sulfur dioxide
One of the oxides of sulfur. Sulfur dioxide (SO
2 ), which is probably the most common
cause of the deterioration of paper, is produced
when sulfur, or materials containing sulfur e.g.,
coal, oil—is burned in air. It is present in
varying amounts in the atmosphere of almost all
industrialized nations. Sulfur dioxide, even at
higher concentrations than are normally found in
the atmosphere, is not in itself harmful to paper
or bookbinding materials, but it can be converted
into highly destructive sulfuric acid, either by
being oxidized to sulfur trioxide (SO 3
), and then into sulfuric acid (SO 3 +
H 2 O = H 2 SO 4
), or by reaction of SO 2 with water to
form sulfurous acid, which in turn oxidizes to
form sulfuric acid (H 2 SO 3
+ H 2 O = H 2 SO
4 + 2H). Some authorities maintain that
this conversion of sulfur dioxide to sulfuric acid
requires the presence of small amounts of copper
or iron, i.e., minute particles of metal broken
from the refining apparatus and entering the pulp,
as is almost always the case with papers produced
during the past 150 to 200 years; whereas others
maintain that the process will take place
regardless of metallic content and that the copper
and/or iron impurities simply function as
catalysts and thus quicken the rate of
deterioration.
Archival papers should be protected against
atmospheric sulfur dioxide. insofar as possible,
by: 1) removing dangerous or potentially dangerous
impurities, e.g., catalytic materials, from the
paper, or, alternatively, manufacturing paper free
from them; 2) the addition of alkaline and
buffering agents to neutralize any sulfuric acid
that does form; 3) destroying the chemical
(catalytic) activity of the metallic impurities by
the use of inhibitors, such as magnesium salts;
and 4) storing paper in an atmosphere as free as
possible from sulfur dioxide.
Sulfur dioxide is also used as a reducing agent in
the preparation of basic chrome tanning liquor
from sodium dichromate in acid solution: Na
2 CrO 7 + H 2 O +
3SO 2 = 2Cr(OH)SO 4 + Na
2 SO 4 . (193 , 198 , 248 )
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