As soi" drops, so do all other sulfur species with positive oxidation states. The solution species
of sulfur are then controlled either by metal sultides or by rhombic sulfur. A
further discussion of these controls is given in Section 17.4.
17.2 DISSOCIATION OF SU LFUR ACIDS
The dissociation constants of the various sulfur acids are given by Reactions
16 through 28 of Table 17.2. These acids are discussed in the order in which
they appear in the table.
Sulfuric acid (HZSOZ) is a strong acid and is essentially fully dissociated
to SO? throughout the pH range of soils. Reaction 17 of Table 17.2 indicates
that at pH 1.98 50% of total sulfate would be present as HSO; , and this
percentage decreases by a factor of 10 for each unit increase in pH.
Sulfurous acid (HZSOQ) is much weaker. At pH 7.18 equal amounts of
HSO and sog- are present and their ratio changes by a factor of 10 for
each unit change in pH. Although sulfurous acid may be added to soils. it IS
unstable and Qnnn rlimnnears. In most soils it nxidizes to SO? 7. whereas in