Phenylhydrazine is able to react with carbonyl groups to give coloured
compounds. Paper sheets can be reacted with this reagent or modifications of
it (e.g.4-nitrophenylhydrazine) and the colour produced can be measured by
reflectance spectroscopy. The colour may also be measured by dissolving the
paper and measuring the absorbance of the solution. Lignin interferes badly
with this test.
A test that measures the quantity of carboxyl groups uses methylene blue
absorption. A sample of the paper is immersed in a solution of methylene
blue and the absorbance of the dye solution at 620 nm is measured before
immersion of the paper and at the end of the experiment. The amount of sam-
ple needed to produce 50% exhaustion of the dye solution is used to calculate
the carboxyl content.
Chemical reagents can dissolve cellulose. Three of these are aqueous solu-
tions containing copper ethylenediamine, cuprammonium hydroxide or cad-
mium ethylenediamine (cadoxen). The solutions all cause some damage to
the cellulose by alkaline degradation during dissolution. Some workers per-
form a pre-treatment in which oxidised groups are reduced, which decreases
the amount of alkaline degradation considerably. Measurements of cellulose
solutions in the copper-containing reagents should ideally be performed in
the absence of air as oxygen can cause degradation of the cellulose. The
cadmium-containing reagent is very toxic and for that reason is less fre-
quently used than it used to be, however, any reader of the paper conservation
research literature will find it mentioned more often than the others men-
tioned here. The higher the DP of the cellulose, the greater is the viscosity of
the solution.
Although DP can be determined by measuring the viscosity of cellulose
solutions, this is a skilled business and needs much practice. For those labo-
ratories with the money for expensive equipment, a better option may be size
exclusion chromatography (SEC). Much work in conservation research has
been carried out in cadoxen solutions, however, cadoxen is aggressive to SEC
columns and more recent work has used LiCl/dimethyl acetamide. The size
exclusion column holds back larger molecules and when the cellulose
emerges from the column, the smallest molecules come out first. A detector
that measures the amount of material emerging from the column enables the
user to see not only the average DP but how the DPs are distributed about the
average. The column can be calibrated with solutions containing carbohy-
drates or other polymers of known DP.
Fourier transform infrared (FTIR) spectroscopy produces a spectrum for
paper with many peaks. Each peak corresponds to a vibration in a chem-
ical bond. The frequency of infrared peaks is usually measured in the unit
cm^1 , the number of wavelengths which can be fitted into a centimetre. Peaks
can be attributed to types of chemical bond; for example, a peak at 2900 cm^1
Paper 53