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.