environment

pH=7   [ H+ ] = [ OH- ] neutral environment

pH>7   [ H+ ] < [ OH- ] basic environment

Paper reaction is dependent on the raw material that it is made of, the manufacturing technology and later conservation conditions. Paper acidity has a significant effect on the lifespan of paper. The pH of high quality paper with a long lifespan is >6.5. Acidity in the range of 6.0–7.0 is generally considered to be the normal paper acidity. Paper reaction 5.0–6.0 tells us that it is an averagely acidic paper and pH <5.0 shows that the paper is acidic. Acidic paper ages significantly faster, is mechanically less resistant and sensitive to the effect of different harmful environmental factors.

Different methods are used to measure paper reaction.

1) pH determination by paper extraction. A fixed amount of paper (usually 1 gram) is soaked in distilled water at room temperature for 1 hour. Paper with very strong glue bonding and cardboard can be soaked up to 24 hours. The accurate concentration of hydrogen ions is determined in the resulting extract with the help of the pH meter.

2) pH determination by contact method. In order to be measured, the paper is placed on a sponge or a piece of film moistened with distilled water.  Distilled water is also dripped on the paper surface. The reaction of the water drop is measured by the pH-metre with the help of a special flat electrode.  

3) pH determination by indicators. Indicators are chemical compounds which show the pH of the solution by its colour. Solutions, indicator papers and so-called pH-pens are used.  

PH determination by paper extraction is the most accurate method but damages the paper the most. pH determination with the indicators is the fastest and the easiest but also provides the most inaccurate result. The most used pH measurement method in the field of preservation is the contact method.

pH determination by contact method