Geology Reference
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SO 2 4 þ 8H 1 þ 6e
K 4 ¼ 10 36.2
"
S (s) þ 4H 2 O
Reduction of S VI to S II :
SO 2 4 þ 9H 1 þ 8e
HS þ 4H 2 O
K 5 ¼ 10 34.0
"
Reduction of S 0 to S II :
S (s) þ 2H 1 þ 2e
K 6 ¼ 10 4.8
"
H 2 S (aq)
Construction of sloped lines involving a solid phase:
pe ¼ log K 6 /2 - log{H 2 S (aq) }/2 - pH
On the line, assume that S T ¼ log{H 2 S (aq) } ¼ 10 2 mol L 1
so
pe ¼ 3.4-pH.
20
HSO 4 -
10
SO 4 2-
S (s)
0
H 2 S
HS -
-10
0
2
4
6
8
10
12
pH
The information contained in this pe-pH diagram is particularly
useful in the determination of stability zones for solid phase metal
sulfides (Example 3.14). For example, it is the reduction of sulfate to
sulfide which determines the stability field of pyrite (FeS 2 ).
Example 3.14: Construction of a pe-pH diagram for the Fe-O 2 -'S'-
CO 2 -H 2 O open system where { Fe T } is 1 10 6 mol L 1 , { S (aq) T } ¼ 1
10 2 mol L 1 and p CO2 ¼ 10 3 atm.
Reduction of Fe III to Fe II :
Fe 31 þ e
Fe 21
K 1 ¼ 10 13.0
"
Fe(OH) 21 þ e þ H 1
Fe 21 þ H 2 O
K 2 ¼ 10 15.2
"
 
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