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pH

 

 

 

The pH scale is a yardstick used to measure the number of hydrogen ions (H+ [acid]) in solutions. The pH scale runs from 0 to 14. The lower numbers refer to acid solutions, while the higher numbers refer to alkaline or basic solutions. At pH 7 (neutral) the concentration of hydrogen ions equals the concentration of hydroxide ions. Any solution with a pH lower than 7 has more hydrogen ions than hydroxide ions in solution. Any solution with a pH higher than 7 has fewer hydrogen ions than hydroxide ions in solution. The pH scale is a logarithmic progression. This means numbers on the pH scale are based on powers of ten. A pH of 2 therefore, indicates ten times fewer hydrogen ions than a pH of 1, pH 3 has ten times fewer hydrogen ions than pH 2 and one hundred times fewer hydrogen ions than pH 1. A pH of 4 has ten times fewer hydrogen ions than pH 3, one hundred times fewer hydrogen ions than pH 2, and one thousand times fewer than pH 1. Since we have seen how hydrogen ions break the bonds holding the cellulose chain together, and since pH is the measurement of these acid ions, are we, therefore, able to specify a paper with a pH of 7.0 or higher with the expectation that it will be archival? The answer, unfortunately, is no! For one thing weak acids may not be fully disassociated. Therefore, you do not always get an accurate picture of acids present by measuring pH. Let us imagine someone offered you a brown kraft paperboard. It is purportedly acid free and, in fact, the pH is 8.0. On further examination you discover that the paperboard contains no alkaline buffering such as calcium carbonate. Now the paperboard is, in a technical sense and at least initially, acid free. However, this paperboard should not be used for archival preservation. There is no alkaline buffer present to neutralize the acids from pollutants in the surrounding environment, and the paper is full of lignin which will break down and form acids which will sever the bonds holding the cellulose chain together. Adding alkaline buffering to a paper which is full of lignin will not keep this paper acid free. Remember, if one half to one percent of the cellulose bonds are broken, the paper will be virtually useless. It also will be a source of acid which can migrate to and damage adjacent materials. Therefore, we should never rely only on the term acid free to specify a paper we intend to use for conservation purposes. It is important to know the pH of a paper product but pH must be used in conjunction with other specifications to be meaningful. We will look at the specifications required to insure a paper is archival, shortly.