a hundred folds but very weak papers will only endure a few folds and for
these the test is not suitable.
10.2 Chemical Methods
The alternative to measuring the physical attributes of paper is to measure the
chemical properties of the paper. As with physical tests, care has to be taken
that the sample of paper is representative of the entire sample, for example, in
an old book the edges of a book block are often more degraded than the
centre because the edge has absorbed air pollutants and is often of higher
acidity. The popular methods will now be briefly described, readers should
see fuller descriptions before attempting any of these methods.
Although some papers are acidic when new, ageing generally increases acid-
ity and pH can be used to monitor the progress of ageing. There are several
methods for measuring the pH of paper including pH-sensitive dyes that can be
placed onto the paper, and surface pH electrodes that can be placed directly on
the surface of the dampened paper. However, the best method is to macerate
paper in pure water and measure the pH of the aqueous extract after a period.
There are several standard methods that are similar. Typically, they mix 1 g of
paper with 50 mL water (hot or at room temperature) and measure pH after 1 hour.
The solubility of paper carbohydrates in sodium hydroxide (NaOH) of vari-
ous strengths is a useful method for both characterising new paper and fol-
lowing degradation. One method uses a 17.5% solution of NaOH to treat paper,
so that, of the carbohydrate component, only cellulose of DP200 remains
undissolved, and lignin and mineral fillers remain largely unaffected. The
insoluble carbohydrate is called -cellulose. The -cellulose content will
decrease on ageing. After the 17.5% NaOH treatment the solution contains
- and -cellulose. The -cellulose, which has a DP of 14–200, is soluble until
the alkali is neutralised, while the -cellulose remains in solution after neu-
tralisation; it has a DP of less than 10. Materials other than paper carbohydrates,
e.g. lignin, interfere with the results.
A related method involves measuring the solubility of the paper in 1%
NaOH at 100°C after a period of time, e.g.60 min. The solubility is propor-
tional to the copper number (see below) that is itself proportional to the num-
ber of carbonyl groups in the paper, a measure of the oxidation and
hydrolysis of the paper. This method has also been used on lignin-containing
fibres of various kinds with some success.
The copper number of paper may be obtained using copper(II) solution,
some of which is reduced to Cu(I) oxide as the carbonyl groups are oxidised
to carboxylic acid. There are several variants of the test, one of which
involves treating the produced Cu(I) oxide with phosphomolybdic acid. The
reduced molybdenum is then titrated with potassium permanganate.
52 Chapter 3