Table 17.2) so the indicated HZSO activity is approximately half the total
sulfur. The b branch of these lines corresponds to the redox region where
SOQ' is stable, or ZS = soii The equilibrium reaction used to construct
the b lines is
soir + 8e` +10H+ ;-; HZSO + 4H20 log KO = 40.67 (17.9)
from which
log H280 = 40.67 + log soi- - 2pH - 8(pe + pH) (17.10)
For pH 7.0 and 10`3 M SO`, Eq. 17.10 simplifies to
log H250 = ` +
From Fig. 17.3 it is apparent that S(rhombic) cannot form in soils of H 7 0
having only 10`3 M total sulfur in solution because such soils w ld
be undersaturated with res t t ' Ou J “BFS
1 ' iss pee 0 S(fh0mblC). Onlv when total sulfur in solution exceeds 1()`1'55 M is it possible for S(rhombic) to form. Higher
levels of soluble sulfur can lead to the precipitation of elemental sulfur. For example, a 1 M solution of sulfur can precipitate S(rhombic) in the pe + pH
range of 2.60 to 3.63 as indicated by the dashed lines that rise above the S(rhombic) line in Fig. 17.3.
As shown in Section 17.2, H2S(g) can be lost to the atmosphere at low redox. The partial pressure of H2S(g) is obtained from the relationship
HZSO ;;**_ H2S(g) log K’ = 0.99
Thus the partial pressure of H 2S(g) in atmospheres is equal to 10 times the molar activity of HZSO. Thus the H2S(g) lines in Fig. 17.3 would lieone log unit above the H 2 SO lines.
The formation of elemental sulfur in soils is highlv unlikely for several
reasons: (1) the pe + pH must be between 2.5 and (2) total sulfur in
solution must exceed 10"?55 M, and (3) the system must be confined.
otherwise H S ` ` '
2 (g) will escape. Furthermore, the formation of S(rhombic)